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Tensile Properties and Fracture Mechanisms of Corn Bract for Corn Peeling Device Design

Tensile Properties and Fracture Mechanisms of Corn Bract for Corn Peeling Device Design agriculture Article Tensile Properties and Fracture Mechanisms of Corn Bract for Corn Peeling Device Design 1 , 2 1 , 2 , 2 , 3 Zhenye Li , Jun Fu * and Xiwen Luo Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130025, China; zhenye19@mails.jlu.edu.cn College of Biological and Agricultural Engineering, Jilin University, Changchun 130025, China; xwluo@scau.edu.cn College of Engineering, South China Agricultural University, Guangzhou 510642, China * Correspondence: fu_jun@jlu.edu.cn Abstract: This paper describes the physical and tensile properties of corn bracts during a whole harvest period by using two corn cultivars, aiming to realize efficient peeling with minimum energy performance and decrease the incidence of ear damage. The value range and change rule of tensile properties were obtained by combining mechanical experiments and numerical statistics. Meanwhile, mathematical models were established for tensile properties depending on bract moisture content and bract thickness. The experimental results show that the tensile properties of leaf blade were affected by the orientation between pulling force and longitudinal vein, and that parallel orientation was greater than perpendicular. Further, the tensile properties of leaf sheaths depended on the angle between pulling force and natural growth direction of the bract in the following order: 0 > 90 > 180 . A larger pulling force angle can improve the probability of bract fracturing at the root of leaf sheaths, which helps reduce bract residue on the peduncle. In addition, the fracture mechanisms of leaf blades and sheaths were expressed from physiological and morphological perspectives. The experimental results are believed to be able to provide theoretical guidance by which to design and optimize Citation: Li, Z.; Fu, J.; Luo, X. Tensile Properties and Fracture Mechanisms corn-peeling devices. of Corn Bract for Corn Peeling Device Design. Agriculture 2021, 11, 796. Keywords: corn bract; tensile properties; fracture properties; mechanical peeling https://doi.org/10.3390/agriculture Academic Editor: Andrea Colantoni 1. Introduction Corn (Zea mays L.) is one of the most versatile cereal crops, and has multiple applica- Received: 12 July 2021 tions as food consumption, industrial raw material, and feeding stuff. The rapid increase Accepted: 18 August 2021 of corn acreage results in a higher requirement for corn harvest quality [1]. Corn peeling is Published: 20 August 2021 the key link in corn ear harvest, and directly affects the storage and follow-up processing of corn ears [2,3]. However, ears are often subjected to excessive extrusion and friction due to Publisher’s Note: MDPI stays neutral an inappropriate parameter setting of the peeling unit, which leads to serious mechanical with regard to jurisdictional claims in damage [4,5]. Mechanical damage caused by external loading is the main factor behind a published maps and institutional affil- decline in quality and the total production of corn, and can cause serious financial losses iations. in the corn industry [6]. Therefore, under the premise of bract removal, an investigation into how to reduce and prevent the mechanical damage caused by excessive peeling force deserves special attention. Mechanical properties of plant leaves are crucial to understanding leaf fracture mecha- Copyright: © 2021 by the authors. nisms and the designs of leaf removal machinery [7,8]. Figure 1 shows a schematic diagram Licensee MDPI, Basel, Switzerland. of corn peeling. During the peeling operation, two peeling rollers rotate toward each other This article is an open access article and tilt downward at a specific angle. In the direction parallel to the axis of the peeling distributed under the terms and roller, the corn ear slides down along the groove of two rollers under its own gravity and conditions of the Creative Commons the force of delivery device, while the corn bract is subjected to an upward friction force. Attribution (CC BY) license (https:// Meanwhile, in the direction perpendicular to the axis of the peeling roller, the relative creativecommons.org/licenses/by/ rotating peeling rollers apply different friction torques to the corn ear due to the different 4.0/). Agriculture 2021, 11, 796. https://doi.org/10.3390/agriculture11080796 https://www.mdpi.com/journal/agriculture Agriculture 2021, 11, x FOR PEER REVIEW 2 of 15 Agriculture 2021, 11, 796 2 of 15 the relative rotating peeling rollers apply different friction torques to the corn ear due to the different materials and surface morphologies, which makes the corn ear rotate around materials and surface morphologies, which makes the corn ear rotate around its axis. In the its axis. In the process of movement, the peeling rollers grab and pull the bract under the process of movement, the peeling rollers grab and pull the bract under the action of friction. action of friction. Once the force provided by the rollers is greater than the fracture force Once the force provided by the rollers is greater than the fracture force of the bract, the of the bract, the bract is pulled off and separated from the corn ear. Consequently, testing bract is pulled off and separated from the corn ear. Consequently, testing the mechanical the mechanical properties of corn bract and determining the mechanical conditions for properties of corn bract and determining the mechanical conditions for completing the completing the peeling operation are of great significance to realizing high efficiency and peeling operation are of great significance to realizing high efficiency and low loss during low loss during corn peeling. corn peeling. Figure 1. Schematic diagram of mechanical corn peeling. Figure 1. Schematic diagram of mechanical corn peeling. Several research efforts have reported on the mechanical properties of various plant Several research efforts have reported on the mechanical properties of various plant leav leaves. es. R Resear esearchers b chers believe elieve tthat hat p plant lant lea leaves ves are are u usually sually ani anisotr sotrop opic ic in d in dif ifffer erent ent d dir irect ections, ions, thus their mecha thus their mechanical nicalpr prop operties erties rest rest with with the the posi position tion a andndir d di ection rection of the of the applied appliload ed loa [9d ]. Kohyama et al. [10] revealed that the mechanical strength of a cabbage leaf depends on [9]. Kohyama et al. [10] revealed that the mechanical strength of a cabbage leaf depends vein orientation, and that strength proved to be greater when the vein orientation was on vein orientation, and that strength proved to be greater when the vein orientation was parallel to the test direction. The same result was found by Toole for lettuce [11]. On parallel to the test direction. The same result was found by Toole for lettuce [11]. On the the other hand, there is some controversy regarding the influence of moisture content other hand, there is some controversy regarding the influence of moisture content on the on the mechanical properties of biomass in the literature. Kneebone et al. [12] proposed mechanical properties of biomass in the literature. Kneebone et al. [12] proposed that dry that dry or withered leaves tend to be somewhat stronger than succulent leaves. On the or withered leaves tend to be somewhat stronger than succulent leaves. On the contrary, contrary, Tang et al. [13] pointed out the mean tensile strength of a rice leaf gradually Tang et al. [13] pointed out the mean tensile strength of a rice leaf gradually decreases decreases with decreasing moisture content. In other respects, Jacobs et al. [14] found that with decreasing moisture content. In other respects, Jacobs et al. [14] found that the cross- the cross-sectional area was the main factor affecting the differences of fracture force and sectional area was the main factor affecting the differences of fracture force and fracture fracture energy among species. Overall, the mechanical properties of plant leaves are highly energy among species. Overall, the mechanical properties of plant leaves are highly influ- influenced by moisture content, the cross-sectional area, and the position and direction of enced by moisture content, the cross-sectional area, and the position and direction of the the applied load. To our knowledge, relatively few comprehensive studies have been done applied load. To our knowledge, relatively few comprehensive studies have been done on on the tensile properties of corn bract that could help realize bract removal with minimum the tensile properties of corn bract that could help realize bract removal with minimum energy performance to reduce ear mechanical damage during peeling operations [15,16]. energy performance to reduce ear mechanical damage during peeling operations [15,16]. To address the above issues, the physical and tensile properties of corn bract were To address the above issues, the physical and tensile properties of corn bract were tested using two corn varieties after entering the harvest period. The purpose of this study tested using two corn varieties after entering the harvest period. The purpose of this study was to quantify and compare the effects of bract moisture content, bract thickness, and was to quantify and compare the effects of bract moisture content, bract thickness, and tensile loading direction on the tensile properties of leaf blades and sheaths throughout tensile loading direction on the tensile properties of leaf blades and sheaths throughout the harvest period. Mathematical models were established for the fracture force, tensile the harvest period. Mathematical models were established for the fracture force, tensile strength, and fracture energy of the leaf blades and sheaths depending on bract moisture strength, and fracture energy of the leaf blades and sheaths depending on bract moisture content and bract thickness. The models may be used to guide the design and parameter content and bract thickness. The models may be used to guide the design and parameter setting of corn peeling devices. In addition, the fracture modes and fracture mechanisms of setting of corn peeling devices. In addition, the fracture modes and fracture mechanisms the leaf blades and sheaths under different tensile loading directions are expressed from of the leaf blades and sheaths under different tensile loading directions are expressed from the perspectives of plant physiological and morphological characteristics. the perspectives of plant physiological and morphological characteristics. 2. Materials and Methods 2.1. Corn Materials Two corn varieties (Feitian 358 and Jidan 558) that are widely planted in Northeast China were selected as experimental varieties. Feitian 358 was cultivated by the Dunhuang Agriculture 2021, 11, x FOR PEER REVIEW 3 of 15 2. Materials and Methods Agriculture 2021, 11, 796 3 of 15 2.1. Corn Materials Two corn varieties (Feitian 358 and Jidan 558) that are widely planted in Northeast China were selected as experimental varieties. Feitian 358 was cultivated by the Dunhuang Seed Industry Co., Ltd., Wuhan, China. Jidan 558 is a native corn variety bred Seed Industry Co., Ltd., Wuhan, China. Jidan 558 is a native corn variety bred by the by the Corn Research Institute of Jilin Academy of Agricultural Sciences, Jilin, China. The Corn Research Institute of Jilin Academy of Agricultural Sciences, Jilin, China. The two two corn varieties are generally sown in late April to early May, and it takes 128–131 days corn varieties are generally sown in late April to early May, and it takes 128–131 days from seedling emergence to maturity. After corn grains had fully matured, the corn ears from seedling emergence to maturity. After corn grains had fully matured, the corn ears with bracts used in this study were harvested on five different days in harvest season (24 with bracts used in this study were harvested on five different days in harvest season September, 1 October, 8 October, 15 October and 22 October 2020) from the Agricultural (24 September, 1 October, 8 October, 15 October and 22 October 2020) from the Agricultural 0  0 Experimental Base of Jilin University (N 43°56′, E 125°14′) in Changchun, Jilin province, Experimental Base of Jilin University (N 43 56 , E 125 14 ) in Changchun, Jilin province, China. The harvested ears were stored in a sealed bag in a shaded place. All tensile tests China. The harvested ears were stored in a sealed bag in a shaded place. All tensile tests were completed within two days of harvesting. were completed within two days of harvesting. As shown in Figure 2, a corn bract is composed of leaf blades and leaf sheaths [17]. As shown in Figure 2, a corn bract is composed of leaf blades and leaf sheaths [17]. Overlapped bracts grow on the peduncle nodes and wrap the bare ear. Field investigation Overlapped bracts grow on the peduncle nodes and wrap the bare ear. Field investigation revealed that the fracture positions of the bract mainly occurred somewhere on the leaf revealed that the fracture positions of the bract mainly occurred somewhere on the leaf blade or at the junction between leafs and peduncle during the peeling operation [18]. blade or at the junction between leafs and peduncle during the peeling operation [18]. Ther Therefore efore, ,in inthis this study study, the , thleaf e leaf blades blades and and leaf le sheaths af sheaths of corn of corn bracts bracts w were selected ere selected as two as test two test posi positionstfor ions the for the tensi tensile test. le test. Figure 2. Section diagram of morphological structure of a corn ear. Figure 2. Section diagram of morphological structure of a corn ear. 2.2. Equipment and Methods 2.2. Equipment and Methods 2.2.1. Physical Properties Test Method 2.2.1. Physical Properties Test Method To determine the physical properties of corn materials, 50 ears each of Feitian 358 and To determine the physical properties of corn materials, 50 ears each of Feitian 358 Jidan 558 were randomly selected for measuring from five harvest days. For each corn and Jidan 558 were randomly selected for measuring from five harvest days. For each corn ear, the length, width at half-length, blade thickness, sheath thickness of the third bract, ear, the length, width at half-length, blade thickness, sheath thickness of the third bract, and the total number of bracts were measured. The mean value and standard deviation and the total number of bracts were measured. The mean value and standard deviation (SD) were calculated. Meanwhile, the moisture content (w.b.) (i.e., wet basis) of the kernel, (SD) were calculated. Meanwhile, the moisture content (w.b.) (i.e., wet basis) of the kernel, bract, and peduncle from five harvest days were determined by using a DZF-6050 electric bract, and peduncle from five harvest days were determined by using a DZF-6050 electric thermostatic drying oven (Rongshida Electric Equipment Co., Ltd., Kunshan, China) at thermostatic drying oven (Rongshida Electric Equipment Co., Ltd., Kunshan, China) at 105  1 C during 24 h. In addition, to compare the physical properties of different layers 105 ± 1°C during 24 h. In addition, to compare the physical properties of different layers of bract, the blade thickness, sheath thickness, and moisture content (w.b.) of the second to of bract, the blade thickness, sheath thickness, and moisture content (w.b.) of the second sixth bracts of the two varieties harvested on 1 October 2020 were measured. to sixth bracts of the two varieties harvested on 1 October 2020 were measured. 2.2.2. Tensile Test Method Tensile test is the most common and effective method for studying the tensile proper- ties of materials [19–21]. The specimen preparation process of this study was as follows: each leaf blade was cut into two specimens for the blade tensile test. A rectangular speci- men (30 mm in width and 120 mm in length) was cut along the longitudinal vein direction from the central part of the top leaf blade for the longitudinal tensile test (LTT). Another rectangular specimen (30 mm in width and 90 mm in length) was taken perpendicular to Agriculture 2021, 11, x FOR PEER REVIEW 4 of 15 2.2.2. Tensile Test Method Tensile test is the most common and effective method for studying the tensile prop- erties of materials [19–21]. The specimen preparation process of this study was as follows: Agriculture 2021, 11, 796 4 of 15 each leaf blade was cut into two specimens for the blade tensile test. A rectangular speci- men (30 mm in width and 120 mm in length) was cut along the longitudinal vein direction from the central part of the top leaf blade for the longitudinal tensile test (LTT). Another the longitudinal vein direction from the central part of the remaining leaf blade for the rectangular specimen (30 mm in width and 90 mm in length) was taken perpendicular to transverse the longitud tensile inal vein test d (TTT). irection f Next, rom t a peduncle he centrawas l part taken of th of e frema the ear ining along leaf bl with ade bract for tfor he the sheath transverse tensile test tensile test. The(TTT). connection Next, length a peduncle was between leaf taken sheath off the ear and peduncle along with bract was contr for olled the sheath tensile test. The connection length between leaf sheath and peduncle was con- at 10 mm, and other redundant bracts were removed. Ten test specimens were prepared for each trolled test atgr 10 oup. mmThe , and ot prepar her red ed specimens undant brwer acts wer e sequ e rem entially oved. Te labelled n test and specim sealed ens were in airtight prepared for each test group. The prepared specimens were sequentially labelled and bags, then stored at the optimum condition of 4 C to retain moisture. sealed in airtight bags, then stored at the optimum condition of 4 °C to retain moisture. The tensile tests were performed on a Universal Testing Machine (D-NS type, Chang- The tensile tests were performed on a Universal Testing Machine (D-NS type, Chang- chun Institute of Mechanical Science Co., Ltd., Jilin, China), which attached to a load chun Institute of Mechanical Science Co., Ltd., Jilin, China), which attached to a load cell cell (range of 10 kN, accuracy of 0.5%), as shown in Figure 3. The test specimen was (range of 10 kN, accuracy of 0.5%), as shown in Figure 3. The test specimen was fixed to fixed to the upper beam and the moving beam of the testing machine by a mechanical the upper beam and the moving beam of the testing machine by a mechanical fixture. The fixture. The moving beam moved downward at a constant speed of 10 mm/min until the moving beam moved downward at a constant speed of 10 mm/min until the specimen specimen fractured. A test in which the specimen slid off the fixture or broke in the fixture fractured. A test in which the specimen slid off the fixture or broke in the fixture was was considered invalid. The single trial was repeated 10 times, and the mean value and considered invalid. The single trial was repeated 10 times, and the mean value and stand- standard deviation of each test index were calculated. The load-displacement curve was ard deviation of each test index were calculated. The load-displacement curve was plotted plotted for each effective test, from which the fracture force (F ), tensile strength (s ), and for each effective test, from which the fracture force (Fb), tensile strength (σt), and fracture fracture energy (E ) were determined. energy (Eg) were determined. Figure 3. Setup for tensile test. Figure 3. Setup for tensile test. Fracture force is the maximum force born by a specimen at fracture failure under the Fracture force is the maximum force born by a specimen at fracture failure under the tensile condition. Tensile strength is defined as the fracture force per unit cross-sectional tensile condition. Tensile strength is defined as the fracture force per unit cross-sectional area of specimen. Thereinto, the product of the average thickness (leaf blade or leaf sheath) area of specimen. Thereinto, the product of the average thickness (leaf blade or leaf sheath) and the specimen width was used as the estimated value of original cross-sectional area and the specimen width was used as the estimated value of original cross-sectional area of of the specimen. Tensile strength was calculated using the following equation [9,19,22]. the specimen. Tensile strength was calculated using the following equation [9,19,22]. σ = , b (1) s = , (1) wt ⋅ w t where s is the tensile strength (MPa), F is the fracture force (N), w is the specimen width t b (mm), and t is the average specimen thickness (mm). Fracture energy is the energy consumption required for material fracture, and it is the internal energy caused by material deformation under an external load. Fracture energy is described as the area under the load-displacement curve between the test origin and maximum load point. It may be expressed mathematically as follows [14,23,24]: n=i1 F + F n+1 E =  D D , (2) ( ) g n+1 n n=0 Agriculture 2021, 11, 796 5 of 15 where E is the fracture energy (J), F and F are values of tensile load (N), and D and g n n+1 n D are values of the specimen’s elongation (mm). n+1 2.3. Statistical Analysis Experimental data were analyzed applying regression analysis and one-way analysis of variance (ANOVA) using SPSS 24 software (SPSS Inc., Chicago, IL, USA). Duncan’s multiple range test was used to compare the mean values, and the significance level was set at 0.05 [20]. 3. Results and Discussion 3.1. Physical Properties of Corn Materials The physical properties of Feitian 358 and Jidan 558 are presented in Table 1. From 24 September to 22 October 2020, the moisture contents of the kernel, bract and peduncle decreased by 41.82%, 68.21%, and 21.94% for Feitian 358, respectively, and showed a decrease of 44.95%, 75.63%, and 33.33% for Jidan 558, respectively. With the delay of harvest day, the moisture content of each part of the ear showed a decreasing trend. Table 1. Physical properties of corn materials used in this study (mean  SD). Harvest Date (in the Year 2020) Physical Properties Cultivars 24 September 1 October 8 October 15 October 22 October Blade thickness (mm) 0.33  0.03 0.33  0.04 0.31  0.03 0.28  0.02 0.27  0.02 Sheath thickness (mm) 0.57  0.07 0.56  0.04 0.54  0.05 0.51  0.04 0.50  0.02 Bract length (cm) 21.84  3.05 21.35  2.32 21.12  2.15 20.86  1.98 20.83  2.22 Feitian Bract width (cm) 9.92  1.65 9.56  1.78 9.63  1.23 9.34  1.77 9.13  1.22 Kernel moisture content (%) 42.35 39.47 33.62 29.74 24.64 Bract moisture content (%) 65.84 52.97 35.93 25.68 20.93 Peduncle moisture content (%) 82.59 83.74 75.58 70.21 64.47 Blade thickness (mm) 0.38  0.03 0.37  0.03 0.35  0.04 0.30  0.04 0.30  0.03 Sheath thickness (mm) 0.81  0.07 0.79  0.03 0.78  0.06 0.75  0.03 0.73  0.02 Bract length (cm) 23.23  2.85 22.72  2.58 22.56  3.02 22.10  2.25 22.12  2.12 Jidan 558 Bract width (cm) 10.77  2.11 10.35  1.86 10.22  1.58 9.82  1.89 9.65  1.65 Kernel moisture content (%) 45.67 42.36 33.23 27.87 25.14 Bract moisture content (%) 72.43 58.73 38.19 21.77 17.65 Peduncle moisture content (%) 86.17 82.23 74.98 63.26 57.45 The length, width, blade thickness, and sheath thickness of the third bract all decreased with the delay of harvest day for both corn varieties, which might have been caused by the decrease in bract moisture content. An explanation for this could be that the pore spaces of leaf are filled with water when leaf moisture content is high, while the water in the pores dries when leaf moisture content decreases, so the leaf size becomes smaller [25]. Vincent et al. [21] reported a similar conclusion that ryegrass leaves thinned as the leaves dried. The average thickness and moisture content of the second to sixth bracts measured on 1 October 2020 are listed in Table 2. For the two varieties, the average thicknesses of leaf blades and sheaths decreased gradually from outer to inner layers, whereas the average moisture content increased firstly and then decreased from outer to inner layers. The total number of bracts for Feitian 358 was between 6 to 10 layers, and 76% of the ears had 7 layers. The total number of bracts for Jidan 558 ranged from 6 to 11 layers, and 52% of the ears had 8 layers. The outermost bract was often damaged by direct contact with the external environment. Therefore, the second to sixth bracts were used to study the effect of bract thickness on the tensile properties. Agriculture 2021, 11, 796 6 of 15 Table 2. Physical properties of the second to sixth bracts used in this study (mean  SD). Layer Feitian 358 Jidan 558 Sheath Moisture Blade Sheath Moisture Thickness Thickness Content Thickness Thickness Content (mm) (mm) (%) (mm) (mm) (%) 2 0.35  0.05 0.68  0.07 49.76 0.38  0.04 0.88  0.06 54.82 3 0.33  0.03 0.56  0.04 52.97 0.37  0.04 0.79  0.03 58.73 4 0.29  0.04 0.50  0.03 54.13 0.35  0.02 0.70  0.04 59.25 5 0.25  0.02 0.49  0.05 54.84 0.32  0.03 0.65  0.05 60.63 6 0.22  0.02 0.47  0.02 53.21 0.26  0.02 0.62  0.03 60.13 3.2. Tensile Properties In this section, the tensile properties, fracture modes, and fracture mechanisms of leaf blades and leaf sheaths are described. The effect of bract moisture content, bract thickness, and tensile loading direction are analyzed. 3.2.1. Tensile Test Results of Leaf Blade The values of mean fracture force, tensile strength, and fracture energy of leaf blades under two directions, as well as the mathematical models of these properties as a function of moisture content and blade thickness, are presented in Tables 3 and 4. Table 3. Tensile properties of the third leaf blade from five harvest days (mean  SD) and repeated measures analysis of variance. Parallel to the Vein Perpendicular to the Vein Date—Bract Tensile Tensile Moisture Fracture Force Fracture Energy Fracture Force Fracture Energy Strength Strength Content (%) 3 3 (N) (10 J) (N) (10 J) (MPa) (MPa) Feitian 358 09/24—65.84 42.33 d  6.46 4.28 d  0.65 212.72 b  23.72 5.13 d  0.80 0.52 d  0.08 9.67 bc  1.33 10/01—52.97 51.55 d  5.45 5.21 d  0.55 254.19 a  25.46 7.26 cd  1.38 0.73 d  0.14 11.99 a  0.99 10/08—35.93 67.51 c  8.02 7.26 c  0.86 227.68 ab  33.56 9.68 c  1.42 1.04 c  0.15 11.21 ab  1.18 10/15—25.68 79.87 b  8.28 9.51 b  0.99 208.08 b  28.04 12.57 b  2.73 1.50 b  0.33 10.6 ab  1.51 10/22—20.93 93.34 a  10.93 11.52 a  1.35 143.90 c  17.56 15.27 a  2.45 1.88 a  0.30 8.85 c  1.19 F-Value 32.951 ** 52.706 ** 12.100 ** 22.783 ** 31.465 ** 4.895 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 F = 136.026 2.493 mc + 0.016 mc , R = 0.980 F = 23.971 0.515 mc + 0.004 mc , R = 0.965 b b 2 2 4 2 2 s = 19.210 0.455 mc + 0.003 mc , R = 0.983 s = 3.326 0.086 mc + 6.689  10 mc , R = 0.969 t t 2 2 2 2 E = 61.796 + 13.298 mc 0.139 mc , R = 0.813 E = 1.465 + 0.473 mc 0.005 mc , R = 0.827 g g Jidan 558 09/24—72.43 36.39 c  5.29 3.19 e  0.46 169.98 b  20.48 4.93 c  0.87 0.43 c  0.08 9.37 a  1.17 10/01—58.73 67.88 b  8.58 6.12 d  0.77 231.79 a  26.53 5.49 c  0.97 0.49 c  0.08 10.78 a  1.43 10/08—38.19 77.84 b  7.12 7.41 c  0.68 c 204.67 a  23.91 6.89 c  1.53 0.66 c  0.15 10.60 a  1.80 10/15—21.77 91.49 a  8.48 10.17 b  0.94 159.82 b  15.48 11.52 b  2.03 1.28 b  0.23 8.89 ab  1.29 10/22—17.65 101.27 a  9.16 11.25 a  1.02 142.83 b  19.43 14.71 a  1.97 1.63 a  0.22 7.47 b  0.98 F-Value 50.926 ** 81.559 ** 13.957 ** 37.257 ** 53.154 ** 4.932 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 F = 101.646 0.082 mc 0.011 mc , R = 0.897 F = 22.976 0.597 mc + 0.005 mc , R = 0.943 b b 2 2 4 2 2 s = 13.555 0.153 mc + 2.027 mc , R = 0.936 s = 2.699 0.076 mc + 6.202  10 mc , R = 0.958 t t 2 2 2 2 E = 12.164 + 8.759 mc 0.090 mc , R = 0.849 E = 2.959 + 0.329 mc 0.003 mc , R = 0.947 g g ** Extreme significance (p < 0.01), p = probability value. F = Fisher ’s variance ratio, R = coefficient of multiple determination, mc = bract moisture content. Mean values with different letters in each column indicate that these values have statistically significant differences (p < 0.05). Agriculture 2021, 11, 796 7 of 15 Tensile Properties of the Third Leaf Blade in Different Harvest Days The longitudinal and transverse tensile properties of leaf blades of the two cultivars showed the same change rule with the delay of harvest day (Table 3). The mean fracture force and tensile strength increased, while the mean fracture energy increased firstly then decreased. With decreasing moisture content, the leaves became more brittle and more sensitive, and decreased in tensile properties, which were considered as a product of analysis [13]. During the harvest period of this study, the bract moisture contents decreased by 68.21% and 75.63% for Feitian 358 and Jidan 558, respectively, while the mean tensile strengths of leaf blades in the LTT increased by 1.69 and 2.53 times, respectively, and those in the TTT ere increased by 2.62 and 2.79 times, respectively. Fiber almost never became brittle by drying [12]. One possible explanation was that, due to water scarcity, the maximum lignification of the vascular system resulted in thickening of the secondary wall, thereby increasing the mechanical strength of the bract [26]. The typical force-displacement curves of the blade tensile test are plotted in Figure 4, wherein Figure 4a,b show the longitudinal tensile curves, and Figure 4c,d show the trans- verse tensile curves. Although the elongations of blade specimens varied greatly for different harvest days for both corn varieties, the overall variation showed a predictable trend. The force-displacement curves showed that the maximum elongation of leaf blades decreased with a delay of harvest day, while the opposite occurred to the changing tendency of fracture force. This might have been due to the fact that reduced moisture content of the bract caused a decrease in toughness of fiber, and led to a decrease in tensile elongation. Table 4. Tensile properties of the second to sixth leaf blades (mean  SD) and repeated measures analysis of variance. Parallel to the Vein Perpendicular to the Vein Layer—Blade Tensile Tensile Thickness Fracture Force Fracture Energy Fracture Force Fracture Energy Strength Strength (mm) 3 3 (N) (10 J) (N) (10 J) (MPa) (MPa) Feitian 358 2—0.35 65.85 a  8.44 6.33 a  0.83 230.22 c  21.19 9.87 a  1.75 0.95 a  0.17 11.13 a  2.23 3—0.33 51.55 b  5.45 5.21 a  0.55 254.19 bc  25.46 7.26 b  1.38 0.73 a  0.14 11.99 a  0.99 4—0.29 44.91 bc  7.24 5.20 a  0.84 333.99 a  25.89 6.33 bc  1.39 0.73 a  0.16 10.82 a  1.60 5—0.25 41.13 bc  5.81 5.42 a  0.76 283.15 b  27.19 5.68 bc  1.87 0.75 a  0.25 8.16 b  1.75 6—0.22 36.27 c  6.05 5.50 a  0.92 168.82 d  16.35 4.65 c  0.67 0.70 a  0.10 6.20 b  1.68 F-Value 14.688 ** 1.717 ns 33.952 ** 9.072 ** 1.658 ns 10.049 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 F = 119.979 739.343 t + 1656.243 t , R = 0.865 F = 16.513 108.177 t + 251.593 t , R = 0.820 b b 2 3 2 3 s = 82.687 + 1008.270 t 3791.638 t + 4682.032 t , s = 19.751 + 228.407 t 839.886 t + 1018.612 t , t t 2 2 R = 0.708 R = 0.758 2 2 2 2 E = 2357.100 + 18539.085 t 32039.386 t , R = 0.915 E = 36.054 + 284.373 t 424.697 t , R = 0.936 g g Jidan 558 2—0.38 74.71 a  10.07 6.50 a  0.86 207.68 b  22.12 7.07 a  1.29 0.62 a  0.11 10.46 ab  1.47 3—0.37 67.88 ab  8.58 6.12 a  0.77 231.79 b  26.53 5.49 b  0.97 0.49 a  0.08 10.78 ab  1.43 4—0.35 62.62 bc  8.16 5.96 a  0.78 321.80 a  27.73 5.31 bc  1.34 0.51 a  0.13 12.16 a  1.97 5—0.32 56.44 cd  7.58 5.95 a  0.80 319.45 a  24.23 4.38 c  1.17 b 0.46 a  0.12 9.58 b  1.45 6—0.26 50.72 d  5.54 6.50 a  0.79 206.34 b  20.66 3.72 c  0.90 0.48 a  0.12 3.93 c  0.69 F-Value 6.699 ** 0.700 ns 28.838 ** 6.140 ** 1.416 ns 23.777 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 F = 149.504 760.441 t + 1465.634 t , R = 0.969 F = 19.376 116.695 t + 217.824 t , R = 0.814 b b 2 3 s = 27.322 + 369.264 t 1313.395 t + 1513.255 t , 2 2 s = 2.269 12.077 t + 20.029 t , R = 0.355 R = 0.879 2 2 2 2 E = 3117.082 + 21450.115 t 33363.837 t , R = 0.907 E = 96.848 + 612.748 t 867.574 t , R = 0.946 g g ** Extreme significance (p < 0.01), ns = not significant (p > 0.05), p = probability value. F = Fisher ’s variance ratio, R = coefficient of multiple determination, t = blade thickness. Mean values with different letters in each column indicate that these values have statistically significant differences (p < 0.05). Agriculture 2021, 11, x FOR PEER REVIEW 7 of 15 Table 3. Tensile properties of the third leaf blade from five harvest days (mean ± SD) and repeated measures analysis of variance. Date—Bract Parallel to the Vein Perpendicular to the Vein Moisture Con- Fracture Force Tensile Fracture Energy Fracture Force Tensile Fracture Energy −3 −3 tent (%) (N) Strength (MPa) (×10 J) (N) Strength (MPa) (×10 J) Feitian 358 09/24—65.84 42.33 d ± 6.46 4.28 d ± 0.65 212.72 b ± 23.72 5.13 d ± 0.80 0.52 d ± 0.08 9.67 bc ± 1.33 10/01—52.97 51.55 d ± 5.45 5.21 d ± 0.55 254.19 a ± 25.46 7.26 cd ± 1.38 0.73 d ± 0.14 11.99 a ± 0.99 10/08—35.93 67.51 c ± 8.02 7.26 c ± 0.86 227.68 ab ± 33.56 9.68 c ± 1.42 1.04 c ± 0.15 11.21 ab ± 1.18 10/15—25.68 79.87 b ± 8.28 9.51 b ± 0.99 208.08 b ± 28.04 12.57 b ± 2.73 1.50 b ± 0.33 10.6 ab ± 1.51 10/22—20.93 93.34 a ± 10.93 11.52 a ± 1.35 143.90 c ± 17.56 15.27 a ± 2.45 1.88 a ± 0.30 8.85 c ± 1.19 F-Value 32.951 ** 52.706 ** 12.100 ** 22.783 ** 31.465 ** 4.895 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 Fb = 136.026 − 2.493 mc + 0.016 mc , R = 0.980 Fb = 23.971 − 0.515 mc + 0.004 mc , R = 0.965 2 2 −4 2 2 σt = 19.210 − 0.455 mc + 0.003 mc , R = 0.983 σt = 3.326 − 0.086 mc + 6.689 × 10 mc , R = 0.969 2 2 2 2 Eg = −61.796 + 13.298 mc − 0.139 mc , R = 0.813 Eg = 1.465 + 0.473 mc − 0.005 mc , R = 0.827 Jidan 558 09/24—72.43 36.39 c ± 5.29 3.19 e ± 0.46 169.98 b ± 20.48 4.93 c ± 0.87 0.43 c ± 0.08 9.37 a ± 1.17 10/01—58.73 67.88 b ± 8.58 6.12 d ± 0.77 231.79 a ± 26.53 5.49 c ± 0.97 0.49 c ± 0.08 10.78 a ± 1.43 10/08—38.19 77.84 b ± 7.12 7.41 c ± 0.68 c 204.67 a ± 23.91 6.89 c ± 1.53 0.66 c ± 0.15 10.60 a ± 1.80 10/15—21.77 91.49 a ± 8.48 10.17 b ± 0.94 159.82 b ± 15.48 11.52 b ± 2.03 1.28 b ± 0.23 8.89 ab ± 1.29 10/22—17.65 101.27 a ± 9.16 11.25 a ± 1.02 142.83 b ± 19.43 14.71 a ± 1.97 1.63 a ± 0.22 7.47 b ± 0.98 F-Value 50.926 ** 81.559 ** 13.957 ** 37.257 ** 53.154 ** 4.932 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 Fb = 101.646 − 0.082 mc − 0.011 mc , R = 0.897 Fb = 22.976 − 0.597 mc + 0.005 mc , R = 0.943 2 2 −4 2 2 σt = 13.555 − 0.153 mc + 2.027 mc , R = 0.936 σt = 2.699 − 0.076 mc + 6.202 × 10 mc , R = 0.958 2 2 2 2 Eg = 12.164 + 8.759 mc − 0.090 mc , R = 0.849 Eg = 2.959 + 0.329 mc − 0.003 mc , R = 0.947 ** Extreme significance (p < 0.01), p = probability value. F = Fisher’s variance ratio, R = coefficient of multiple determina- Agriculture 2021, 11, 796 8 of 15 tion, mc = bract moisture content. Mean values with different letters in each column indicate that these values have statis- tically significant differences (p < 0.05). Agriculture 2021, 11, x FOR PEER REVIEW 8 of 15 (a) (b) (c) (d) Figure 4. Force-displacement curves of the tensile test of the third leaf blade from five harvest days during the 2020 harvest Figure 4. Force-displacement curves of the tensile test of the third leaf blade from five harvest days during the 2020 harvest season: (a) longitudinal tensile curves of Feitian 358; (b) longitudinal tensile curves of Jidan 558; (c) transverse tensile season: (a) longitudinal tensile curves of Feitian 358; (b) longitudinal tensile curves of Jidan 558; (c) transverse tensile curves curves of Feitian 358; (d) transverse tensile curves of Jidan 558. of Feitian 358; (d) transverse tensile curves of Jidan 558. Tensile Properties of the Second to Sixth Leaf Blades on 1 October 2020 Tensile Properties of the Second to Sixth Leaf Blades on 1 October 2020 The longitudinal and transverse tensile fracture forces of leaf blades showed a de- The longitudinal and transverse tensile fracture forces of leaf blades showed a de- creasing trend from outer to inner layers for both varieties (Table 4). This was because the creasing trend from outer to inner layers for both varieties (Table 4). This was because the cross-sectional area gradually decreased from outer to inner layers when the specimen cross-sectional area gradually decreased from outer to inner layers when the specimen width was constant. The researches proved that the fracture force has a strong correlation width was constant. The researches proved that the fracture force has a strong correlation with the cross-sectional area [27,28]. However, whether LTT or TTT, the tensile strengths with the cross-sectional area [27,28]. However, whether LTT or TTT, the tensile strengths of of each layer of leaf blades had little difference except for the second layer, which was each layer of leaf blades had little difference except for the second layer, which was larger larger for both varieties. Moisture content might have been the main reason for this. As for both varieties. Moisture content might have been the main reason for this. As shown shown in Table 2, the moisture content of the second bract was relatively low. In addition, in Table 2, the moisture content of the second bract was relatively low. In addition, the the fracture energy increased firstly and then decreased from outer to inner layers. Ac- fracture energy increased firstly and then decreased from outer to inner layers. According cording to the Equation (2), we inferred it was the specimen elongation that resulted in to the Equation (2), we inferred it was the specimen elongation that resulted in the maxi- the maximum fracture energy occurring in the middle layer. In fact, we did find that the mum fracture energy occurring in the middle layer. In fact, we did find that the specimen specimen elongation of the middle layer was larger, possibly as a result of moisture con- elongation of the middle layer was larger, possibly as a result of moisture content and tent and blade thickness. One-way ANOVA showed that blade thickness had a significant blade thickness. One-way ANOVA showed that blade thickness had a significant effect on effect fractur on e f for ract ceu and re force fractur and e ener fractgy ure en (p <er 0.01), gy (pbut < 0.01) the , ef but the eff fect on the ect tensile on the tensil strength e strength was not was not obvious (p > 0.05). Our results agree well with findings of Jacobs et al. [14] on the fracture properties of seven tropical grasses. According to the results of the blade tensile test, the load increased for both direc- tions, with an increase of specimen elongation before decreasing abruptly due to specimen fracture. The mean fracture force and tensile strength of the leaf blade in the LTT were about 8.95 times, and the mean fracture energy was about 24.03 times as much as that in the TTT. Therefore, the leaf blade was prone to transverse tearing during the mechanical peeling. The result of field investigation showed that most leaf blades were torn at differ- ent degrees in the transverse direction after mechanical peeling, which provides support- ing evidence for our experimental result. Agriculture 2021, 11, 796 9 of 15 obvious (p > 0.05). Our results agree well with findings of Jacobs et al. [14] on the fracture properties of seven tropical grasses. According to the results of the blade tensile test, the load increased for both directions, with an increase of specimen elongation before decreasing abruptly due to specimen fracture. The mean fracture force and tensile strength of the leaf blade in the LTT were about 8.95 times, and the mean fracture energy was about 24.03 times as much as that in the TTT. Therefore, the leaf blade was prone to transverse tearing during the mechanical peeling. The result of field investigation showed that most leaf blades were torn at different degrees in the transverse direction after mechanical peeling, which provides supporting evidence for our experimental result. 3.2.2. Tensile Test Results of Leaf Sheath As illustrated in Table 5, bract moisture content had a significant effect on the tensile properties of the leaf sheaths (p < 0.01), and sheath thickness had a significant effect on the fracture force and fracture energy (p < 0.01) but no effect on tensile strength (p > 0.05). The results of tensile properties of leaf sheath under three directions, and the regression relationships between these properties and moisture content (or sheath thickness) are presented in Figures 5 and 6. In the following paragraphs, the effects of each factor on the tensile properties of leaf sheaths are comprehensively discussed. Table 5. Repeated measures analysis of variance for tensile properties of leaf sheaths. F-Value (Feitian 358) F-Value (Jidan 558) Influence Measured Factor Variables 0 90 180 0 90 180 Bract F 19.533 ** 33.198 ** 27.477 ** 56.744 ** 71.471 ** 37.258 ** moisture s 45.234 ** 27.348 ** 23.340 ** 21.007 ** 23.648 ** 54.087 ** content E 13.456 ** 6.963 ** 4.516 ** 15.423 ** 5.959 ** 4.732 ** F 15.064 ** 7.884 ** 4.713 ** 21.529 ** 6.415 ** 6.052 ** Sheath s 1.386 ns 0.881 ns 0.914 ns 1.857 ns 1.520 ns 0.747 ns thickness E 37.559 ** 24.741 ** 18.132 ** 28.098 ** 33.745 ** 14.856 ** F = Fisher ’s variance ratio. ** Extreme significance (p < 0.01), ns = not significant (p > 0.05), p = probability value. Tensile Properties of the Third Leaf Sheath from Different Harvest Days With the delay of harvest day, the mean fracture force and tensile strength that made the third leaf sheath separate from the peduncle both increased with decreasing moisture content for the two cultivars, while the mean fracture energy increased firstly and then decreased (Figure 5). The fracture force (or tensile strength) of the leaf sheaths for Feitian 358 and Jidan 558 satisfied the quadratic polynomial relationship (R > 0.926) and linear relationship (R > 0.966) with moisture content, respectively. Meanwhile, the fracture en- ergy of the leaf sheaths for the two cultivars satisfied the quadratic polynomial relationship (R > 0.856) with moisture content. When the leaf sheaths from five harvest days were pulled at angles of 0 , 90 , and 180 (the angles between the pulling force and the natural growth direction of bract), the mean fracture forces required to remove the third leaf sheaths were 31.79, 11.78, and 8.01 N, respectively; the mean tensile strengths were 4.89, 1.82, and 3 3 1.23 MPa, respectively; the mean fracture energies were 90.14  10 , 29.53  10 , and 15.33  10 J, respectively. Agriculture 2021, 11, x FOR PEER REVIEW 10 of 15 Tensile Properties of the Third Leaf Sheath from Different Harvest Days With the delay of harvest day, the mean fracture force and tensile strength that made the third leaf sheath separate from the peduncle both increased with decreasing moisture content for the two cultivars, while the mean fracture energy increased firstly and then decreased (Figure 5). The fracture force (or tensile strength) of the leaf sheaths for Feitian 358 and Jidan 558 satisfied the quadratic polynomial relationship (R > 0.926) and linear relationship (R > 0.966) with moisture content, respectively. Meanwhile, the fracture en- ergy of the leaf sheaths for the two cultivars satisfied the quadratic polynomial relation- ship (R > 0.856) with moisture content. When the leaf sheaths from five harvest days were pulled at angles of 0°, 90°, and 180° (the angles between the pulling force and the natural growth direction of bract), the mean fracture forces required to remove the third leaf sheaths were 31.79, 11.78, and 8.01 N, respectively; the mean tensile strengths were 4.89, Agriculture 2021, 11, 796 10 of 15 −3 −3 1.82, and 1.23 MPa, respectively; the mean fracture energies were 90.14 × 10 , 29.53 × 10 , −3 and 15.33 × 10 J, respectively. (a) (b) (c) Figure 5. Relationships between tensile properties and moisture content: (a) fracture force; (b) tensile strength; (c) fracture Figure 5. Relationships between tensile properties and moisture content: (a) fracture force; (b) tensile strength; energy. (c) fracture energy. Tensile Properties of the Second to Sixth Leaf Sheaths on 1 October 2020 As shown in Figure 6, the mean fracture force that separated the leaf sheath from the peduncle increased from inner to outer layers for the two cultivars. Sheath thickness had no significant effect on the tensile strength of the leaf sheaths (p > 0.05), and there was no obvious rule in the variation of tensile strength. The experimental results showed that the mean tensile strength of the outer layer was higher than that of the inner layer when the tensile angles were 0 and 90 , while the mean tensile strength of each layer had little difference when the tensile angle was 180 . The fracture energy increased firstly and then decreased from outer to inner layers. The fracture force of the leaf sheaths for the two cultivars satisfied the quadratic polynomial relationship (0 , R > 0.995) and the linear relationship (90 and 180 , R > 0.935) with sheath thickness. Meanwhile, the mean tensile strength and fracture energy of the leaf sheaths for the two cultivars satisfied the cubic 2 2 polynomial relationship (R > 0.326) and the quadratic polynomial relationship (R > 0.860) with sheath thickness, respectively. When the leaf sheaths were pulled at angles of 0 , 90 , and 180 , the mean fracture forces required to remove the second to sixth leaf sheaths were 23.40, 8.18, and 5.51 N, respectively; the mean tensile strengths were 3.67, 1.28, and Agriculture 2021, 11, x FOR PEER REVIEW 11 of 15 Tensile Properties of the Second to Sixth Leaf Sheaths on 1 October 2020 As shown in Figure 6, the mean fracture force that separated the leaf sheath from the peduncle increased from inner to outer layers for the two cultivars. Sheath thickness had no significant effect on the tensile strength of the leaf sheaths (p > 0.05), and there was no obvious rule in the variation of tensile strength. The experimental results showed that the mean tensile strength of the outer layer was higher than that of the inner layer when the tensile angles were 0° and 90°, while the mean tensile strength of each layer had little difference when the tensile angle was 180°. The fracture energy increased firstly and then decreased from outer to inner layers. The fracture force of the leaf sheaths for the two cultivars satisfied the quadratic polynomial relationship (0°, R > 0.995) and the linear re- lationship (90° and 180°, R > 0.935) with sheath thickness. Meanwhile, the mean tensile strength and fracture energy of the leaf sheaths for the two cultivars satisfied the cubic 2 2 polynomial relationship (R > 0.326) and the quadratic polynomial relationship (R > 0.860) Agriculture 2021, 11, 796 11 of 15 with sheath thickness, respectively. When the leaf sheaths were pulled at angles of 0°, 90°, and 180°, the mean fracture forces required to remove the second to sixth leaf sheaths were 23.40, 8.18, and 5.51 N, respectively; the mean tensile strengths were 3.67, 1.28, and −3 3 −3 3 0.86 MPa, respectively; the mean fracture energies were 93.00 × 10 , 29.26 × 10 , and 15.30 0.86 MPa, respectively; the mean fracture energies were 93.00  10 , 29.26  10 , and −3 3 × 10 J, respectively. 15.30  10 J, respectively. According to the above analysis, among the three directions of sheath tensile test, the According to the above analysis, among the three directions of sheath tensile test, the tensile property values were largest when pulled the leaf sheaths at an angle of 0°, and tensile property values were largest when pulled the leaf sheaths at an angle of 0 , and smal smallest lest whe when n pul pulling ling t the he lea leaf f she sheaths aths at at an an an ang gle le of of 1 180 80°.. Unde Under r tthe he s same ame ttest est co conditions, nditions, t the he m mean ean fractur fracture e for force ce (or (or tensile tensilestr stength) rengthof ) of 0 0° was wa2.83 s 2.83 and and 4.23 4.2times 3 time as s as much muc as h a that s thof at 90 and 180 , respectively. Meanwhile, the mean fracture energy of 0 was about 3.14 and of 90° and 180°, respectively. Meanwhile, the mean fracture energy of 0° was about 3.14 and 6.17 6. times 17 tias memuch s as m as uch that as of th90 at of 9 and 0°180 and 1 , respectively 80°, respec.tiTher vely. The efore,ref the ore, t fractur he fr e act foru ce re fo of leaf rce sheaths could be reduced by increasing the pulling force angle on the bract. It was reported of leaf sheaths could be reduced by increasing the pulling force angle on the bract. It was in the literature that the fracture force of sugarcane leaf showed a similar decreasing trend reported in the literature that the fracture force of sugarcane leaf showed a similar de- with an increase of pulling force angle [29]. creasing trend with an increase of pulling force angle [29]. Agriculture 2021, 11, x FOR PEER REVIEW 12 of 15 (a) (b) (c) Figure 6. Relationships between tensile properties and sheath thickness: (a) fracture force; (b) tensile strength; (c) fracture Figure 6. Relationships between tensile properties and sheath thickness: (a) fracture force; (b) tensile strength; energy. (c) fracture energy. 3.2.3. Vein Traits and Fracture Modes of Bract 3.2.3. Vein Traits and Fracture Modes of Bract Figur Figure e 7 7 shows shows the the surfac surfacee im image age o off aa corn cornbract bract taken wi taken with th aa digi digital talcamera. camera. Corn Corn bract bract has the has the pattern pattern of of parallel parallel vei veins ntypical s typica to l to monocotyl monocotyledonous edonous pl plants. ants. The The longitudinal longitudi- nal veins arranged in sequence are parallel to each other. There are mesophyll and small transverse veins between two longitudinal veins. Figure 7. Vein traits of corn bract. Figure 8 shows the surface and cross-sectional morphology of a corn bract observed using scanning electron microscopy. It could be seen that the longitudinal veins of the corn bract were about two to three times thicker than the surrounding mesophyll tissue. This highlighted the shortcoming of using a single thickness value in the tensile strength formula. The actual tensile strength of the bract was larger than the experimental data. Therefore, the tensile strength of a bract should be set slightly higher when the experi- mental data are applied to the actual machine parameter setting. Even so, the data still reflected the variation of tensile strength of bracts throughout the harvest period. Agriculture 2021, 11, x FOR PEER REVIEW 12 of 15 (c) Figure 6. Relationships between tensile properties and sheath thickness: (a) fracture force; (b) tensile strength; (c) fracture energy. Agriculture 2021, 11, 796 12 of 15 3.2.3. Vein Traits and Fracture Modes of Bract Figure 7 shows the surface image of a corn bract taken with a digital camera. Corn bract has the pattern of parallel veins typical to monocotyledonous plants. The longitudi- veins arranged in sequence are parallel to each other. There are mesophyll and small nal veins arranged in sequence are parallel to each other. There are mesophyll and small transverse veins between two longitudinal veins. transverse veins between two longitudinal veins. Figure 7. Vein traits of corn bract. Figure 7. Vein traits of corn bract. Figure 8 shows the surface and cross-sectional morphology of a corn bract observed Figure 8 shows the surface and cross-sectional morphology of a corn bract observed using scanning electron microscopy. It could be seen that the longitudinal veins of the using scanning electron microscopy. It could be seen that the longitudinal veins of the corn corn bra bract ct were a were about bout two two to to three ti three times mes th thicker icker than than the the surro surrounding unding mesophyll tissu mesophyll tissue. e. This This h highlighted ighlighted t the he sho shortcoming rtcoming o offusing using aa single singlethickness thickness v value aluein inthe the t tensile ensilestr stength rength formula. formula. The a The actual ctual tensi tensile le strength of strength of the the bract bract was lar was larger ger than t than the he experimen experimental tal d data. ata. Ther Therefore efore, the , the tensile tensile str stre ength ngth of a br of a bract should act should be set be slightly set slight higher ly higher when when the experimental the experi- Agriculture 2021, 11, x FOR PEER REVIEW 13 of 15 data menta are l da applied ta are to appli theeact d to the a ual machine ctual parameter machine pa setting. rameter setti Even so, ng. Even the data so, the da still reflected ta still the variation of tensile strength of bracts throughout the harvest period. reflected the variation of tensile strength of bracts throughout the harvest period. (a) (b) Figure 8. Surface and cross-sectional morphology of corn bracts: (a) surface morphology of corn bracts; (b) cross-sectional Figure 8. Surface and cross-sectional morphology of corn bracts: (a) surface morphology of corn bracts; (b) cross- morphology of corn bracts. EHT = extra high tension, SE = secondary electron, Mag = magnification times, WD = work sectional morphology of corn bracts. EHT = extra high tension, SE = secondary electron, Mag = magnification times, distance. WD = work distance. The crack propagation behavior of blade specimens under two vein orientations are The crack propagation behavior of blade specimens under two vein orientations presented in Figure 9. The blade specimen distributed the tensile load on each solid lon- are presented in Figure 9. The blade specimen distributed the tensile load on each solid gitudinal vein when the leaf blade was pulled with a force parallel to the longitudinal longitudinal vein when the leaf blade was pulled with a force parallel to the longitudinal vein. As the load increased, the stress spread rapidly to the remaining longitudinal veins vein. As the load increased, the stress spread rapidly to the remaining longitudinal veins when a longitudinal vein broke. Once respective fracture strengths of the remaining lon- when a longitudinal vein broke. Once respective fracture strengths of the remaining gitudinal veins were exceeded, catastrophic failure occurred to the whole specimen. In longitudinal veins were exceeded, catastrophic failure occurred to the whole specimen. In addition, as shown in Figure 4a,b, the decline curves remained linear after catastrophic addition, as shown in Figure 4a,b, the decline curves remained linear after catastrophic failur failure o e off t the he blade blade specim specimen. en. This This ind indicates icates th that at mos most t of t ofhthe e long longi itudin tudinal al veins veins broke a broke l- most simultaneously. Additionally, the locations of fracture point of each longitudinal vein were relatively random, and might be far apart due to the different physical proper- ties of each position on the leaf blade. Thus, the fracture shape of the blade specimen might have been an irregular broken line [13]. It was observed that the fracture shape of the blade specimen was relatively regular in the TTT. Small transverse veins provided little resistance. The fracture mode was the mesophyll tissue torn along the longitudinal vein direction. The crack occurred between two adjacent longitudinal veins, then rapidly propagated until specimen fracture. (a) (b) Figure 9. Crack propagation modes of leaf blade under two vein orientations: (a) fracture mode of leaf blades in the LTT; (b) fracture mode of leaf blades in the TTT. When pulling the leaf sheath at an angle of 0°, the fracture mode of leaf sheaths was similar to that of leaf blades in the LTT. The fracture position was usually not at the root Agriculture 2021, 11, x FOR PEER REVIEW 13 of 15 (a) (b) Figure 8. Surface and cross-sectional morphology of corn bracts: (a) surface morphology of corn bracts; (b) cross-sectional morphology of corn bracts. EHT = extra high tension, SE = secondary electron, Mag = magnification times, WD = work distance. The crack propagation behavior of blade specimens under two vein orientations are presented in Figure 9. The blade specimen distributed the tensile load on each solid lon- gitudinal vein when the leaf blade was pulled with a force parallel to the longitudinal vein. As the load increased, the stress spread rapidly to the remaining longitudinal veins when a longitudinal vein broke. Once respective fracture strengths of the remaining lon- gitudinal veins were exceeded, catastrophic failure occurred to the whole specimen. In addition, as shown in Figure 4a,b, the decline curves remained linear after catastrophic failure of the blade specimen. This indicates that most of the longitudinal veins broke al- most simultaneously. Additionally, the locations of fracture point of each longitudinal Agriculture 2021, 11, 796 13 of 15 vein were relatively random, and might be far apart due to the different physical proper- ties of each position on the leaf blade. Thus, the fracture shape of the blade specimen might have been an irregular broken line [13]. almost simultaneously. Additionally, the locations of fracture point of each longitudinal It was observed that the fracture shape of the blade specimen was relatively regular vein were relatively random, and might be far apart due to the different physical properties in the TTT. Small transverse veins provided little resistance. The fracture mode was the of each position on the leaf blade. Thus, the fracture shape of the blade specimen might mesophyll tissue torn along the longitudinal vein direction. The crack occurred between have been an irregular broken line [13]. two adjacent longitudinal veins, then rapidly propagated until specimen fracture. (a) (b) Figure 9. Crack propagation modes of leaf blade under two vein orientations: (a) fracture mode of leaf blades in the LTT; Figure 9. Crack propagation modes of leaf blade under two vein orientations: (a) fracture mode of leaf blades in the LTT; (b) fracture mode of leaf blades in the TTT. (b) fracture mode of leaf blades in the TTT. When pulling the leaf sheath at an angle of 0°, the fracture mode of leaf sheaths was It was observed that the fracture shape of the blade specimen was relatively regular similar to that of leaf blades in the LTT. The fracture position was usually not at the root in the TTT. Small transverse veins provided little resistance. The fracture mode was the mesophyll tissue torn along the longitudinal vein direction. The crack occurred between two adjacent longitudinal veins, then rapidly propagated until specimen fracture. When pulling the leaf sheath at an angle of 0 , the fracture mode of leaf sheaths was similar to that of leaf blades in the LTT. The fracture position was usually not at the root of the leaf sheath, but slightly away from the root or somewhere on the leaf blade. This was because, when the pulling force angle was 0 , the mean tensile strength of the leaf sheath was closest to that of the leaf blade in the LTT, especially at the early stage of harvest. When pulling the leaf sheath at an angle of 180 , the tensile strength of the leaf sheath was relatively small, and the bract fractured along the root of the leaf sheath. The abrupt change of fracture behavior might have been caused by the stress of surface fibers transformed from a tensile stress to a bending stress, which resulted in changes to the crack initiation and propagation mechanism [30]. 4. Conclusions This paper presented the physical and tensile properties of corn bracts for two corn cultivars, Feitian 358 and Jidan 558. The results of physical properties showed that bract dimensions and moisture contents of the two cultivars varied with the changes of harvest day and bract layer. One-way ANOVA showed that bract moisture content creates a significant effect on the fracture force, tensile strength, and fracture energy, while bract thickness has a significant effect only on the fracture force and fracture energy (p < 0.01). As moisture content decreased, the tensile strengths of the leaf blades and sheaths both increased for the two cultivars. However, the tensile strengths of the leaf blades and sheaths did not noticeably change for different thicknesses. Vein orientation had a significant effect on the tensile properties of the leaf blades. A crack passed through every longitudinal vein in the LTT, while cracks usually occurred in the mesophyll between two adjacent longitudinal veins in the TTT. The tensile strength of Agriculture 2021, 11, 796 14 of 15 leaf blades in the LTT was about 8.95 times as much as that in the TTT. Therefore, the leaf blades were more prone to transverse tearing during the mechanical peeling. The tensile properties of leaf sheaths depended on the angle between pulling force and the natural growth direction of the bract. The tensile properties decreased gradually with an increase in pulling force angle. Hence, increasing the pulling force angle exerted by the peeling component on the bract could reduce the peeling power consumption and improve the efficiency and quality of mechanical peeling. Further, a larger pulling force angle could improve the probability of bract fracturing at the root of the leaf sheath, which is beneficial to reducing bract residue on the peeled ear. In this study, the test specimens were collected from five harvest days from different weeks after physiological maturity of the crop, coinciding with the normal harvest season. The test results could represent an actual material situation from the harvest period. Author Contributions: Conceptualization, J.F.; methodology, J.F.; validation, Z.L.; investigation, Z.L.; resources, J.F.; writing—original draft preparation, Z.L.; writing—review and editing, J.F.; supervision, X.L.; project administration, J.F.; funding acquisition, J.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the thirteenth five-year science and technology project of Jilin Provincial Education Department (No. JJKH20190144KJ). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data and models presented in this study are available on demand from the first author at (zhenye19@mails.jlu.edu.cn). Acknowledgments: The authors are grateful for the corn provided by the Agricultural Experimental Base of Jilin University. 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Tensile Properties and Fracture Mechanisms of Corn Bract for Corn Peeling Device Design

Li, Zhenye; Fu, Jun; Luo, Xiwen
Agriculture , Volume 11 (8) – Aug 20, 2021
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agriculture Article Tensile Properties and Fracture Mechanisms of Corn Bract for Corn Peeling Device Design 1 , 2 1 , 2 , 2 , 3 Zhenye Li , Jun Fu * and Xiwen Luo Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130025, China; zhenye19@mails.jlu.edu.cn College of Biological and Agricultural Engineering, Jilin University, Changchun 130025, China; xwluo@scau.edu.cn College of Engineering, South China Agricultural University, Guangzhou 510642, China * Correspondence: fu_jun@jlu.edu.cn Abstract: This paper describes the physical and tensile properties of corn bracts during a whole harvest period by using two corn cultivars, aiming to realize efficient peeling with minimum energy performance and decrease the incidence of ear damage. The value range and change rule of tensile properties were obtained by combining mechanical experiments and numerical statistics. Meanwhile, mathematical models were established for tensile properties depending on bract moisture content and bract thickness. The experimental results show that the tensile properties of leaf blade were affected by the orientation between pulling force and longitudinal vein, and that parallel orientation was greater than perpendicular. Further, the tensile properties of leaf sheaths depended on the angle between pulling force and natural growth direction of the bract in the following order: 0 > 90 > 180 . A larger pulling force angle can improve the probability of bract fracturing at the root of leaf sheaths, which helps reduce bract residue on the peduncle. In addition, the fracture mechanisms of leaf blades and sheaths were expressed from physiological and morphological perspectives. The experimental results are believed to be able to provide theoretical guidance by which to design and optimize Citation: Li, Z.; Fu, J.; Luo, X. Tensile Properties and Fracture Mechanisms corn-peeling devices. of Corn Bract for Corn Peeling Device Design. Agriculture 2021, 11, 796. Keywords: corn bract; tensile properties; fracture properties; mechanical peeling https://doi.org/10.3390/agriculture Academic Editor: Andrea Colantoni 1. Introduction Corn (Zea mays L.) is one of the most versatile cereal crops, and has multiple applica- Received: 12 July 2021 tions as food consumption, industrial raw material, and feeding stuff. The rapid increase Accepted: 18 August 2021 of corn acreage results in a higher requirement for corn harvest quality [1]. Corn peeling is Published: 20 August 2021 the key link in corn ear harvest, and directly affects the storage and follow-up processing of corn ears [2,3]. However, ears are often subjected to excessive extrusion and friction due to Publisher’s Note: MDPI stays neutral an inappropriate parameter setting of the peeling unit, which leads to serious mechanical with regard to jurisdictional claims in damage [4,5]. Mechanical damage caused by external loading is the main factor behind a published maps and institutional affil- decline in quality and the total production of corn, and can cause serious financial losses iations. in the corn industry [6]. Therefore, under the premise of bract removal, an investigation into how to reduce and prevent the mechanical damage caused by excessive peeling force deserves special attention. Mechanical properties of plant leaves are crucial to understanding leaf fracture mecha- Copyright: © 2021 by the authors. nisms and the designs of leaf removal machinery [7,8]. Figure 1 shows a schematic diagram Licensee MDPI, Basel, Switzerland. of corn peeling. During the peeling operation, two peeling rollers rotate toward each other This article is an open access article and tilt downward at a specific angle. In the direction parallel to the axis of the peeling distributed under the terms and roller, the corn ear slides down along the groove of two rollers under its own gravity and conditions of the Creative Commons the force of delivery device, while the corn bract is subjected to an upward friction force. Attribution (CC BY) license (https:// Meanwhile, in the direction perpendicular to the axis of the peeling roller, the relative creativecommons.org/licenses/by/ rotating peeling rollers apply different friction torques to the corn ear due to the different 4.0/). Agriculture 2021, 11, 796. https://doi.org/10.3390/agriculture11080796 https://www.mdpi.com/journal/agriculture Agriculture 2021, 11, x FOR PEER REVIEW 2 of 15 Agriculture 2021, 11, 796 2 of 15 the relative rotating peeling rollers apply different friction torques to the corn ear due to the different materials and surface morphologies, which makes the corn ear rotate around materials and surface morphologies, which makes the corn ear rotate around its axis. In the its axis. In the process of movement, the peeling rollers grab and pull the bract under the process of movement, the peeling rollers grab and pull the bract under the action of friction. action of friction. Once the force provided by the rollers is greater than the fracture force Once the force provided by the rollers is greater than the fracture force of the bract, the of the bract, the bract is pulled off and separated from the corn ear. Consequently, testing bract is pulled off and separated from the corn ear. Consequently, testing the mechanical the mechanical properties of corn bract and determining the mechanical conditions for properties of corn bract and determining the mechanical conditions for completing the completing the peeling operation are of great significance to realizing high efficiency and peeling operation are of great significance to realizing high efficiency and low loss during low loss during corn peeling. corn peeling. Figure 1. Schematic diagram of mechanical corn peeling. Figure 1. Schematic diagram of mechanical corn peeling. Several research efforts have reported on the mechanical properties of various plant Several research efforts have reported on the mechanical properties of various plant leav leaves. es. R Resear esearchers b chers believe elieve tthat hat p plant lant lea leaves ves are are u usually sually ani anisotr sotrop opic ic in d in dif ifffer erent ent d dir irect ections, ions, thus their mecha thus their mechanical nicalpr prop operties erties rest rest with with the the posi position tion a andndir d di ection rection of the of the applied appliload ed loa [9d ]. Kohyama et al. [10] revealed that the mechanical strength of a cabbage leaf depends on [9]. Kohyama et al. [10] revealed that the mechanical strength of a cabbage leaf depends vein orientation, and that strength proved to be greater when the vein orientation was on vein orientation, and that strength proved to be greater when the vein orientation was parallel to the test direction. The same result was found by Toole for lettuce [11]. On parallel to the test direction. The same result was found by Toole for lettuce [11]. On the the other hand, there is some controversy regarding the influence of moisture content other hand, there is some controversy regarding the influence of moisture content on the on the mechanical properties of biomass in the literature. Kneebone et al. [12] proposed mechanical properties of biomass in the literature. Kneebone et al. [12] proposed that dry that dry or withered leaves tend to be somewhat stronger than succulent leaves. On the or withered leaves tend to be somewhat stronger than succulent leaves. On the contrary, contrary, Tang et al. [13] pointed out the mean tensile strength of a rice leaf gradually Tang et al. [13] pointed out the mean tensile strength of a rice leaf gradually decreases decreases with decreasing moisture content. In other respects, Jacobs et al. [14] found that with decreasing moisture content. In other respects, Jacobs et al. [14] found that the cross- the cross-sectional area was the main factor affecting the differences of fracture force and sectional area was the main factor affecting the differences of fracture force and fracture fracture energy among species. Overall, the mechanical properties of plant leaves are highly energy among species. Overall, the mechanical properties of plant leaves are highly influ- influenced by moisture content, the cross-sectional area, and the position and direction of enced by moisture content, the cross-sectional area, and the position and direction of the the applied load. To our knowledge, relatively few comprehensive studies have been done applied load. To our knowledge, relatively few comprehensive studies have been done on on the tensile properties of corn bract that could help realize bract removal with minimum the tensile properties of corn bract that could help realize bract removal with minimum energy performance to reduce ear mechanical damage during peeling operations [15,16]. energy performance to reduce ear mechanical damage during peeling operations [15,16]. To address the above issues, the physical and tensile properties of corn bract were To address the above issues, the physical and tensile properties of corn bract were tested using two corn varieties after entering the harvest period. The purpose of this study tested using two corn varieties after entering the harvest period. The purpose of this study was to quantify and compare the effects of bract moisture content, bract thickness, and was to quantify and compare the effects of bract moisture content, bract thickness, and tensile loading direction on the tensile properties of leaf blades and sheaths throughout tensile loading direction on the tensile properties of leaf blades and sheaths throughout the harvest period. Mathematical models were established for the fracture force, tensile the harvest period. Mathematical models were established for the fracture force, tensile strength, and fracture energy of the leaf blades and sheaths depending on bract moisture strength, and fracture energy of the leaf blades and sheaths depending on bract moisture content and bract thickness. The models may be used to guide the design and parameter content and bract thickness. The models may be used to guide the design and parameter setting of corn peeling devices. In addition, the fracture modes and fracture mechanisms of setting of corn peeling devices. In addition, the fracture modes and fracture mechanisms the leaf blades and sheaths under different tensile loading directions are expressed from of the leaf blades and sheaths under different tensile loading directions are expressed from the perspectives of plant physiological and morphological characteristics. the perspectives of plant physiological and morphological characteristics. 2. Materials and Methods 2.1. Corn Materials Two corn varieties (Feitian 358 and Jidan 558) that are widely planted in Northeast China were selected as experimental varieties. Feitian 358 was cultivated by the Dunhuang Agriculture 2021, 11, x FOR PEER REVIEW 3 of 15 2. Materials and Methods Agriculture 2021, 11, 796 3 of 15 2.1. Corn Materials Two corn varieties (Feitian 358 and Jidan 558) that are widely planted in Northeast China were selected as experimental varieties. Feitian 358 was cultivated by the Dunhuang Seed Industry Co., Ltd., Wuhan, China. Jidan 558 is a native corn variety bred Seed Industry Co., Ltd., Wuhan, China. Jidan 558 is a native corn variety bred by the by the Corn Research Institute of Jilin Academy of Agricultural Sciences, Jilin, China. The Corn Research Institute of Jilin Academy of Agricultural Sciences, Jilin, China. The two two corn varieties are generally sown in late April to early May, and it takes 128–131 days corn varieties are generally sown in late April to early May, and it takes 128–131 days from seedling emergence to maturity. After corn grains had fully matured, the corn ears from seedling emergence to maturity. After corn grains had fully matured, the corn ears with bracts used in this study were harvested on five different days in harvest season (24 with bracts used in this study were harvested on five different days in harvest season September, 1 October, 8 October, 15 October and 22 October 2020) from the Agricultural (24 September, 1 October, 8 October, 15 October and 22 October 2020) from the Agricultural 0  0 Experimental Base of Jilin University (N 43°56′, E 125°14′) in Changchun, Jilin province, Experimental Base of Jilin University (N 43 56 , E 125 14 ) in Changchun, Jilin province, China. The harvested ears were stored in a sealed bag in a shaded place. All tensile tests China. The harvested ears were stored in a sealed bag in a shaded place. All tensile tests were completed within two days of harvesting. were completed within two days of harvesting. As shown in Figure 2, a corn bract is composed of leaf blades and leaf sheaths [17]. As shown in Figure 2, a corn bract is composed of leaf blades and leaf sheaths [17]. Overlapped bracts grow on the peduncle nodes and wrap the bare ear. Field investigation Overlapped bracts grow on the peduncle nodes and wrap the bare ear. Field investigation revealed that the fracture positions of the bract mainly occurred somewhere on the leaf revealed that the fracture positions of the bract mainly occurred somewhere on the leaf blade or at the junction between leafs and peduncle during the peeling operation [18]. blade or at the junction between leafs and peduncle during the peeling operation [18]. Ther Therefore efore, ,in inthis this study study, the , thleaf e leaf blades blades and and leaf le sheaths af sheaths of corn of corn bracts bracts w were selected ere selected as two as test two test posi positionstfor ions the for the tensi tensile test. le test. Figure 2. Section diagram of morphological structure of a corn ear. Figure 2. Section diagram of morphological structure of a corn ear. 2.2. Equipment and Methods 2.2. Equipment and Methods 2.2.1. Physical Properties Test Method 2.2.1. Physical Properties Test Method To determine the physical properties of corn materials, 50 ears each of Feitian 358 and To determine the physical properties of corn materials, 50 ears each of Feitian 358 Jidan 558 were randomly selected for measuring from five harvest days. For each corn and Jidan 558 were randomly selected for measuring from five harvest days. For each corn ear, the length, width at half-length, blade thickness, sheath thickness of the third bract, ear, the length, width at half-length, blade thickness, sheath thickness of the third bract, and the total number of bracts were measured. The mean value and standard deviation and the total number of bracts were measured. The mean value and standard deviation (SD) were calculated. Meanwhile, the moisture content (w.b.) (i.e., wet basis) of the kernel, (SD) were calculated. Meanwhile, the moisture content (w.b.) (i.e., wet basis) of the kernel, bract, and peduncle from five harvest days were determined by using a DZF-6050 electric bract, and peduncle from five harvest days were determined by using a DZF-6050 electric thermostatic drying oven (Rongshida Electric Equipment Co., Ltd., Kunshan, China) at thermostatic drying oven (Rongshida Electric Equipment Co., Ltd., Kunshan, China) at 105  1 C during 24 h. In addition, to compare the physical properties of different layers 105 ± 1°C during 24 h. In addition, to compare the physical properties of different layers of bract, the blade thickness, sheath thickness, and moisture content (w.b.) of the second to of bract, the blade thickness, sheath thickness, and moisture content (w.b.) of the second sixth bracts of the two varieties harvested on 1 October 2020 were measured. to sixth bracts of the two varieties harvested on 1 October 2020 were measured. 2.2.2. Tensile Test Method Tensile test is the most common and effective method for studying the tensile proper- ties of materials [19–21]. The specimen preparation process of this study was as follows: each leaf blade was cut into two specimens for the blade tensile test. A rectangular speci- men (30 mm in width and 120 mm in length) was cut along the longitudinal vein direction from the central part of the top leaf blade for the longitudinal tensile test (LTT). Another rectangular specimen (30 mm in width and 90 mm in length) was taken perpendicular to Agriculture 2021, 11, x FOR PEER REVIEW 4 of 15 2.2.2. Tensile Test Method Tensile test is the most common and effective method for studying the tensile prop- erties of materials [19–21]. The specimen preparation process of this study was as follows: Agriculture 2021, 11, 796 4 of 15 each leaf blade was cut into two specimens for the blade tensile test. A rectangular speci- men (30 mm in width and 120 mm in length) was cut along the longitudinal vein direction from the central part of the top leaf blade for the longitudinal tensile test (LTT). Another the longitudinal vein direction from the central part of the remaining leaf blade for the rectangular specimen (30 mm in width and 90 mm in length) was taken perpendicular to transverse the longitud tensile inal vein test d (TTT). irection f Next, rom t a peduncle he centrawas l part taken of th of e frema the ear ining along leaf bl with ade bract for tfor he the sheath transverse tensile test tensile test. The(TTT). connection Next, length a peduncle was between leaf taken sheath off the ear and peduncle along with bract was contr for olled the sheath tensile test. The connection length between leaf sheath and peduncle was con- at 10 mm, and other redundant bracts were removed. Ten test specimens were prepared for each trolled test atgr 10 oup. mmThe , and ot prepar her red ed specimens undant brwer acts wer e sequ e rem entially oved. Te labelled n test and specim sealed ens were in airtight prepared for each test group. The prepared specimens were sequentially labelled and bags, then stored at the optimum condition of 4 C to retain moisture. sealed in airtight bags, then stored at the optimum condition of 4 °C to retain moisture. The tensile tests were performed on a Universal Testing Machine (D-NS type, Chang- The tensile tests were performed on a Universal Testing Machine (D-NS type, Chang- chun Institute of Mechanical Science Co., Ltd., Jilin, China), which attached to a load chun Institute of Mechanical Science Co., Ltd., Jilin, China), which attached to a load cell cell (range of 10 kN, accuracy of 0.5%), as shown in Figure 3. The test specimen was (range of 10 kN, accuracy of 0.5%), as shown in Figure 3. The test specimen was fixed to fixed to the upper beam and the moving beam of the testing machine by a mechanical the upper beam and the moving beam of the testing machine by a mechanical fixture. The fixture. The moving beam moved downward at a constant speed of 10 mm/min until the moving beam moved downward at a constant speed of 10 mm/min until the specimen specimen fractured. A test in which the specimen slid off the fixture or broke in the fixture fractured. A test in which the specimen slid off the fixture or broke in the fixture was was considered invalid. The single trial was repeated 10 times, and the mean value and considered invalid. The single trial was repeated 10 times, and the mean value and stand- standard deviation of each test index were calculated. The load-displacement curve was ard deviation of each test index were calculated. The load-displacement curve was plotted plotted for each effective test, from which the fracture force (F ), tensile strength (s ), and for each effective test, from which the fracture force (Fb), tensile strength (σt), and fracture fracture energy (E ) were determined. energy (Eg) were determined. Figure 3. Setup for tensile test. Figure 3. Setup for tensile test. Fracture force is the maximum force born by a specimen at fracture failure under the Fracture force is the maximum force born by a specimen at fracture failure under the tensile condition. Tensile strength is defined as the fracture force per unit cross-sectional tensile condition. Tensile strength is defined as the fracture force per unit cross-sectional area of specimen. Thereinto, the product of the average thickness (leaf blade or leaf sheath) area of specimen. Thereinto, the product of the average thickness (leaf blade or leaf sheath) and the specimen width was used as the estimated value of original cross-sectional area and the specimen width was used as the estimated value of original cross-sectional area of of the specimen. Tensile strength was calculated using the following equation [9,19,22]. the specimen. Tensile strength was calculated using the following equation [9,19,22]. σ = , b (1) s = , (1) wt ⋅ w t where s is the tensile strength (MPa), F is the fracture force (N), w is the specimen width t b (mm), and t is the average specimen thickness (mm). Fracture energy is the energy consumption required for material fracture, and it is the internal energy caused by material deformation under an external load. Fracture energy is described as the area under the load-displacement curve between the test origin and maximum load point. It may be expressed mathematically as follows [14,23,24]: n=i1 F + F n+1 E =  D D , (2) ( ) g n+1 n n=0 Agriculture 2021, 11, 796 5 of 15 where E is the fracture energy (J), F and F are values of tensile load (N), and D and g n n+1 n D are values of the specimen’s elongation (mm). n+1 2.3. Statistical Analysis Experimental data were analyzed applying regression analysis and one-way analysis of variance (ANOVA) using SPSS 24 software (SPSS Inc., Chicago, IL, USA). Duncan’s multiple range test was used to compare the mean values, and the significance level was set at 0.05 [20]. 3. Results and Discussion 3.1. Physical Properties of Corn Materials The physical properties of Feitian 358 and Jidan 558 are presented in Table 1. From 24 September to 22 October 2020, the moisture contents of the kernel, bract and peduncle decreased by 41.82%, 68.21%, and 21.94% for Feitian 358, respectively, and showed a decrease of 44.95%, 75.63%, and 33.33% for Jidan 558, respectively. With the delay of harvest day, the moisture content of each part of the ear showed a decreasing trend. Table 1. Physical properties of corn materials used in this study (mean  SD). Harvest Date (in the Year 2020) Physical Properties Cultivars 24 September 1 October 8 October 15 October 22 October Blade thickness (mm) 0.33  0.03 0.33  0.04 0.31  0.03 0.28  0.02 0.27  0.02 Sheath thickness (mm) 0.57  0.07 0.56  0.04 0.54  0.05 0.51  0.04 0.50  0.02 Bract length (cm) 21.84  3.05 21.35  2.32 21.12  2.15 20.86  1.98 20.83  2.22 Feitian Bract width (cm) 9.92  1.65 9.56  1.78 9.63  1.23 9.34  1.77 9.13  1.22 Kernel moisture content (%) 42.35 39.47 33.62 29.74 24.64 Bract moisture content (%) 65.84 52.97 35.93 25.68 20.93 Peduncle moisture content (%) 82.59 83.74 75.58 70.21 64.47 Blade thickness (mm) 0.38  0.03 0.37  0.03 0.35  0.04 0.30  0.04 0.30  0.03 Sheath thickness (mm) 0.81  0.07 0.79  0.03 0.78  0.06 0.75  0.03 0.73  0.02 Bract length (cm) 23.23  2.85 22.72  2.58 22.56  3.02 22.10  2.25 22.12  2.12 Jidan 558 Bract width (cm) 10.77  2.11 10.35  1.86 10.22  1.58 9.82  1.89 9.65  1.65 Kernel moisture content (%) 45.67 42.36 33.23 27.87 25.14 Bract moisture content (%) 72.43 58.73 38.19 21.77 17.65 Peduncle moisture content (%) 86.17 82.23 74.98 63.26 57.45 The length, width, blade thickness, and sheath thickness of the third bract all decreased with the delay of harvest day for both corn varieties, which might have been caused by the decrease in bract moisture content. An explanation for this could be that the pore spaces of leaf are filled with water when leaf moisture content is high, while the water in the pores dries when leaf moisture content decreases, so the leaf size becomes smaller [25]. Vincent et al. [21] reported a similar conclusion that ryegrass leaves thinned as the leaves dried. The average thickness and moisture content of the second to sixth bracts measured on 1 October 2020 are listed in Table 2. For the two varieties, the average thicknesses of leaf blades and sheaths decreased gradually from outer to inner layers, whereas the average moisture content increased firstly and then decreased from outer to inner layers. The total number of bracts for Feitian 358 was between 6 to 10 layers, and 76% of the ears had 7 layers. The total number of bracts for Jidan 558 ranged from 6 to 11 layers, and 52% of the ears had 8 layers. The outermost bract was often damaged by direct contact with the external environment. Therefore, the second to sixth bracts were used to study the effect of bract thickness on the tensile properties. Agriculture 2021, 11, 796 6 of 15 Table 2. Physical properties of the second to sixth bracts used in this study (mean  SD). Layer Feitian 358 Jidan 558 Sheath Moisture Blade Sheath Moisture Thickness Thickness Content Thickness Thickness Content (mm) (mm) (%) (mm) (mm) (%) 2 0.35  0.05 0.68  0.07 49.76 0.38  0.04 0.88  0.06 54.82 3 0.33  0.03 0.56  0.04 52.97 0.37  0.04 0.79  0.03 58.73 4 0.29  0.04 0.50  0.03 54.13 0.35  0.02 0.70  0.04 59.25 5 0.25  0.02 0.49  0.05 54.84 0.32  0.03 0.65  0.05 60.63 6 0.22  0.02 0.47  0.02 53.21 0.26  0.02 0.62  0.03 60.13 3.2. Tensile Properties In this section, the tensile properties, fracture modes, and fracture mechanisms of leaf blades and leaf sheaths are described. The effect of bract moisture content, bract thickness, and tensile loading direction are analyzed. 3.2.1. Tensile Test Results of Leaf Blade The values of mean fracture force, tensile strength, and fracture energy of leaf blades under two directions, as well as the mathematical models of these properties as a function of moisture content and blade thickness, are presented in Tables 3 and 4. Table 3. Tensile properties of the third leaf blade from five harvest days (mean  SD) and repeated measures analysis of variance. Parallel to the Vein Perpendicular to the Vein Date—Bract Tensile Tensile Moisture Fracture Force Fracture Energy Fracture Force Fracture Energy Strength Strength Content (%) 3 3 (N) (10 J) (N) (10 J) (MPa) (MPa) Feitian 358 09/24—65.84 42.33 d  6.46 4.28 d  0.65 212.72 b  23.72 5.13 d  0.80 0.52 d  0.08 9.67 bc  1.33 10/01—52.97 51.55 d  5.45 5.21 d  0.55 254.19 a  25.46 7.26 cd  1.38 0.73 d  0.14 11.99 a  0.99 10/08—35.93 67.51 c  8.02 7.26 c  0.86 227.68 ab  33.56 9.68 c  1.42 1.04 c  0.15 11.21 ab  1.18 10/15—25.68 79.87 b  8.28 9.51 b  0.99 208.08 b  28.04 12.57 b  2.73 1.50 b  0.33 10.6 ab  1.51 10/22—20.93 93.34 a  10.93 11.52 a  1.35 143.90 c  17.56 15.27 a  2.45 1.88 a  0.30 8.85 c  1.19 F-Value 32.951 ** 52.706 ** 12.100 ** 22.783 ** 31.465 ** 4.895 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 F = 136.026 2.493 mc + 0.016 mc , R = 0.980 F = 23.971 0.515 mc + 0.004 mc , R = 0.965 b b 2 2 4 2 2 s = 19.210 0.455 mc + 0.003 mc , R = 0.983 s = 3.326 0.086 mc + 6.689  10 mc , R = 0.969 t t 2 2 2 2 E = 61.796 + 13.298 mc 0.139 mc , R = 0.813 E = 1.465 + 0.473 mc 0.005 mc , R = 0.827 g g Jidan 558 09/24—72.43 36.39 c  5.29 3.19 e  0.46 169.98 b  20.48 4.93 c  0.87 0.43 c  0.08 9.37 a  1.17 10/01—58.73 67.88 b  8.58 6.12 d  0.77 231.79 a  26.53 5.49 c  0.97 0.49 c  0.08 10.78 a  1.43 10/08—38.19 77.84 b  7.12 7.41 c  0.68 c 204.67 a  23.91 6.89 c  1.53 0.66 c  0.15 10.60 a  1.80 10/15—21.77 91.49 a  8.48 10.17 b  0.94 159.82 b  15.48 11.52 b  2.03 1.28 b  0.23 8.89 ab  1.29 10/22—17.65 101.27 a  9.16 11.25 a  1.02 142.83 b  19.43 14.71 a  1.97 1.63 a  0.22 7.47 b  0.98 F-Value 50.926 ** 81.559 ** 13.957 ** 37.257 ** 53.154 ** 4.932 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 F = 101.646 0.082 mc 0.011 mc , R = 0.897 F = 22.976 0.597 mc + 0.005 mc , R = 0.943 b b 2 2 4 2 2 s = 13.555 0.153 mc + 2.027 mc , R = 0.936 s = 2.699 0.076 mc + 6.202  10 mc , R = 0.958 t t 2 2 2 2 E = 12.164 + 8.759 mc 0.090 mc , R = 0.849 E = 2.959 + 0.329 mc 0.003 mc , R = 0.947 g g ** Extreme significance (p < 0.01), p = probability value. F = Fisher ’s variance ratio, R = coefficient of multiple determination, mc = bract moisture content. Mean values with different letters in each column indicate that these values have statistically significant differences (p < 0.05). Agriculture 2021, 11, 796 7 of 15 Tensile Properties of the Third Leaf Blade in Different Harvest Days The longitudinal and transverse tensile properties of leaf blades of the two cultivars showed the same change rule with the delay of harvest day (Table 3). The mean fracture force and tensile strength increased, while the mean fracture energy increased firstly then decreased. With decreasing moisture content, the leaves became more brittle and more sensitive, and decreased in tensile properties, which were considered as a product of analysis [13]. During the harvest period of this study, the bract moisture contents decreased by 68.21% and 75.63% for Feitian 358 and Jidan 558, respectively, while the mean tensile strengths of leaf blades in the LTT increased by 1.69 and 2.53 times, respectively, and those in the TTT ere increased by 2.62 and 2.79 times, respectively. Fiber almost never became brittle by drying [12]. One possible explanation was that, due to water scarcity, the maximum lignification of the vascular system resulted in thickening of the secondary wall, thereby increasing the mechanical strength of the bract [26]. The typical force-displacement curves of the blade tensile test are plotted in Figure 4, wherein Figure 4a,b show the longitudinal tensile curves, and Figure 4c,d show the trans- verse tensile curves. Although the elongations of blade specimens varied greatly for different harvest days for both corn varieties, the overall variation showed a predictable trend. The force-displacement curves showed that the maximum elongation of leaf blades decreased with a delay of harvest day, while the opposite occurred to the changing tendency of fracture force. This might have been due to the fact that reduced moisture content of the bract caused a decrease in toughness of fiber, and led to a decrease in tensile elongation. Table 4. Tensile properties of the second to sixth leaf blades (mean  SD) and repeated measures analysis of variance. Parallel to the Vein Perpendicular to the Vein Layer—Blade Tensile Tensile Thickness Fracture Force Fracture Energy Fracture Force Fracture Energy Strength Strength (mm) 3 3 (N) (10 J) (N) (10 J) (MPa) (MPa) Feitian 358 2—0.35 65.85 a  8.44 6.33 a  0.83 230.22 c  21.19 9.87 a  1.75 0.95 a  0.17 11.13 a  2.23 3—0.33 51.55 b  5.45 5.21 a  0.55 254.19 bc  25.46 7.26 b  1.38 0.73 a  0.14 11.99 a  0.99 4—0.29 44.91 bc  7.24 5.20 a  0.84 333.99 a  25.89 6.33 bc  1.39 0.73 a  0.16 10.82 a  1.60 5—0.25 41.13 bc  5.81 5.42 a  0.76 283.15 b  27.19 5.68 bc  1.87 0.75 a  0.25 8.16 b  1.75 6—0.22 36.27 c  6.05 5.50 a  0.92 168.82 d  16.35 4.65 c  0.67 0.70 a  0.10 6.20 b  1.68 F-Value 14.688 ** 1.717 ns 33.952 ** 9.072 ** 1.658 ns 10.049 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 F = 119.979 739.343 t + 1656.243 t , R = 0.865 F = 16.513 108.177 t + 251.593 t , R = 0.820 b b 2 3 2 3 s = 82.687 + 1008.270 t 3791.638 t + 4682.032 t , s = 19.751 + 228.407 t 839.886 t + 1018.612 t , t t 2 2 R = 0.708 R = 0.758 2 2 2 2 E = 2357.100 + 18539.085 t 32039.386 t , R = 0.915 E = 36.054 + 284.373 t 424.697 t , R = 0.936 g g Jidan 558 2—0.38 74.71 a  10.07 6.50 a  0.86 207.68 b  22.12 7.07 a  1.29 0.62 a  0.11 10.46 ab  1.47 3—0.37 67.88 ab  8.58 6.12 a  0.77 231.79 b  26.53 5.49 b  0.97 0.49 a  0.08 10.78 ab  1.43 4—0.35 62.62 bc  8.16 5.96 a  0.78 321.80 a  27.73 5.31 bc  1.34 0.51 a  0.13 12.16 a  1.97 5—0.32 56.44 cd  7.58 5.95 a  0.80 319.45 a  24.23 4.38 c  1.17 b 0.46 a  0.12 9.58 b  1.45 6—0.26 50.72 d  5.54 6.50 a  0.79 206.34 b  20.66 3.72 c  0.90 0.48 a  0.12 3.93 c  0.69 F-Value 6.699 ** 0.700 ns 28.838 ** 6.140 ** 1.416 ns 23.777 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 F = 149.504 760.441 t + 1465.634 t , R = 0.969 F = 19.376 116.695 t + 217.824 t , R = 0.814 b b 2 3 s = 27.322 + 369.264 t 1313.395 t + 1513.255 t , 2 2 s = 2.269 12.077 t + 20.029 t , R = 0.355 R = 0.879 2 2 2 2 E = 3117.082 + 21450.115 t 33363.837 t , R = 0.907 E = 96.848 + 612.748 t 867.574 t , R = 0.946 g g ** Extreme significance (p < 0.01), ns = not significant (p > 0.05), p = probability value. F = Fisher ’s variance ratio, R = coefficient of multiple determination, t = blade thickness. Mean values with different letters in each column indicate that these values have statistically significant differences (p < 0.05). Agriculture 2021, 11, x FOR PEER REVIEW 7 of 15 Table 3. Tensile properties of the third leaf blade from five harvest days (mean ± SD) and repeated measures analysis of variance. Date—Bract Parallel to the Vein Perpendicular to the Vein Moisture Con- Fracture Force Tensile Fracture Energy Fracture Force Tensile Fracture Energy −3 −3 tent (%) (N) Strength (MPa) (×10 J) (N) Strength (MPa) (×10 J) Feitian 358 09/24—65.84 42.33 d ± 6.46 4.28 d ± 0.65 212.72 b ± 23.72 5.13 d ± 0.80 0.52 d ± 0.08 9.67 bc ± 1.33 10/01—52.97 51.55 d ± 5.45 5.21 d ± 0.55 254.19 a ± 25.46 7.26 cd ± 1.38 0.73 d ± 0.14 11.99 a ± 0.99 10/08—35.93 67.51 c ± 8.02 7.26 c ± 0.86 227.68 ab ± 33.56 9.68 c ± 1.42 1.04 c ± 0.15 11.21 ab ± 1.18 10/15—25.68 79.87 b ± 8.28 9.51 b ± 0.99 208.08 b ± 28.04 12.57 b ± 2.73 1.50 b ± 0.33 10.6 ab ± 1.51 10/22—20.93 93.34 a ± 10.93 11.52 a ± 1.35 143.90 c ± 17.56 15.27 a ± 2.45 1.88 a ± 0.30 8.85 c ± 1.19 F-Value 32.951 ** 52.706 ** 12.100 ** 22.783 ** 31.465 ** 4.895 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 Fb = 136.026 − 2.493 mc + 0.016 mc , R = 0.980 Fb = 23.971 − 0.515 mc + 0.004 mc , R = 0.965 2 2 −4 2 2 σt = 19.210 − 0.455 mc + 0.003 mc , R = 0.983 σt = 3.326 − 0.086 mc + 6.689 × 10 mc , R = 0.969 2 2 2 2 Eg = −61.796 + 13.298 mc − 0.139 mc , R = 0.813 Eg = 1.465 + 0.473 mc − 0.005 mc , R = 0.827 Jidan 558 09/24—72.43 36.39 c ± 5.29 3.19 e ± 0.46 169.98 b ± 20.48 4.93 c ± 0.87 0.43 c ± 0.08 9.37 a ± 1.17 10/01—58.73 67.88 b ± 8.58 6.12 d ± 0.77 231.79 a ± 26.53 5.49 c ± 0.97 0.49 c ± 0.08 10.78 a ± 1.43 10/08—38.19 77.84 b ± 7.12 7.41 c ± 0.68 c 204.67 a ± 23.91 6.89 c ± 1.53 0.66 c ± 0.15 10.60 a ± 1.80 10/15—21.77 91.49 a ± 8.48 10.17 b ± 0.94 159.82 b ± 15.48 11.52 b ± 2.03 1.28 b ± 0.23 8.89 ab ± 1.29 10/22—17.65 101.27 a ± 9.16 11.25 a ± 1.02 142.83 b ± 19.43 14.71 a ± 1.97 1.63 a ± 0.22 7.47 b ± 0.98 F-Value 50.926 ** 81.559 ** 13.957 ** 37.257 ** 53.154 ** 4.932 ** Regression relationships (parallel) Regression relationships (perpendicular) 2 2 2 2 Fb = 101.646 − 0.082 mc − 0.011 mc , R = 0.897 Fb = 22.976 − 0.597 mc + 0.005 mc , R = 0.943 2 2 −4 2 2 σt = 13.555 − 0.153 mc + 2.027 mc , R = 0.936 σt = 2.699 − 0.076 mc + 6.202 × 10 mc , R = 0.958 2 2 2 2 Eg = 12.164 + 8.759 mc − 0.090 mc , R = 0.849 Eg = 2.959 + 0.329 mc − 0.003 mc , R = 0.947 ** Extreme significance (p < 0.01), p = probability value. F = Fisher’s variance ratio, R = coefficient of multiple determina- Agriculture 2021, 11, 796 8 of 15 tion, mc = bract moisture content. Mean values with different letters in each column indicate that these values have statis- tically significant differences (p < 0.05). Agriculture 2021, 11, x FOR PEER REVIEW 8 of 15 (a) (b) (c) (d) Figure 4. Force-displacement curves of the tensile test of the third leaf blade from five harvest days during the 2020 harvest Figure 4. Force-displacement curves of the tensile test of the third leaf blade from five harvest days during the 2020 harvest season: (a) longitudinal tensile curves of Feitian 358; (b) longitudinal tensile curves of Jidan 558; (c) transverse tensile season: (a) longitudinal tensile curves of Feitian 358; (b) longitudinal tensile curves of Jidan 558; (c) transverse tensile curves curves of Feitian 358; (d) transverse tensile curves of Jidan 558. of Feitian 358; (d) transverse tensile curves of Jidan 558. Tensile Properties of the Second to Sixth Leaf Blades on 1 October 2020 Tensile Properties of the Second to Sixth Leaf Blades on 1 October 2020 The longitudinal and transverse tensile fracture forces of leaf blades showed a de- The longitudinal and transverse tensile fracture forces of leaf blades showed a de- creasing trend from outer to inner layers for both varieties (Table 4). This was because the creasing trend from outer to inner layers for both varieties (Table 4). This was because the cross-sectional area gradually decreased from outer to inner layers when the specimen cross-sectional area gradually decreased from outer to inner layers when the specimen width was constant. The researches proved that the fracture force has a strong correlation width was constant. The researches proved that the fracture force has a strong correlation with the cross-sectional area [27,28]. However, whether LTT or TTT, the tensile strengths with the cross-sectional area [27,28]. However, whether LTT or TTT, the tensile strengths of of each layer of leaf blades had little difference except for the second layer, which was each layer of leaf blades had little difference except for the second layer, which was larger larger for both varieties. Moisture content might have been the main reason for this. As for both varieties. Moisture content might have been the main reason for this. As shown shown in Table 2, the moisture content of the second bract was relatively low. In addition, in Table 2, the moisture content of the second bract was relatively low. In addition, the the fracture energy increased firstly and then decreased from outer to inner layers. Ac- fracture energy increased firstly and then decreased from outer to inner layers. According cording to the Equation (2), we inferred it was the specimen elongation that resulted in to the Equation (2), we inferred it was the specimen elongation that resulted in the maxi- the maximum fracture energy occurring in the middle layer. In fact, we did find that the mum fracture energy occurring in the middle layer. In fact, we did find that the specimen specimen elongation of the middle layer was larger, possibly as a result of moisture con- elongation of the middle layer was larger, possibly as a result of moisture content and tent and blade thickness. One-way ANOVA showed that blade thickness had a significant blade thickness. One-way ANOVA showed that blade thickness had a significant effect on effect fractur on e f for ract ceu and re force fractur and e ener fractgy ure en (p <er 0.01), gy (pbut < 0.01) the , ef but the eff fect on the ect tensile on the tensil strength e strength was not was not obvious (p > 0.05). Our results agree well with findings of Jacobs et al. [14] on the fracture properties of seven tropical grasses. According to the results of the blade tensile test, the load increased for both direc- tions, with an increase of specimen elongation before decreasing abruptly due to specimen fracture. The mean fracture force and tensile strength of the leaf blade in the LTT were about 8.95 times, and the mean fracture energy was about 24.03 times as much as that in the TTT. Therefore, the leaf blade was prone to transverse tearing during the mechanical peeling. The result of field investigation showed that most leaf blades were torn at differ- ent degrees in the transverse direction after mechanical peeling, which provides support- ing evidence for our experimental result. Agriculture 2021, 11, 796 9 of 15 obvious (p > 0.05). Our results agree well with findings of Jacobs et al. [14] on the fracture properties of seven tropical grasses. According to the results of the blade tensile test, the load increased for both directions, with an increase of specimen elongation before decreasing abruptly due to specimen fracture. The mean fracture force and tensile strength of the leaf blade in the LTT were about 8.95 times, and the mean fracture energy was about 24.03 times as much as that in the TTT. Therefore, the leaf blade was prone to transverse tearing during the mechanical peeling. The result of field investigation showed that most leaf blades were torn at different degrees in the transverse direction after mechanical peeling, which provides supporting evidence for our experimental result. 3.2.2. Tensile Test Results of Leaf Sheath As illustrated in Table 5, bract moisture content had a significant effect on the tensile properties of the leaf sheaths (p < 0.01), and sheath thickness had a significant effect on the fracture force and fracture energy (p < 0.01) but no effect on tensile strength (p > 0.05). The results of tensile properties of leaf sheath under three directions, and the regression relationships between these properties and moisture content (or sheath thickness) are presented in Figures 5 and 6. In the following paragraphs, the effects of each factor on the tensile properties of leaf sheaths are comprehensively discussed. Table 5. Repeated measures analysis of variance for tensile properties of leaf sheaths. F-Value (Feitian 358) F-Value (Jidan 558) Influence Measured Factor Variables 0 90 180 0 90 180 Bract F 19.533 ** 33.198 ** 27.477 ** 56.744 ** 71.471 ** 37.258 ** moisture s 45.234 ** 27.348 ** 23.340 ** 21.007 ** 23.648 ** 54.087 ** content E 13.456 ** 6.963 ** 4.516 ** 15.423 ** 5.959 ** 4.732 ** F 15.064 ** 7.884 ** 4.713 ** 21.529 ** 6.415 ** 6.052 ** Sheath s 1.386 ns 0.881 ns 0.914 ns 1.857 ns 1.520 ns 0.747 ns thickness E 37.559 ** 24.741 ** 18.132 ** 28.098 ** 33.745 ** 14.856 ** F = Fisher ’s variance ratio. ** Extreme significance (p < 0.01), ns = not significant (p > 0.05), p = probability value. Tensile Properties of the Third Leaf Sheath from Different Harvest Days With the delay of harvest day, the mean fracture force and tensile strength that made the third leaf sheath separate from the peduncle both increased with decreasing moisture content for the two cultivars, while the mean fracture energy increased firstly and then decreased (Figure 5). The fracture force (or tensile strength) of the leaf sheaths for Feitian 358 and Jidan 558 satisfied the quadratic polynomial relationship (R > 0.926) and linear relationship (R > 0.966) with moisture content, respectively. Meanwhile, the fracture en- ergy of the leaf sheaths for the two cultivars satisfied the quadratic polynomial relationship (R > 0.856) with moisture content. When the leaf sheaths from five harvest days were pulled at angles of 0 , 90 , and 180 (the angles between the pulling force and the natural growth direction of bract), the mean fracture forces required to remove the third leaf sheaths were 31.79, 11.78, and 8.01 N, respectively; the mean tensile strengths were 4.89, 1.82, and 3 3 1.23 MPa, respectively; the mean fracture energies were 90.14  10 , 29.53  10 , and 15.33  10 J, respectively. Agriculture 2021, 11, x FOR PEER REVIEW 10 of 15 Tensile Properties of the Third Leaf Sheath from Different Harvest Days With the delay of harvest day, the mean fracture force and tensile strength that made the third leaf sheath separate from the peduncle both increased with decreasing moisture content for the two cultivars, while the mean fracture energy increased firstly and then decreased (Figure 5). The fracture force (or tensile strength) of the leaf sheaths for Feitian 358 and Jidan 558 satisfied the quadratic polynomial relationship (R > 0.926) and linear relationship (R > 0.966) with moisture content, respectively. Meanwhile, the fracture en- ergy of the leaf sheaths for the two cultivars satisfied the quadratic polynomial relation- ship (R > 0.856) with moisture content. When the leaf sheaths from five harvest days were pulled at angles of 0°, 90°, and 180° (the angles between the pulling force and the natural growth direction of bract), the mean fracture forces required to remove the third leaf sheaths were 31.79, 11.78, and 8.01 N, respectively; the mean tensile strengths were 4.89, Agriculture 2021, 11, 796 10 of 15 −3 −3 1.82, and 1.23 MPa, respectively; the mean fracture energies were 90.14 × 10 , 29.53 × 10 , −3 and 15.33 × 10 J, respectively. (a) (b) (c) Figure 5. Relationships between tensile properties and moisture content: (a) fracture force; (b) tensile strength; (c) fracture Figure 5. Relationships between tensile properties and moisture content: (a) fracture force; (b) tensile strength; energy. (c) fracture energy. Tensile Properties of the Second to Sixth Leaf Sheaths on 1 October 2020 As shown in Figure 6, the mean fracture force that separated the leaf sheath from the peduncle increased from inner to outer layers for the two cultivars. Sheath thickness had no significant effect on the tensile strength of the leaf sheaths (p > 0.05), and there was no obvious rule in the variation of tensile strength. The experimental results showed that the mean tensile strength of the outer layer was higher than that of the inner layer when the tensile angles were 0 and 90 , while the mean tensile strength of each layer had little difference when the tensile angle was 180 . The fracture energy increased firstly and then decreased from outer to inner layers. The fracture force of the leaf sheaths for the two cultivars satisfied the quadratic polynomial relationship (0 , R > 0.995) and the linear relationship (90 and 180 , R > 0.935) with sheath thickness. Meanwhile, the mean tensile strength and fracture energy of the leaf sheaths for the two cultivars satisfied the cubic 2 2 polynomial relationship (R > 0.326) and the quadratic polynomial relationship (R > 0.860) with sheath thickness, respectively. When the leaf sheaths were pulled at angles of 0 , 90 , and 180 , the mean fracture forces required to remove the second to sixth leaf sheaths were 23.40, 8.18, and 5.51 N, respectively; the mean tensile strengths were 3.67, 1.28, and Agriculture 2021, 11, x FOR PEER REVIEW 11 of 15 Tensile Properties of the Second to Sixth Leaf Sheaths on 1 October 2020 As shown in Figure 6, the mean fracture force that separated the leaf sheath from the peduncle increased from inner to outer layers for the two cultivars. Sheath thickness had no significant effect on the tensile strength of the leaf sheaths (p > 0.05), and there was no obvious rule in the variation of tensile strength. The experimental results showed that the mean tensile strength of the outer layer was higher than that of the inner layer when the tensile angles were 0° and 90°, while the mean tensile strength of each layer had little difference when the tensile angle was 180°. The fracture energy increased firstly and then decreased from outer to inner layers. The fracture force of the leaf sheaths for the two cultivars satisfied the quadratic polynomial relationship (0°, R > 0.995) and the linear re- lationship (90° and 180°, R > 0.935) with sheath thickness. Meanwhile, the mean tensile strength and fracture energy of the leaf sheaths for the two cultivars satisfied the cubic 2 2 polynomial relationship (R > 0.326) and the quadratic polynomial relationship (R > 0.860) Agriculture 2021, 11, 796 11 of 15 with sheath thickness, respectively. When the leaf sheaths were pulled at angles of 0°, 90°, and 180°, the mean fracture forces required to remove the second to sixth leaf sheaths were 23.40, 8.18, and 5.51 N, respectively; the mean tensile strengths were 3.67, 1.28, and −3 3 −3 3 0.86 MPa, respectively; the mean fracture energies were 93.00 × 10 , 29.26 × 10 , and 15.30 0.86 MPa, respectively; the mean fracture energies were 93.00  10 , 29.26  10 , and −3 3 × 10 J, respectively. 15.30  10 J, respectively. According to the above analysis, among the three directions of sheath tensile test, the According to the above analysis, among the three directions of sheath tensile test, the tensile property values were largest when pulled the leaf sheaths at an angle of 0°, and tensile property values were largest when pulled the leaf sheaths at an angle of 0 , and smal smallest lest whe when n pul pulling ling t the he lea leaf f she sheaths aths at at an an an ang gle le of of 1 180 80°.. Unde Under r tthe he s same ame ttest est co conditions, nditions, t the he m mean ean fractur fracture e for force ce (or (or tensile tensilestr stength) rengthof ) of 0 0° was wa2.83 s 2.83 and and 4.23 4.2times 3 time as s as much muc as h a that s thof at 90 and 180 , respectively. Meanwhile, the mean fracture energy of 0 was about 3.14 and of 90° and 180°, respectively. Meanwhile, the mean fracture energy of 0° was about 3.14 and 6.17 6. times 17 tias memuch s as m as uch that as of th90 at of 9 and 0°180 and 1 , respectively 80°, respec.tiTher vely. The efore,ref the ore, t fractur he fr e act foru ce re fo of leaf rce sheaths could be reduced by increasing the pulling force angle on the bract. It was reported of leaf sheaths could be reduced by increasing the pulling force angle on the bract. It was in the literature that the fracture force of sugarcane leaf showed a similar decreasing trend reported in the literature that the fracture force of sugarcane leaf showed a similar de- with an increase of pulling force angle [29]. creasing trend with an increase of pulling force angle [29]. Agriculture 2021, 11, x FOR PEER REVIEW 12 of 15 (a) (b) (c) Figure 6. Relationships between tensile properties and sheath thickness: (a) fracture force; (b) tensile strength; (c) fracture Figure 6. Relationships between tensile properties and sheath thickness: (a) fracture force; (b) tensile strength; energy. (c) fracture energy. 3.2.3. Vein Traits and Fracture Modes of Bract 3.2.3. Vein Traits and Fracture Modes of Bract Figur Figure e 7 7 shows shows the the surfac surfacee im image age o off aa corn cornbract bract taken wi taken with th aa digi digital talcamera. camera. Corn Corn bract bract has the has the pattern pattern of of parallel parallel vei veins ntypical s typica to l to monocotyl monocotyledonous edonous pl plants. ants. The The longitudinal longitudi- nal veins arranged in sequence are parallel to each other. There are mesophyll and small transverse veins between two longitudinal veins. Figure 7. Vein traits of corn bract. Figure 8 shows the surface and cross-sectional morphology of a corn bract observed using scanning electron microscopy. It could be seen that the longitudinal veins of the corn bract were about two to three times thicker than the surrounding mesophyll tissue. This highlighted the shortcoming of using a single thickness value in the tensile strength formula. The actual tensile strength of the bract was larger than the experimental data. Therefore, the tensile strength of a bract should be set slightly higher when the experi- mental data are applied to the actual machine parameter setting. Even so, the data still reflected the variation of tensile strength of bracts throughout the harvest period. Agriculture 2021, 11, x FOR PEER REVIEW 12 of 15 (c) Figure 6. Relationships between tensile properties and sheath thickness: (a) fracture force; (b) tensile strength; (c) fracture energy. Agriculture 2021, 11, 796 12 of 15 3.2.3. Vein Traits and Fracture Modes of Bract Figure 7 shows the surface image of a corn bract taken with a digital camera. Corn bract has the pattern of parallel veins typical to monocotyledonous plants. The longitudi- veins arranged in sequence are parallel to each other. There are mesophyll and small nal veins arranged in sequence are parallel to each other. There are mesophyll and small transverse veins between two longitudinal veins. transverse veins between two longitudinal veins. Figure 7. Vein traits of corn bract. Figure 7. Vein traits of corn bract. Figure 8 shows the surface and cross-sectional morphology of a corn bract observed Figure 8 shows the surface and cross-sectional morphology of a corn bract observed using scanning electron microscopy. It could be seen that the longitudinal veins of the using scanning electron microscopy. It could be seen that the longitudinal veins of the corn corn bra bract ct were a were about bout two two to to three ti three times mes th thicker icker than than the the surro surrounding unding mesophyll tissu mesophyll tissue. e. This This h highlighted ighlighted t the he sho shortcoming rtcoming o offusing using aa single singlethickness thickness v value aluein inthe the t tensile ensilestr stength rength formula. formula. The a The actual ctual tensi tensile le strength of strength of the the bract bract was lar was larger ger than t than the he experimen experimental tal d data. ata. Ther Therefore efore, the , the tensile tensile str stre ength ngth of a br of a bract should act should be set be slightly set slight higher ly higher when when the experimental the experi- Agriculture 2021, 11, x FOR PEER REVIEW 13 of 15 data menta are l da applied ta are to appli theeact d to the a ual machine ctual parameter machine pa setting. rameter setti Even so, ng. Even the data so, the da still reflected ta still the variation of tensile strength of bracts throughout the harvest period. reflected the variation of tensile strength of bracts throughout the harvest period. (a) (b) Figure 8. Surface and cross-sectional morphology of corn bracts: (a) surface morphology of corn bracts; (b) cross-sectional Figure 8. Surface and cross-sectional morphology of corn bracts: (a) surface morphology of corn bracts; (b) cross- morphology of corn bracts. EHT = extra high tension, SE = secondary electron, Mag = magnification times, WD = work sectional morphology of corn bracts. EHT = extra high tension, SE = secondary electron, Mag = magnification times, distance. WD = work distance. The crack propagation behavior of blade specimens under two vein orientations are The crack propagation behavior of blade specimens under two vein orientations presented in Figure 9. The blade specimen distributed the tensile load on each solid lon- are presented in Figure 9. The blade specimen distributed the tensile load on each solid gitudinal vein when the leaf blade was pulled with a force parallel to the longitudinal longitudinal vein when the leaf blade was pulled with a force parallel to the longitudinal vein. As the load increased, the stress spread rapidly to the remaining longitudinal veins vein. As the load increased, the stress spread rapidly to the remaining longitudinal veins when a longitudinal vein broke. Once respective fracture strengths of the remaining lon- when a longitudinal vein broke. Once respective fracture strengths of the remaining gitudinal veins were exceeded, catastrophic failure occurred to the whole specimen. In longitudinal veins were exceeded, catastrophic failure occurred to the whole specimen. In addition, as shown in Figure 4a,b, the decline curves remained linear after catastrophic addition, as shown in Figure 4a,b, the decline curves remained linear after catastrophic failur failure o e off t the he blade blade specim specimen. en. This This ind indicates icates th that at mos most t of t ofhthe e long longi itudin tudinal al veins veins broke a broke l- most simultaneously. Additionally, the locations of fracture point of each longitudinal vein were relatively random, and might be far apart due to the different physical proper- ties of each position on the leaf blade. Thus, the fracture shape of the blade specimen might have been an irregular broken line [13]. It was observed that the fracture shape of the blade specimen was relatively regular in the TTT. Small transverse veins provided little resistance. The fracture mode was the mesophyll tissue torn along the longitudinal vein direction. The crack occurred between two adjacent longitudinal veins, then rapidly propagated until specimen fracture. (a) (b) Figure 9. Crack propagation modes of leaf blade under two vein orientations: (a) fracture mode of leaf blades in the LTT; (b) fracture mode of leaf blades in the TTT. When pulling the leaf sheath at an angle of 0°, the fracture mode of leaf sheaths was similar to that of leaf blades in the LTT. The fracture position was usually not at the root Agriculture 2021, 11, x FOR PEER REVIEW 13 of 15 (a) (b) Figure 8. Surface and cross-sectional morphology of corn bracts: (a) surface morphology of corn bracts; (b) cross-sectional morphology of corn bracts. EHT = extra high tension, SE = secondary electron, Mag = magnification times, WD = work distance. The crack propagation behavior of blade specimens under two vein orientations are presented in Figure 9. The blade specimen distributed the tensile load on each solid lon- gitudinal vein when the leaf blade was pulled with a force parallel to the longitudinal vein. As the load increased, the stress spread rapidly to the remaining longitudinal veins when a longitudinal vein broke. Once respective fracture strengths of the remaining lon- gitudinal veins were exceeded, catastrophic failure occurred to the whole specimen. In addition, as shown in Figure 4a,b, the decline curves remained linear after catastrophic failure of the blade specimen. This indicates that most of the longitudinal veins broke al- most simultaneously. Additionally, the locations of fracture point of each longitudinal Agriculture 2021, 11, 796 13 of 15 vein were relatively random, and might be far apart due to the different physical proper- ties of each position on the leaf blade. Thus, the fracture shape of the blade specimen might have been an irregular broken line [13]. almost simultaneously. Additionally, the locations of fracture point of each longitudinal It was observed that the fracture shape of the blade specimen was relatively regular vein were relatively random, and might be far apart due to the different physical properties in the TTT. Small transverse veins provided little resistance. The fracture mode was the of each position on the leaf blade. Thus, the fracture shape of the blade specimen might mesophyll tissue torn along the longitudinal vein direction. The crack occurred between have been an irregular broken line [13]. two adjacent longitudinal veins, then rapidly propagated until specimen fracture. (a) (b) Figure 9. Crack propagation modes of leaf blade under two vein orientations: (a) fracture mode of leaf blades in the LTT; Figure 9. Crack propagation modes of leaf blade under two vein orientations: (a) fracture mode of leaf blades in the LTT; (b) fracture mode of leaf blades in the TTT. (b) fracture mode of leaf blades in the TTT. When pulling the leaf sheath at an angle of 0°, the fracture mode of leaf sheaths was It was observed that the fracture shape of the blade specimen was relatively regular similar to that of leaf blades in the LTT. The fracture position was usually not at the root in the TTT. Small transverse veins provided little resistance. The fracture mode was the mesophyll tissue torn along the longitudinal vein direction. The crack occurred between two adjacent longitudinal veins, then rapidly propagated until specimen fracture. When pulling the leaf sheath at an angle of 0 , the fracture mode of leaf sheaths was similar to that of leaf blades in the LTT. The fracture position was usually not at the root of the leaf sheath, but slightly away from the root or somewhere on the leaf blade. This was because, when the pulling force angle was 0 , the mean tensile strength of the leaf sheath was closest to that of the leaf blade in the LTT, especially at the early stage of harvest. When pulling the leaf sheath at an angle of 180 , the tensile strength of the leaf sheath was relatively small, and the bract fractured along the root of the leaf sheath. The abrupt change of fracture behavior might have been caused by the stress of surface fibers transformed from a tensile stress to a bending stress, which resulted in changes to the crack initiation and propagation mechanism [30]. 4. Conclusions This paper presented the physical and tensile properties of corn bracts for two corn cultivars, Feitian 358 and Jidan 558. The results of physical properties showed that bract dimensions and moisture contents of the two cultivars varied with the changes of harvest day and bract layer. One-way ANOVA showed that bract moisture content creates a significant effect on the fracture force, tensile strength, and fracture energy, while bract thickness has a significant effect only on the fracture force and fracture energy (p < 0.01). As moisture content decreased, the tensile strengths of the leaf blades and sheaths both increased for the two cultivars. However, the tensile strengths of the leaf blades and sheaths did not noticeably change for different thicknesses. Vein orientation had a significant effect on the tensile properties of the leaf blades. A crack passed through every longitudinal vein in the LTT, while cracks usually occurred in the mesophyll between two adjacent longitudinal veins in the TTT. The tensile strength of Agriculture 2021, 11, 796 14 of 15 leaf blades in the LTT was about 8.95 times as much as that in the TTT. Therefore, the leaf blades were more prone to transverse tearing during the mechanical peeling. The tensile properties of leaf sheaths depended on the angle between pulling force and the natural growth direction of the bract. The tensile properties decreased gradually with an increase in pulling force angle. Hence, increasing the pulling force angle exerted by the peeling component on the bract could reduce the peeling power consumption and improve the efficiency and quality of mechanical peeling. Further, a larger pulling force angle could improve the probability of bract fracturing at the root of the leaf sheath, which is beneficial to reducing bract residue on the peeled ear. In this study, the test specimens were collected from five harvest days from different weeks after physiological maturity of the crop, coinciding with the normal harvest season. The test results could represent an actual material situation from the harvest period. Author Contributions: Conceptualization, J.F.; methodology, J.F.; validation, Z.L.; investigation, Z.L.; resources, J.F.; writing—original draft preparation, Z.L.; writing—review and editing, J.F.; supervision, X.L.; project administration, J.F.; funding acquisition, J.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the thirteenth five-year science and technology project of Jilin Provincial Education Department (No. JJKH20190144KJ). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data and models presented in this study are available on demand from the first author at (zhenye19@mails.jlu.edu.cn). Acknowledgments: The authors are grateful for the corn provided by the Agricultural Experimental Base of Jilin University. 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Journal

Agriculture – Multidisciplinary Digital Publishing Institute

Published: Aug 20, 2021

Keywords: corn bract; tensile properties; fracture properties; mechanical peeling

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