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A Smart Washer for Bolt Looseness Monitoring Based on Piezoelectric Active Sensing Method

A Smart Washer for Bolt Looseness Monitoring Based on Piezoelectric Active Sensing Method applied sciences Article A Smart Washer for Bolt Looseness Monitoring Based on Piezoelectric Active Sensing Method Heyue Yin, Tao Wang, Dan Yang *, Shaopeng Liu, Junhua Shao and Yourong Li Key Laboratory of Metallurgical Equipment and Control of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China; yinheyue123@163.com (H.Y.); wangtao77@wust.edu.cn (T.W.); lspmgl@126.com (S.L.); shaojunhua@wust.edu.cn (J.S.); liyourong@wust.edu.cn (Y.L.) * Correspondence: yangdan@wust.edu.cn; Tel.: +86-27-6886-2292 Academic Editor: Gangbing Song Received: 21 September 2016; Accepted: 20 October 2016; Published: 26 October 2016 Abstract: Piezoceramic based active sensing methods have been researched to monitor preload on bolt connections. However, there is a saturation problem involved with this type of method. The transmitted energy is sometimes saturated before the maximum preload which is due to it coming into contact with flat surfaces. When it comes to flat contact surfaces, the true contact area will easily saturate with the preload. The design of a new type of bolt looseness monitoring sensor, a smart washer, is to mitigate the saturation problem. The smart washer is composed of two annular disks with contact surfaces that are machined into convex and concave respectively, to eliminate the complete flat contact surfaces and to reduce the saturation effect. One piezoelectric patch is bonded on the non-contact surface of each annular disk. These two mating annular disks form a smart washer. One of the two piezoelectric patches serves as an actuator to generate an ultrasonic wave that propagates through the contact surface; the other one serves as a sensor to detect the propagated waves. The wave energy propagated through the contact surface is proportional to the true contact area which is determined by the bolt preload. The time reversal method is used to extract the peak of the focused signal as the index of the transmission wave energy; then, the relationship between the signal peak and bolt preload is obtained. Experimental results show that the focused signal peak value changes with the bolt preload and presents an approximate linear relationship when the saturation problem is experienced. The proposed smart washer can monitor the full range of the rated preload. Keywords: bolt loosen; active sensing; smart washer; non-destructive monitoring 1. Introduction Bolt connections are widely used in many industrial fields. Bolt connections are influenced by cyclic loading and various working conditions, and the bolt connections tend to loosen. In practice, accidents have been caused by bolt looseness, and monitoring of the bolt preload force has become important in the engineering field to ensure the safety of the structures. With the development of structural health monitoring (SHM), more theories and technologies are being developed regarding bolt health monitoring and bolt looseness detection [1]. Coin-tap is commonly used in engineering practices, including in the detection of bolt looseness. According to the change of the sound during an impact, engineers estimate whether the bolt is loosened or not. Based on this, Cawley and Adams [2,3] proposed a theory that structural imperfection causes the changes of the response signals, thus the structural damage can be identified according to the width of the time domain curves or the area ratio of the frequency domain. After a series of research, Xiang et al. [4,5] proposed a controlled tap detection method for bolt tightness based on a controlled Appl. Sci. 2016, 6, 320; doi:10.3390/app6110320 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 320 2 of 10 tap detection principle, and the bolt loosening status can be determined quantitatively by using the support vector machine method. However, the tap method cannot provide real time monitoring of bolt looseness. Piezoceramics with such attractive features as low cost, small size, wide bandwidth, and sensing and actuating functions have been widely researched in real-time active vibration control [6–8], stress-wave based communication [9–11], and structural health monitoring [12–16]. Another attractive feature of a piezoceramic transducer is the coupling between its electric impedance and the host structure’s mechanical impedance. The piezoelectric impedance method was proposed to monitor structure health status in 1994 by Liang et al. [17]. The analytical relationship between the piezoelectric impedance and the mechanical impedance of the host structure was established for the first time. Sopon et al. [18] used a piezoelectric transducer (PZT) as an actuator–sensor collectively in conjunction with the numerical model-based spectral element method (SEM) in structural health monitoring to quantitatively detect damage of bolted joints. By analyzing the characteristics of the piezoelectric element under the bolt looseness condition, Gao et al. [19] concluded that the conductance of the piezoelectric patches changes significantly under the high-frequency (more than 50 kHz) voltage excitation. Mascarenas et al. [20] enhanced the PZT to form a washer for monitoring individual bolt looseness by examining the electro-mechanical impedance (EMI) at resonance. Nguyem et al. [21] used multi-channel wireless impedance sensor nodes and multiple PZT-interfaces to monitor bolt loosening in bolted connection. The ultrasonic method is employed in some SHM fields. As a valuable way to monitor bolt looseness, the ultrasonic method is continually improved by many researchers [22]. An ultrasonic instrument for the accurate measurement of bolt stress was described by Joshi in 1984 [23]. Sisman et al. [24] used the ultrasonic method to quantitatively detect the preload between the bolt and the connect surface. Yasui et al. [25] proposed an ultrasonic velocity ratio method for measuring bolt axial load. Jhang et al. [26] used the phase detection technique to measure the TOF (time of flight) of a tone-burst ultrasonic wave; the method improved the precision of the ultrasonic velocity method for measuring bolt stress. The propagation velocity of the ultrasonic method is affected by bolt material, stress and some other factors, and it is not easy to accurately determine the axial force of the bolt. Additionally, it requires a special precision instrument for the measurement, and it is difficult to apply to real time application. The active sensing method utilizes the sensing and actuation capability of the piezoelectric transducers. For this method, at least one pair of the PZT patches is used with one serving as an actuator to generate a stress wave and the other serving as a sensor to detect the stress wave. A damage or change along the wave propagating path will change the detect signal. As a real time method, many researchers have explored this method [27]. Mita and Fujimoto [28] combined the ultrasonic waves and support vector machines method to detect loosened bolts. The experiments show that the proposed methods could identify the location and the level of damage of the loosened bolts. The active sensing-based method was also used in detecting damage of the bolt on a steel bridge [29], and some researchers applied ultrasonic and magneto-elastic active sensors in monitoring bolted joints in a satellite panel [30]. In our team’s previous studies [31,32], the piezoelectric active sensing method with the time reversal technique [33] was used to monitor the bolt looseness. The time reversal method reverses the response signal that is received by the sensor in its time domain. This method can make the signal energy focus on space and time. In these studies, the peak of the time reversal focused signal can represent the wave energy that transmitted through the interface of the bolted joint, and the wave energy is proportional to the true contact area of the bolted joint interface which depends on the bolt tightening force. The experiments show that the focused signal peak amplitudes increase with the increasing blot preload until reaching saturation and the focused signal peak remains stable. When there is no more deformation space at the bolted joint interface under a certain bolt preload, the true contact area does not change any more. Besides, the surface roughness of the bolted joint affects Appl. Sci. 2016, 6, 320 3 of 10 the saturation value; the higher roughness can postpone the saturation value of the bolt preload. When it comes to bolt connection surfaces, the contact roughness is determined by the process and working condition, which cannot be increased artificially. Even if a high roughness is used, it is hard to Appl. Sci. 2016, 6, 320  3 of 10  guarantee the higher roughness precisely for a certain machining process. Therefore, in order to resolve the problem of saturation, a new monitoring structure needs to be developed. The new proposition guarantee  the  higher  roughness  precisely  for  a  certain  machining  process.  Therefore,  in  order  to  is that of a new washer structure for monitoring bolted looseness status based on the piezoelectric resolve  the  problem  of  saturation,  a  new  monitoring  structure  needs  to  be  developed.  The  new  activeproposition sensing method  is that and of a new to deal  wash with er structure the saturation  for monitoring problem.  bolted The looseness new washer  status design  based on will the contr   ol piezoelectric active sensing method and to deal with the saturation problem. The new washer design  the contact area artificially so that the washer can monitor the full range of the rated preload. will control the contact area artificially so that the washer can monitor the full range of the rated  2. Proposed preload.Method   and Principle The new smart washer structure is shown in Figure 1. It is composed of two annular disks, 2. Proposed Method and Principle  the contact surfaces of which are machined into convex and concave, respectively. Two piezoelectric The new smart washer structure is shown in Figure 1. It is composed of two annular disks, the  patches are bonded onto two non-contact surfaces of the proposed smart washer sensor. One contact surfaces of which are machined into convex and concave, respectively. Two piezoelectric  piezoelectric patch serves as an actuator to generate an ultrasonic wave transmitting through the patches  are  bonded  onto  two  non‐contact  surfaces  of  the  proposed  smart  washer  sensor.  One  contact surface, and the other one as a sensor to detect the response signals. Furthermore, the time piezoelectric patch serves as an actuator to generate an ultrasonic wave transmitting through the  reversal contact method  surface, is employed,  and the other which  onecan  as aensur  sensor e atohigh  detect signal  the response to noise signals. ratio. The Furthermore, first response  the time signal   is reversal method is employed, which can ensure a high signal to noise ratio. The first response signal  reversed in the time domain and then reemitted. Then, the reverse focused signal is detected by the is reversed in the time domain and then reemitted. Then, the reverse focused signal is detected by the  sensor piezoelectric patch. In addition, based on the fact that the waves are the energy transmission sensor piezoelectric patch. In addition, based on the fact that the waves are the energy transmission  and the transmitted energy is proportional to the true contact area of the bolted joint interface, and the transmitted energy is proportional to the true contact area of the bolted joint interface, the  the bolted loosening status can be determined by extracting the focused signal peak that has a certain bolted loosening status can be determined by extracting the focused signal peak that has a certain  relationship with the transmitted energy. relationship with the transmitted energy.  Figure 1. Structure sketch.  Figure 1. Structure sketch. In order to describe the relationship between the contact area and loading‐force, an analysis of  In order to describe the relationship between the contact area and loading-force, an analysis of the the  contact  is  necessary.  According  to  the  Classical  Hertz  contact  theory,  two  elasticaxis‐parallel  cylinders’  internal  contact  is  shown  in  Figure  2;  the  contact  surface  shape  of  the  cylinders  is  contact is necessary. According to the Classical Hertz contact theory, two elasticaxis-parallel cylinders’ rectangular [34]. Furthermore, the unit’s pressure on the contact surface is distributed in an elliptic  internal contact is shown in Figure 2; the contact surface shape of the cylinders is rectangular [34]. cylindrical pattern as shown in Figure 3. Hertz theory relates loading force F and contact area S as  Furthermore, the unit’s pressure on the contact surface is distributed in an elliptic cylindrical pattern follows:  as shown in Figure 3. Hertz theory relates loading force F and contact area S as follows: 4FR a= (1)  4FR πLE a = (1) LE R R 1 2 R  (2)  R  R 2 1 R R R = (2) R R 1 1-- μ 1 μ 2 1 = + (3)  E 1 2 2 1 1  1 1 2 = + (3) E E E 4FLR F 2LR Sa =2 L=2 = 4   (4)  r r * * ππ EE 4FLR FLR S = 2aL = 2 = 4 (4) where a is half the width of the rectangle, L is the length of cylinders, E1 and E2 are the elasticity  E E modulus  of  the  two  cylinders,  R1  and  R2  are  the  curvatures  of  the  cylinders,  and μ1  and μ2  are  Poisson’s ratios.   Appl. Sci. 2016, 6, 320 4 of 10 where a is half the width of the rectangle, L is the length of cylinders, E and E are the elasticity 1 2 modulus of the two cylinders, R and R are the curvatures of the cylinders, and  and  are 1 2 1 2 Poisson’s ratios. Appl. Sci. 2016, 6, 320  4 of 10  For the smart washer in this paper, the contact area is an annulus; the existing contact formula is Appl. Sci. 2016, 6, 320  4 of 10  not applicable to this model; after the differential, the annulus can be regarded as being composed of For the smart washer in this paper, the contact area is an annulus; the existing contact formula  For the smart washer in this paper, the contact area is an annulus; the existing contact formula  numerous small rectangles, made up of countless small rectangles; the area of each rectangle is dS; is not applicable to this model; after the differential, the annulus can be regarded as being composed  is not applicable to this model; after the differential, the annulus can be regarded as being composed  and the contact area, S, can be obtained by the equation as follows: of numerous small rectangles, made up of countless small rectangles; the area of each rectangle is dS;  of numerous small rectangles, made up of countless small rectangles; the area of each rectangle is dS;  and the contact area, S, can be obtained by the equation as follows:  and the contact area, S, can be obtained by the equation as follows:  dS = 2adL (5) d=Sa2 dL  (5)  d=Sa2 dL  (5)  Z Z 2r 2r π⋅ 2FrR 2FrR 2r π⋅ S =SS =d dS == 2ad 2La= d4L = 4   (6)  (6) 2FrR òò * SS =d =0 2adL=4   (6)  E E òò * where r is the distance between the center of the washer and the center of the convex and concave, where r is the distance between the center of the washer and the center of the convex and concave, as  where r is the distance between the center of the washer and the center of the convex and concave, as  shown in Figure 1, and 2πr is the perimeter of the annulus center.   as shown in Figure 1, and 2r is the perimeter of the annulus center. shown in Figure 1, and 2πr is the perimeter of the annulus center.   Figure 2. Axis‐parallel cylinders’ internal contact.   Figure 2. Axis-parallel cylinders’ internal contact. Figure 2. Axis‐parallel cylinders’ internal contact.   Figure 3. Contact surface status.  Figure 3. Contact surface status.  Figure 3. Contact surface status. It can be seen that the contact area is decided by the acting force when r and R are given, more  It can be seen that the contact area is decided by the acting force when r and R are given, more  specifically, the area will grow with an increasing load. Therefore, the contact area can be an indicator  It can be seen that the contact area is decided by the acting force when r and R are given, more specifically, the area will grow with an increasing load. Therefore, the contact area can be an indicator  of the loading force.   specifically, the area will grow with an increasing load. Therefore, the contact area can be an indicator of the loading force.   The essence of wave propagation is energy transmission, and the wave leakage and dissipation  of the loading The essenc force.e of wave propagation is energy transmission, and the wave leakage and dissipation  occur at the contact interface. Thus, the energy will be reduced when the wave transmits through the  occur at the contact interface. Thus, the energy will be reduced when the wave transmits through the  The essence of wave propagation is energy transmission, and the wave leakage and dissipation interface. For the washer, the energy received by the PZT1 and PZT2 is the transmitted wave energy  interface. For the washer, the energy received by the PZT1 and PZT2 is the transmitted wave energy  occur at the contact interface. Thus, the energy will be reduced when the wave transmits through the that is transmitted through the convex and concave contact interfaces. It is found that the transmitted  that is transmitted through the convex and concave contact interfaces. It is found that the transmitted  interface. For the washer, the energy received by the PZT1 and PZT2 is the transmitted wave energy energy, E, is proportional to the true contact area, S [35], as follows:  energy, E, is proportional to the true contact area, S [35], as follows:  that is transmitted through the convex and concave contact interfaces. It is found that the transmitted E C S   (7)  E C S (7)  energy, E, is proportional to the true contact area, S [1 35], as follows: The relationship implies that the energy through the contact area increases with the enlargement  The relationship implies that the energy through the contact area increases with the enlargement  of the contact area, when the force increases according to Equation (6). As a result, more energy is  E µ C S (7) of the contact area, when the force increases according to Equation (6). As a result, more energy is  transmitted to the sensor. After considering the relationship in Equation (6), the transmitted energy,  transmitted to the sensor. After considering the relationship in Equation (6), the transmitted energy,  E, can be expressed as follows:  The relationship implies that the energy through the contact area increases with the enlargement E, can be expressed as follows:  of the contact area, when the force increases according to Equation (6). As a result, more energy is Appl. Sci. 2016, 6, 320 5 of 10 transmitted to the sensor. After considering the relationship in Equation (6), the transmitted energy, E, can be expressed as follows: E µ C F (8) Appl. Sci. 2016, 6, 320  5 of 10  In Equations (7) and (8), C and C are constants. It can be seen from Equation (8) that the received 1 2 (8)  E C F   energy is proportional to the square root of the loading force, which is the bolt axial force. Thus, the bolt looseness can be represented by the energy of wave. In Equations (7) and (8), C1 and C2 are constants. It can be seen from Equation (8) that the received  The time reversal method is adopted in this study. Based on the time reversal theory, the energy is proportional to the square root of the loading force, which is the bolt axial force. Thus, the  response signal energy is proportional to the peak amplitude of the focused signal [32], whereupon the bolt looseness can be represented by the energy of wave.   transmitted signal energy can be represented by the focused signal peak amplitude. A higher focused The  time  reversal  method  is  adopted  in  this  study.  Based  on  the  time  reversal  theory,  the  signal response peak amplitude  signal energ means y is praoportional larger transmitted  to the peaksignal  amplitener ude gy of th and e foc au lar sed ger sig bolt nal [32 preload. ], whereupon  the transmitted signal energy can be represented by the focused signal peak amplitude. A higher  3. The Smart Washer and Experimental Procedure focused signal peak amplitude means a larger transmitted signal energy and a larger bolt preload.  The design derives from the asperities of the machined surface. The microcosmic asperities 3. The Smart Washer and Experimental Procedure   are magnified to macroscopic convex and concave. Considering the Hertz contact stress theory The design derives from the asperities of the machined surface. The microcosmic asperities are  and by optimizing the data, the curvatures of the convex and concave are decided. The curvatures magnified to macroscopic convex and concave. Considering the Hertz contact stress theory and by  of the convex and concave are similar sizes to ensure that slippage does not occur between them. optimizing the data, the curvatures of the convex and concave are decided. The curvatures of the  There is a groove on the non-contact surface of the upper-washer and lower-washer, respectively, convex and concave are similar sizes to ensure that slippage does not occur between them. There is a  to place the piezoelectric patch (PZT) that served as an actuator and senor. A certain space between the groove on the non‐contact surface of the upper‐washer and lower‐washer, respectively, to place the  upper-washer and lower-washer is needed to avoid the additional surface contact, except the contact piezoelectric patch (PZT) that served as an actuator and senor. A certain space between the upper‐ of the convex and concave. The purpose of the washer design is to control the contact area artificially washer and lower‐washer is needed to avoid the additional surface contact, except the contact of the  so that convex the washer  and concan cave monitor . The purpose the full  of range the wa of shthe er de rated sign is pr to eload.  control From  the Equation contact are (6), a arti theficia contact lly so area will change that the with washdif er can ferent  monitor curvatur  the es full of range the convex  of the rat and ed concave, preload. Fr orom with  Equation the dif (6 fer ), ent the center contactdistances  area  will change with different curvatures of the convex and concave, or with the different center distances  under the same preload. In order to explore the influence of the central symmetry plane location of the under the same preload. In order to explore the influence of the central symmetry plane location of  convex and concave, two kinds of specimens are designed in the experiment. the convex and concave, two kinds of specimens are designed in the experiment.  The size of the smart washer is designed based on the standard bolt M16, as shown in Figure 4. The size of the smart washer is designed based on the standard bolt M16, as shown in Figure 4.  The outer diameter of the washer is 50 mm and the internal diameter is 17 mm for the M16 bolt The outer diameter of the washer is 50 mm and the internal diameter is 17 mm for the M16 bolt  connection. The curvature of the convex R and the concave R are 6 mm and 10 mm, respectively. 1 2 connection. The curvature of the convex R1 and the concave R2 are 6 mm and 10 mm, respectively.  The detailed sizes of all specimens are listed in Table 1. The above and bottom straight grooves, where The detailed sizes of all specimens are listed in Table 1. The above and bottom straight grooves, where  the piezoelectric patches are placed, are filled with insulating glue to protect the piezoelectric patches. the piezoelectric patches are placed, are filled with insulating glue to protect the piezoelectric patches.  12 12 The piezoelectric constant d and d of the patch surface is 400  10−12 CN and 150 −12 10 CN The  piezoelectric  constant 33  d33 and31  d31  of  the  patch surface is  400  ×  10   CN and −150  ×  10   CN  respectively respectively, , and and the the dimensions  dimensions of of the the piezoelectric  piezoelectric patch patch ar are e 88 mm mm ×  9 mm 9 mm  × 1 mm. 1 mm.    Figure 4. Specimen structure.   Figure 4. Specimen structure. Table 1. Specimen size.  Table 1. Specimen size. A: Center Distance of Convex  No.  B: Outer Diameter/mm   C: Internal Diameter/mm  and Concave/mm  A: Center Distance of Convex No. B: Outer Diameter/mm C: Internal Diameter/mm 1  and Concave/mm 25  50  17  2  29  50  17  1 25 50 17 2 29 50 17 The purpose of the experiment is to verify whether the smart washer can effectively monitor the  bolt looseness. The experimental scheme is shown in Figure 5 and the experimental setup is shown  Appl. Sci. 2016, 6, 320 6 of 10 Appl. Sci. 2016, 6, 320  6 of 10  The purpose of the experiment is to verify whether the smart washer can effectively monitor Appl. Sci. 2016, 6, 320  6 of 10  the bolt looseness. The experimental scheme is shown in Figure 5 and the experimental setup is shown in Figure 6 and an electronic universal testing machine (type CMT5105, SUNS, Shenzhen, China) was  in Figure 6 and an electronic universal testing machine (type CMT5105, SUNS, Shenzhen, China) in Figure 6 and an electronic universal testing machine (type CMT5105, SUNS, Shenzhen, China) was  used to precisely simulate the bolt preload applied on the washer. In the experiment, the loading  was used to precisely simulate the bolt preload applied on the washer. In the experiment, the loading used to precisely simulate the bolt preload applied on the washer. In the experiment, the loading  force  ranged  from  0  to  60,000  N  to  meet  the  rated  load  of  grade  8.8  of  M16  bolt.  A  National  force ranged from 0 to 60,000 N to meet the rated load of grade 8.8 of M16 bolt. A National Instruments force  ranged  from  0  to  60,000  N  to  meet  the  rated  load  of  grade  8.8  of  M16  bolt.  A  National  Instruments multifunction DAQ device, USB‐6361(National Instruments Corporation, Austin, TX,  multifunction DAQ device, USB-6361(National Instruments Corporation, Austin, TX, USA), Instruments multifunction DAQ device, USB‐6361(National Instruments Corporation, Austin, TX,  USA),  was  used in  the  experiment  as  the data acquisition apparatus  which  converted  between  a  was used in the experiment as the data acquisition apparatus which converted between a digital USA),  was  used in  the  experiment  as  the data acquisition apparatus  which  converted  between  a  digital signal and analog signal. For a given bolt load on a washer, a pulse excitation signal was  signal and analog signal. For a given bolt load on a washer, a pulse excitation signal was launched by digital signal and analog signal. For a given bolt load on a washer, a pulse excitation signal was  launched by the LabVIEW in the computer as shown in Figure 7a. The center frequency of the pulse  the LabVIEW in the computer as shown in Figure 7a. The center frequency of the pulse signal was launched by the LabVIEW in the computer as shown in Figure 7a. The center frequency of the pulse  signal was 150 kHz and the amplitude was 10 V. Then, the excitation signal was converted to an  150 kHz and the amplitude was 10 V. Then, the excitation signal was converted to an analog signal signal was 150 kHz and the amplitude was 10 V. Then, the excitation signal was converted to an  analog signal and is sent to excite the PZT1 by the NI USB‐6361. The PZT1 generates an ultrasonic  and is sent to excite the PZT1 by the NI USB-6361. The PZT1 generates an ultrasonic wave under the analog signal and is sent to excite the PZT1 by the NI USB‐6361. The PZT1 generates an ultrasonic  wave under the excitation of the pulse signal as shown in Figure 7a, then the sensor PZT (PZT2)  excitation of the pulse signal as shown in Figure 7a, then the sensor PZT (PZT2) detected the response wave under the excitation of the pulse signal as shown in Figure 7a, then the sensor PZT (PZT2)  detected  the  response  signal  which  was  amplified  by  a  signal  amplifier  (MISTRAS  2/4/6‐C,  signal which was amplified by a signal amplifier (MISTRAS 2/4/6-C, PHYSICAL ACOUSTICS detected  the  response  signal  which  was  amplified  by  a  signal  amplifier  (MISTRAS  2/4/6‐C,  PHYSICAL ACOUSTICS CORPORATION, Princeton, NJ, USA), as shown in Figure 7b. Figure 7c  CORPORATION, Princeton, NJ, USA), as shown in Figure 7b. Figure 7c shows that the recorded PHYSICAL ACOUSTICS CORPORATION, Princeton, NJ, USA), as shown in Figure 7b. Figure 7c  shows that the recorded response signal was reversed in the time domain which reemitted by the  response signal was reversed in the time domain which reemitted by the PZT1. The focused signal, shows that the recorded response signal was reversed in the time domain which reemitted by the  PZT1. The focused signal, as shown in Figure 7d, is detected by PZT2 and was stored in the computer  as shown in Figure 7d, is detected by PZT2 and was stored in the computer for further analysis. PZT1. The focused signal, as shown in Figure 7d, is detected by PZT2 and was stored in the computer  for further analysis. The procedure was repeated under different bolt preloads to obtain the focused  The procedure was repeated under different bolt preloads to obtain the focused signals at different for further analysis. The procedure was repeated under different bolt preloads to obtain the focused  signals at different bolt preloads, and the relationship between the focused signal peak amplitudes  bolt preloads, and the relationship between the focused signal peak amplitudes and the bolt preloads signals at different bolt preloads, and the relationship between the focused signal peak amplitudes  and the bolt preloads was then derived.   was then derived. and the bolt preloads was then derived.   Figure 5. Experimental scheme.  Figure 5. Experimental scheme. Figure 5. Experimental scheme.  (a)  (b) (a)  (b) Figure 6. The experimental setup: (a) Smart washers on the test fixture; (b) Entire setup.  Figure 6. The experimental setup: (a) Smart washers on the test fixture; (b) Entire setup.  Figure 6. The experimental setup: (a) Smart washers on the test fixture; (b) Entire setup. Appl. Sci. 2016, 6, 320 7 of 10 Appl. Sci. 2016, 6, 320  7 of 10  (a)  (b) (c)  (d) Figure 7. Experimental signals. (a) Pulse signal; (b) Response signal; (c) Time reversed signal; (d)  Figure 7. Experimental signals. (a) Pulse signal; (b) Response signal; (c) Time reversed signal; Focused signal.  (d) Focused signal. 4. Experiment Results and Discussion   4. Experiment Results and Discussion The experiments were carried out based on the above‐mentioned procedure and the focused  The experiments were carried out based on the above-mentioned procedure and the focused signals of two specimens under the same conditions were obtained. The relationship between the bolt  signals of two specimens under the same conditions were obtained. The relationship between the preload and the peak of the focused signal extracted from the experimental data are shown in Figures  bolt preload and the peak of the focused signal extracted from the experimental data are shown in 8 and 9. According to the relevant theory of bolt connection, only a very small part of the external  Figures 8 and 9. According to the relevant theory of bolt connection, only a very small part of the load, which is applied on the bolt joint structure, works on the bolt when the bolt preload is enough,  external load, which is applied on the bolt joint structure, works on the bolt when the bolt preload is but  once  the  bolt  preload  is  insufficient,  the  external  load  that  works  on  the  bolt  will  be  greatly  enough, increased but and once will the most bolt lipr keeload ly lead is to insuf  the ficient, failure of the the external  bolt. For load  thethat  bolt works joint struct on the ure, bolt  it iswill  import be gr ant eatly   to ensure that the bolt preload is at a reasonable level. Therefore, for the bolt preload monitoring, the  increased and will most likely lead to the failure of the bolt. For the bolt joint structure, it is important prominent task is to monitor the axial force changes around its rated preload. Thus, it is unnecessary  to ensure that the bolt preload is at a reasonable level. Therefore, for the bolt preload monitoring, to monitor the force which is much smaller than the rated preload, and for this reason, the starting  the prominent task is to monitor the axial force changes around its rated preload. Thus, it is unnecessary value of the abscissa is 20 KN in the experiments.  to monitor the force which is much smaller than the rated preload, and for this reason, the starting Figure 8 shows the experimental result of the first specimen with its center distance of the convex  value of the abscissa is 20 KN in the experiments. and concave being 25 mm from the washer center. It can be found that, with the increase of the bolt  Figure 8 shows the experimental result of the first specimen with its center distance of the convex preload, the focused signal peak increases approximately linearly. This agrees with the analysis in  and concave being 25 mm from the washer center. It can be found that, with the increase of the bolt Section 2 which shows that the peak amplitude increases with the increase of the axial force. The  preload, the focused signal peak increases approximately linearly. This agrees with the analysis in slope of the curve is steep enough to interrogate the change of the bolt axial force. Even at the 60 KN  Section 2 which shows that the peak amplitude increases with the increase of the axial force. The preload, the focused signal peak still kept increasing, thus the designed smart washer can monitor  slope of the curve is steep enough to interrogate the change of the bolt axial force. Even at the 60 KN the  full  range  of  the  M16  bolt.  The  effectiveness  of  the  proposed  design  is  validated  by  this  preload, the focused signal peak still kept increasing, thus the designed smart washer can monitor the experiment.  full range of the M16 bolt. The effectiveness of the proposed design is validated by this experiment. Figure 9 shows the result of specimen 2 (the center distance is 29 mm); the focused signal peak  Figure 9 shows the result of specimen 2 (the center distance is 29 mm); the focused signal peak amplitude  keeps  increasing  and  does  not  reach  saturation.  Nevertheless,  the  curve  has  gently  amplitude keeps increasing and does not reach saturation. Nevertheless, the curve has gently changed changed between 30 and 40 kN, which may be caused by the machining errors of the specimen.  between 30 and 40 kN, which may be caused by the machining errors of the specimen. Appl. Sci. 2016, 6, 320 8 of 10 Appl. Sci. 2016, 6, 320  8 of 10  Appl. Sci. 2016, 6, 320  8 of 10  The experimental results show that the amplitude of the focused signal can indicate the bolt  The experimental results show that the amplitude of the focused signal can indicate the bolt The experimental results show that the amplitude of the focused signal can indicate the bolt  preload obviously, that is to say that the smart washers, based on the piezoelectric active sensing  pr preload eload obviously obviously, , that that isis to to say say that that the the smart smart washers, washers, based based on on the the piezoelectric piezoelectric active active sensing sensing  method and time reversal technique, can effectively monitor the bolt looseness in the related preload  method and time reversal technique, can effectively monitor the bolt looseness in the related preload method and time reversal technique, can effectively monitor the bolt looseness in the related preload  range. By comparing the experimental results of the two specimens, it is shown that a small change  range. range. By By compari comparing ng the the experimental experimental rresults esults of of the the two two specimens, specimens, it it is is shown shown that that aa small small change change  in the contact location will not obviously affect the contact area change under the applied preload  in the contact location will not obviously affect the contact area change under the applied preload in the contact location will not obviously affect the contact area change under the applied preload  range and thus will not affect the transmitted wave energy change. For the M16 bolt, the designed  range range and and thus thus will will not not af aff fect ect the the ttra ransmitted nsmitted wave wave ener energy gy change. change. For For the the M16 M16 bolt, bolt, the the designed designed  smart washer is sufficient to measure 20 kN~60 kN axial force which covers the rated preload range  smart smart washer washer isissuf  sufi ffic cient ientto tomeasur  measure e 20 20 kN~60  kN~60 kN  kN axial  axiafor l force ce which  whiccovers h covers the the rated  rated pr eload preloa range d range of  of the bolt, and the change of amplitude with obvious axial force. Hence, for the different types of  the of the bolt, bolt, and and the the change  chanof geamplitude  of amplitud with e with obvious  obvio axial us axi foral ce. force Hence, . Hence for,the  fordif  thfer e di ent ffetypes rent types of bolt,  of  bolt, by designing a washer that is different in size, the entire rated preload measurement of the bolt  by bolt, designing  by designing a washer  a washer that is tha diftfer  is ent differ in ent size, in the  size entir , the e entire rated pr raeload ted preload measur measurement ement of the bolt  of the can bolt be  can be fulfilled successfully.  fulfilled can be fu successfully lfilled success . fully.  Figure 8. The peak amplitude of the focused signal under different preloads of the first specimen.  Figure 8. The peak amplitude of the focused signal under different preloads of the first specimen. Figure 8. The peak amplitude of the focused signal under different preloads of the first specimen.  Figure 9. The peak amplitude of the focused signal under different preloads of the second specimen.  Figure 9. The peak amplitude of the focused signal under different preloads of the second specimen.  Figure 9. The peak amplitude of the focused signal under different preloads of the second specimen. 5. Conclusions  5. Conclusions  5. Conclusions As  previously  stated,  a  smart  washer  based  on  the  piezoelectric  active  sensing  method  was  As  previously  stated,  a  smart  washer  based  on  the  piezoelectric  active  sensing  method  was  As previously stated, a smart washer based on the piezoelectric active sensing method was proposed for bolt looseness monitoring. The applicability and effectiveness of the smart washer has  proposed for bolt looseness monitoring. The applicability and effectiveness of the smart washer has  proposed for bolt looseness monitoring. The applicability and effectiveness of the smart washer has been presented by experimental studies. Based on the theory that the change of the transmitted wave  been presented by experimental studies. Based on the theory that the change of the transmitted wave  been presented by experimental studies. Based on the theory that the change of the transmitted wave energy, through a bolted joint interface, depends on the bolt preload force, the bolt loosening status  energy, through a bolted joint interface, depends on the bolt preload force, the bolt loosening status  energy, through a bolted joint interface, depends on the bolt preload force, the bolt loosening status can be deduced by observing the change of the peak amplitude of focused signals. The experiment  can be deduced by observing the change of the peak amplitude of focused signals. The experiment  can be deduced by observing the change of the peak amplitude of focused signals. The experiment results show that the focused signal peak amplitude has an approximate linear correlation with the  results show that the focused signal peak amplitude has an approximate linear correlation with the  results show that the focused signal peak amplitude has an approximate linear correlation with the bolt preload.  bolt preload.  bolt preload. Further researches are needed to design the different curvature radii of the convex and concave  Further researches are needed to design the different curvature radii of the convex and concave  surfaces and to determine the sizes of the smart washer, in order to form a series of smart washers  surfaces and to determine the sizes of the smart washer, in order to form a series of smart washers  Appl. Sci. 2016, 6, 320 9 of 10 Further researches are needed to design the different curvature radii of the convex and concave surfaces and to determine the sizes of the smart washer, in order to form a series of smart washers that can measure the full range of the rated preload of different types of bolts. Lastly, through theoretical analysis and numerical simulation, the smart washer structure can be optimized. Acknowledgments: This research was supported by the National Natural Science Foundation of China under Grants No. 51375354 and No. 51278084, the Natural Science Foundation of Hubei Province under Grants No. 2015CFB306, and the Research Project of Hubei Education under Grants No. Q20131104 and No. Q20151103. Author Contributions: Heyue Yin, Tao Wang, Dan Yang and Yourong Li conceived and designed the experiments; Heyue Yin, Shaopeng Liu and Junhua Shao performed the experiments; Heyue Yin, Tao Wang and Dan Yang analyzed the data; Tao Wang and Junhua Shao contributed reagents/materials/analysis tools; Heyue Yin, Tao Wang, Dan Yang, Shaopeng Liu, Junhua Shao and Yourong Li wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. References 1. Wang, T.; Song, G.; Liu, S.; Li, Y.; Xiao, H. Review of bolted connection monitoring. Int. J. Distrib. Sens. Netw. 2013. [CrossRef] 2. Cawley, P.; Adams, R.D. The mechanics of the coin-tap method of non-destructive testing. J. Sound Vib. 1988, 122, 299–316. [CrossRef] 3. Cawley, P.; Adams, R.D. Sensitivity of the coin-tap method of nondestructive testing. Mater. Eval. 1989, 47, 558–563. 4. Xiang, Z.; Dai, X.; Zhang, Y.; Lu, Q. The tap-scan method for damage detection of bridge structures. Interact. 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A Smart Washer for Bolt Looseness Monitoring Based on Piezoelectric Active Sensing Method

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applied sciences Article A Smart Washer for Bolt Looseness Monitoring Based on Piezoelectric Active Sensing Method Heyue Yin, Tao Wang, Dan Yang *, Shaopeng Liu, Junhua Shao and Yourong Li Key Laboratory of Metallurgical Equipment and Control of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China; yinheyue123@163.com (H.Y.); wangtao77@wust.edu.cn (T.W.); lspmgl@126.com (S.L.); shaojunhua@wust.edu.cn (J.S.); liyourong@wust.edu.cn (Y.L.) * Correspondence: yangdan@wust.edu.cn; Tel.: +86-27-6886-2292 Academic Editor: Gangbing Song Received: 21 September 2016; Accepted: 20 October 2016; Published: 26 October 2016 Abstract: Piezoceramic based active sensing methods have been researched to monitor preload on bolt connections. However, there is a saturation problem involved with this type of method. The transmitted energy is sometimes saturated before the maximum preload which is due to it coming into contact with flat surfaces. When it comes to flat contact surfaces, the true contact area will easily saturate with the preload. The design of a new type of bolt looseness monitoring sensor, a smart washer, is to mitigate the saturation problem. The smart washer is composed of two annular disks with contact surfaces that are machined into convex and concave respectively, to eliminate the complete flat contact surfaces and to reduce the saturation effect. One piezoelectric patch is bonded on the non-contact surface of each annular disk. These two mating annular disks form a smart washer. One of the two piezoelectric patches serves as an actuator to generate an ultrasonic wave that propagates through the contact surface; the other one serves as a sensor to detect the propagated waves. The wave energy propagated through the contact surface is proportional to the true contact area which is determined by the bolt preload. The time reversal method is used to extract the peak of the focused signal as the index of the transmission wave energy; then, the relationship between the signal peak and bolt preload is obtained. Experimental results show that the focused signal peak value changes with the bolt preload and presents an approximate linear relationship when the saturation problem is experienced. The proposed smart washer can monitor the full range of the rated preload. Keywords: bolt loosen; active sensing; smart washer; non-destructive monitoring 1. Introduction Bolt connections are widely used in many industrial fields. Bolt connections are influenced by cyclic loading and various working conditions, and the bolt connections tend to loosen. In practice, accidents have been caused by bolt looseness, and monitoring of the bolt preload force has become important in the engineering field to ensure the safety of the structures. With the development of structural health monitoring (SHM), more theories and technologies are being developed regarding bolt health monitoring and bolt looseness detection [1]. Coin-tap is commonly used in engineering practices, including in the detection of bolt looseness. According to the change of the sound during an impact, engineers estimate whether the bolt is loosened or not. Based on this, Cawley and Adams [2,3] proposed a theory that structural imperfection causes the changes of the response signals, thus the structural damage can be identified according to the width of the time domain curves or the area ratio of the frequency domain. After a series of research, Xiang et al. [4,5] proposed a controlled tap detection method for bolt tightness based on a controlled Appl. Sci. 2016, 6, 320; doi:10.3390/app6110320 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 320 2 of 10 tap detection principle, and the bolt loosening status can be determined quantitatively by using the support vector machine method. However, the tap method cannot provide real time monitoring of bolt looseness. Piezoceramics with such attractive features as low cost, small size, wide bandwidth, and sensing and actuating functions have been widely researched in real-time active vibration control [6–8], stress-wave based communication [9–11], and structural health monitoring [12–16]. Another attractive feature of a piezoceramic transducer is the coupling between its electric impedance and the host structure’s mechanical impedance. The piezoelectric impedance method was proposed to monitor structure health status in 1994 by Liang et al. [17]. The analytical relationship between the piezoelectric impedance and the mechanical impedance of the host structure was established for the first time. Sopon et al. [18] used a piezoelectric transducer (PZT) as an actuator–sensor collectively in conjunction with the numerical model-based spectral element method (SEM) in structural health monitoring to quantitatively detect damage of bolted joints. By analyzing the characteristics of the piezoelectric element under the bolt looseness condition, Gao et al. [19] concluded that the conductance of the piezoelectric patches changes significantly under the high-frequency (more than 50 kHz) voltage excitation. Mascarenas et al. [20] enhanced the PZT to form a washer for monitoring individual bolt looseness by examining the electro-mechanical impedance (EMI) at resonance. Nguyem et al. [21] used multi-channel wireless impedance sensor nodes and multiple PZT-interfaces to monitor bolt loosening in bolted connection. The ultrasonic method is employed in some SHM fields. As a valuable way to monitor bolt looseness, the ultrasonic method is continually improved by many researchers [22]. An ultrasonic instrument for the accurate measurement of bolt stress was described by Joshi in 1984 [23]. Sisman et al. [24] used the ultrasonic method to quantitatively detect the preload between the bolt and the connect surface. Yasui et al. [25] proposed an ultrasonic velocity ratio method for measuring bolt axial load. Jhang et al. [26] used the phase detection technique to measure the TOF (time of flight) of a tone-burst ultrasonic wave; the method improved the precision of the ultrasonic velocity method for measuring bolt stress. The propagation velocity of the ultrasonic method is affected by bolt material, stress and some other factors, and it is not easy to accurately determine the axial force of the bolt. Additionally, it requires a special precision instrument for the measurement, and it is difficult to apply to real time application. The active sensing method utilizes the sensing and actuation capability of the piezoelectric transducers. For this method, at least one pair of the PZT patches is used with one serving as an actuator to generate a stress wave and the other serving as a sensor to detect the stress wave. A damage or change along the wave propagating path will change the detect signal. As a real time method, many researchers have explored this method [27]. Mita and Fujimoto [28] combined the ultrasonic waves and support vector machines method to detect loosened bolts. The experiments show that the proposed methods could identify the location and the level of damage of the loosened bolts. The active sensing-based method was also used in detecting damage of the bolt on a steel bridge [29], and some researchers applied ultrasonic and magneto-elastic active sensors in monitoring bolted joints in a satellite panel [30]. In our team’s previous studies [31,32], the piezoelectric active sensing method with the time reversal technique [33] was used to monitor the bolt looseness. The time reversal method reverses the response signal that is received by the sensor in its time domain. This method can make the signal energy focus on space and time. In these studies, the peak of the time reversal focused signal can represent the wave energy that transmitted through the interface of the bolted joint, and the wave energy is proportional to the true contact area of the bolted joint interface which depends on the bolt tightening force. The experiments show that the focused signal peak amplitudes increase with the increasing blot preload until reaching saturation and the focused signal peak remains stable. When there is no more deformation space at the bolted joint interface under a certain bolt preload, the true contact area does not change any more. Besides, the surface roughness of the bolted joint affects Appl. Sci. 2016, 6, 320 3 of 10 the saturation value; the higher roughness can postpone the saturation value of the bolt preload. When it comes to bolt connection surfaces, the contact roughness is determined by the process and working condition, which cannot be increased artificially. Even if a high roughness is used, it is hard to Appl. Sci. 2016, 6, 320  3 of 10  guarantee the higher roughness precisely for a certain machining process. Therefore, in order to resolve the problem of saturation, a new monitoring structure needs to be developed. The new proposition guarantee  the  higher  roughness  precisely  for  a  certain  machining  process.  Therefore,  in  order  to  is that of a new washer structure for monitoring bolted looseness status based on the piezoelectric resolve  the  problem  of  saturation,  a  new  monitoring  structure  needs  to  be  developed.  The  new  activeproposition sensing method  is that and of a new to deal  wash with er structure the saturation  for monitoring problem.  bolted The looseness new washer  status design  based on will the contr   ol piezoelectric active sensing method and to deal with the saturation problem. The new washer design  the contact area artificially so that the washer can monitor the full range of the rated preload. will control the contact area artificially so that the washer can monitor the full range of the rated  2. Proposed preload.Method   and Principle The new smart washer structure is shown in Figure 1. It is composed of two annular disks, 2. Proposed Method and Principle  the contact surfaces of which are machined into convex and concave, respectively. Two piezoelectric The new smart washer structure is shown in Figure 1. It is composed of two annular disks, the  patches are bonded onto two non-contact surfaces of the proposed smart washer sensor. One contact surfaces of which are machined into convex and concave, respectively. Two piezoelectric  piezoelectric patch serves as an actuator to generate an ultrasonic wave transmitting through the patches  are  bonded  onto  two  non‐contact  surfaces  of  the  proposed  smart  washer  sensor.  One  contact surface, and the other one as a sensor to detect the response signals. Furthermore, the time piezoelectric patch serves as an actuator to generate an ultrasonic wave transmitting through the  reversal contact method  surface, is employed,  and the other which  onecan  as aensur  sensor e atohigh  detect signal  the response to noise signals. ratio. The Furthermore, first response  the time signal   is reversal method is employed, which can ensure a high signal to noise ratio. The first response signal  reversed in the time domain and then reemitted. Then, the reverse focused signal is detected by the is reversed in the time domain and then reemitted. Then, the reverse focused signal is detected by the  sensor piezoelectric patch. In addition, based on the fact that the waves are the energy transmission sensor piezoelectric patch. In addition, based on the fact that the waves are the energy transmission  and the transmitted energy is proportional to the true contact area of the bolted joint interface, and the transmitted energy is proportional to the true contact area of the bolted joint interface, the  the bolted loosening status can be determined by extracting the focused signal peak that has a certain bolted loosening status can be determined by extracting the focused signal peak that has a certain  relationship with the transmitted energy. relationship with the transmitted energy.  Figure 1. Structure sketch.  Figure 1. Structure sketch. In order to describe the relationship between the contact area and loading‐force, an analysis of  In order to describe the relationship between the contact area and loading-force, an analysis of the the  contact  is  necessary.  According  to  the  Classical  Hertz  contact  theory,  two  elasticaxis‐parallel  cylinders’  internal  contact  is  shown  in  Figure  2;  the  contact  surface  shape  of  the  cylinders  is  contact is necessary. According to the Classical Hertz contact theory, two elasticaxis-parallel cylinders’ rectangular [34]. Furthermore, the unit’s pressure on the contact surface is distributed in an elliptic  internal contact is shown in Figure 2; the contact surface shape of the cylinders is rectangular [34]. cylindrical pattern as shown in Figure 3. Hertz theory relates loading force F and contact area S as  Furthermore, the unit’s pressure on the contact surface is distributed in an elliptic cylindrical pattern follows:  as shown in Figure 3. Hertz theory relates loading force F and contact area S as follows: 4FR a= (1)  4FR πLE a = (1) LE R R 1 2 R  (2)  R  R 2 1 R R R = (2) R R 1 1-- μ 1 μ 2 1 = + (3)  E 1 2 2 1 1  1 1 2 = + (3) E E E 4FLR F 2LR Sa =2 L=2 = 4   (4)  r r * * ππ EE 4FLR FLR S = 2aL = 2 = 4 (4) where a is half the width of the rectangle, L is the length of cylinders, E1 and E2 are the elasticity  E E modulus  of  the  two  cylinders,  R1  and  R2  are  the  curvatures  of  the  cylinders,  and μ1  and μ2  are  Poisson’s ratios.   Appl. Sci. 2016, 6, 320 4 of 10 where a is half the width of the rectangle, L is the length of cylinders, E and E are the elasticity 1 2 modulus of the two cylinders, R and R are the curvatures of the cylinders, and  and  are 1 2 1 2 Poisson’s ratios. Appl. Sci. 2016, 6, 320  4 of 10  For the smart washer in this paper, the contact area is an annulus; the existing contact formula is Appl. Sci. 2016, 6, 320  4 of 10  not applicable to this model; after the differential, the annulus can be regarded as being composed of For the smart washer in this paper, the contact area is an annulus; the existing contact formula  For the smart washer in this paper, the contact area is an annulus; the existing contact formula  numerous small rectangles, made up of countless small rectangles; the area of each rectangle is dS; is not applicable to this model; after the differential, the annulus can be regarded as being composed  is not applicable to this model; after the differential, the annulus can be regarded as being composed  and the contact area, S, can be obtained by the equation as follows: of numerous small rectangles, made up of countless small rectangles; the area of each rectangle is dS;  of numerous small rectangles, made up of countless small rectangles; the area of each rectangle is dS;  and the contact area, S, can be obtained by the equation as follows:  and the contact area, S, can be obtained by the equation as follows:  dS = 2adL (5) d=Sa2 dL  (5)  d=Sa2 dL  (5)  Z Z 2r 2r π⋅ 2FrR 2FrR 2r π⋅ S =SS =d dS == 2ad 2La= d4L = 4   (6)  (6) 2FrR òò * SS =d =0 2adL=4   (6)  E E òò * where r is the distance between the center of the washer and the center of the convex and concave, where r is the distance between the center of the washer and the center of the convex and concave, as  where r is the distance between the center of the washer and the center of the convex and concave, as  shown in Figure 1, and 2πr is the perimeter of the annulus center.   as shown in Figure 1, and 2r is the perimeter of the annulus center. shown in Figure 1, and 2πr is the perimeter of the annulus center.   Figure 2. Axis‐parallel cylinders’ internal contact.   Figure 2. Axis-parallel cylinders’ internal contact. Figure 2. Axis‐parallel cylinders’ internal contact.   Figure 3. Contact surface status.  Figure 3. Contact surface status.  Figure 3. Contact surface status. It can be seen that the contact area is decided by the acting force when r and R are given, more  It can be seen that the contact area is decided by the acting force when r and R are given, more  specifically, the area will grow with an increasing load. Therefore, the contact area can be an indicator  It can be seen that the contact area is decided by the acting force when r and R are given, more specifically, the area will grow with an increasing load. Therefore, the contact area can be an indicator  of the loading force.   specifically, the area will grow with an increasing load. Therefore, the contact area can be an indicator of the loading force.   The essence of wave propagation is energy transmission, and the wave leakage and dissipation  of the loading The essenc force.e of wave propagation is energy transmission, and the wave leakage and dissipation  occur at the contact interface. Thus, the energy will be reduced when the wave transmits through the  occur at the contact interface. Thus, the energy will be reduced when the wave transmits through the  The essence of wave propagation is energy transmission, and the wave leakage and dissipation interface. For the washer, the energy received by the PZT1 and PZT2 is the transmitted wave energy  interface. For the washer, the energy received by the PZT1 and PZT2 is the transmitted wave energy  occur at the contact interface. Thus, the energy will be reduced when the wave transmits through the that is transmitted through the convex and concave contact interfaces. It is found that the transmitted  that is transmitted through the convex and concave contact interfaces. It is found that the transmitted  interface. For the washer, the energy received by the PZT1 and PZT2 is the transmitted wave energy energy, E, is proportional to the true contact area, S [35], as follows:  energy, E, is proportional to the true contact area, S [35], as follows:  that is transmitted through the convex and concave contact interfaces. It is found that the transmitted E C S   (7)  E C S (7)  energy, E, is proportional to the true contact area, S [1 35], as follows: The relationship implies that the energy through the contact area increases with the enlargement  The relationship implies that the energy through the contact area increases with the enlargement  of the contact area, when the force increases according to Equation (6). As a result, more energy is  E µ C S (7) of the contact area, when the force increases according to Equation (6). As a result, more energy is  transmitted to the sensor. After considering the relationship in Equation (6), the transmitted energy,  transmitted to the sensor. After considering the relationship in Equation (6), the transmitted energy,  E, can be expressed as follows:  The relationship implies that the energy through the contact area increases with the enlargement E, can be expressed as follows:  of the contact area, when the force increases according to Equation (6). As a result, more energy is Appl. Sci. 2016, 6, 320 5 of 10 transmitted to the sensor. After considering the relationship in Equation (6), the transmitted energy, E, can be expressed as follows: E µ C F (8) Appl. Sci. 2016, 6, 320  5 of 10  In Equations (7) and (8), C and C are constants. It can be seen from Equation (8) that the received 1 2 (8)  E C F   energy is proportional to the square root of the loading force, which is the bolt axial force. Thus, the bolt looseness can be represented by the energy of wave. In Equations (7) and (8), C1 and C2 are constants. It can be seen from Equation (8) that the received  The time reversal method is adopted in this study. Based on the time reversal theory, the energy is proportional to the square root of the loading force, which is the bolt axial force. Thus, the  response signal energy is proportional to the peak amplitude of the focused signal [32], whereupon the bolt looseness can be represented by the energy of wave.   transmitted signal energy can be represented by the focused signal peak amplitude. A higher focused The  time  reversal  method  is  adopted  in  this  study.  Based  on  the  time  reversal  theory,  the  signal response peak amplitude  signal energ means y is praoportional larger transmitted  to the peaksignal  amplitener ude gy of th and e foc au lar sed ger sig bolt nal [32 preload. ], whereupon  the transmitted signal energy can be represented by the focused signal peak amplitude. A higher  3. The Smart Washer and Experimental Procedure focused signal peak amplitude means a larger transmitted signal energy and a larger bolt preload.  The design derives from the asperities of the machined surface. The microcosmic asperities 3. The Smart Washer and Experimental Procedure   are magnified to macroscopic convex and concave. Considering the Hertz contact stress theory The design derives from the asperities of the machined surface. The microcosmic asperities are  and by optimizing the data, the curvatures of the convex and concave are decided. The curvatures magnified to macroscopic convex and concave. Considering the Hertz contact stress theory and by  of the convex and concave are similar sizes to ensure that slippage does not occur between them. optimizing the data, the curvatures of the convex and concave are decided. The curvatures of the  There is a groove on the non-contact surface of the upper-washer and lower-washer, respectively, convex and concave are similar sizes to ensure that slippage does not occur between them. There is a  to place the piezoelectric patch (PZT) that served as an actuator and senor. A certain space between the groove on the non‐contact surface of the upper‐washer and lower‐washer, respectively, to place the  upper-washer and lower-washer is needed to avoid the additional surface contact, except the contact piezoelectric patch (PZT) that served as an actuator and senor. A certain space between the upper‐ of the convex and concave. The purpose of the washer design is to control the contact area artificially washer and lower‐washer is needed to avoid the additional surface contact, except the contact of the  so that convex the washer  and concan cave monitor . The purpose the full  of range the wa of shthe er de rated sign is pr to eload.  control From  the Equation contact are (6), a arti theficia contact lly so area will change that the with washdif er can ferent  monitor curvatur  the es full of range the convex  of the rat and ed concave, preload. Fr orom with  Equation the dif (6 fer ), ent the center contactdistances  area  will change with different curvatures of the convex and concave, or with the different center distances  under the same preload. In order to explore the influence of the central symmetry plane location of the under the same preload. In order to explore the influence of the central symmetry plane location of  convex and concave, two kinds of specimens are designed in the experiment. the convex and concave, two kinds of specimens are designed in the experiment.  The size of the smart washer is designed based on the standard bolt M16, as shown in Figure 4. The size of the smart washer is designed based on the standard bolt M16, as shown in Figure 4.  The outer diameter of the washer is 50 mm and the internal diameter is 17 mm for the M16 bolt The outer diameter of the washer is 50 mm and the internal diameter is 17 mm for the M16 bolt  connection. The curvature of the convex R and the concave R are 6 mm and 10 mm, respectively. 1 2 connection. The curvature of the convex R1 and the concave R2 are 6 mm and 10 mm, respectively.  The detailed sizes of all specimens are listed in Table 1. The above and bottom straight grooves, where The detailed sizes of all specimens are listed in Table 1. The above and bottom straight grooves, where  the piezoelectric patches are placed, are filled with insulating glue to protect the piezoelectric patches. the piezoelectric patches are placed, are filled with insulating glue to protect the piezoelectric patches.  12 12 The piezoelectric constant d and d of the patch surface is 400  10−12 CN and 150 −12 10 CN The  piezoelectric  constant 33  d33 and31  d31  of  the  patch surface is  400  ×  10   CN and −150  ×  10   CN  respectively respectively, , and and the the dimensions  dimensions of of the the piezoelectric  piezoelectric patch patch ar are e 88 mm mm ×  9 mm 9 mm  × 1 mm. 1 mm.    Figure 4. Specimen structure.   Figure 4. Specimen structure. Table 1. Specimen size.  Table 1. Specimen size. A: Center Distance of Convex  No.  B: Outer Diameter/mm   C: Internal Diameter/mm  and Concave/mm  A: Center Distance of Convex No. B: Outer Diameter/mm C: Internal Diameter/mm 1  and Concave/mm 25  50  17  2  29  50  17  1 25 50 17 2 29 50 17 The purpose of the experiment is to verify whether the smart washer can effectively monitor the  bolt looseness. The experimental scheme is shown in Figure 5 and the experimental setup is shown  Appl. Sci. 2016, 6, 320 6 of 10 Appl. Sci. 2016, 6, 320  6 of 10  The purpose of the experiment is to verify whether the smart washer can effectively monitor Appl. Sci. 2016, 6, 320  6 of 10  the bolt looseness. The experimental scheme is shown in Figure 5 and the experimental setup is shown in Figure 6 and an electronic universal testing machine (type CMT5105, SUNS, Shenzhen, China) was  in Figure 6 and an electronic universal testing machine (type CMT5105, SUNS, Shenzhen, China) in Figure 6 and an electronic universal testing machine (type CMT5105, SUNS, Shenzhen, China) was  used to precisely simulate the bolt preload applied on the washer. In the experiment, the loading  was used to precisely simulate the bolt preload applied on the washer. In the experiment, the loading used to precisely simulate the bolt preload applied on the washer. In the experiment, the loading  force  ranged  from  0  to  60,000  N  to  meet  the  rated  load  of  grade  8.8  of  M16  bolt.  A  National  force ranged from 0 to 60,000 N to meet the rated load of grade 8.8 of M16 bolt. A National Instruments force  ranged  from  0  to  60,000  N  to  meet  the  rated  load  of  grade  8.8  of  M16  bolt.  A  National  Instruments multifunction DAQ device, USB‐6361(National Instruments Corporation, Austin, TX,  multifunction DAQ device, USB-6361(National Instruments Corporation, Austin, TX, USA), Instruments multifunction DAQ device, USB‐6361(National Instruments Corporation, Austin, TX,  USA),  was  used in  the  experiment  as  the data acquisition apparatus  which  converted  between  a  was used in the experiment as the data acquisition apparatus which converted between a digital USA),  was  used in  the  experiment  as  the data acquisition apparatus  which  converted  between  a  digital signal and analog signal. For a given bolt load on a washer, a pulse excitation signal was  signal and analog signal. For a given bolt load on a washer, a pulse excitation signal was launched by digital signal and analog signal. For a given bolt load on a washer, a pulse excitation signal was  launched by the LabVIEW in the computer as shown in Figure 7a. The center frequency of the pulse  the LabVIEW in the computer as shown in Figure 7a. The center frequency of the pulse signal was launched by the LabVIEW in the computer as shown in Figure 7a. The center frequency of the pulse  signal was 150 kHz and the amplitude was 10 V. Then, the excitation signal was converted to an  150 kHz and the amplitude was 10 V. Then, the excitation signal was converted to an analog signal signal was 150 kHz and the amplitude was 10 V. Then, the excitation signal was converted to an  analog signal and is sent to excite the PZT1 by the NI USB‐6361. The PZT1 generates an ultrasonic  and is sent to excite the PZT1 by the NI USB-6361. The PZT1 generates an ultrasonic wave under the analog signal and is sent to excite the PZT1 by the NI USB‐6361. The PZT1 generates an ultrasonic  wave under the excitation of the pulse signal as shown in Figure 7a, then the sensor PZT (PZT2)  excitation of the pulse signal as shown in Figure 7a, then the sensor PZT (PZT2) detected the response wave under the excitation of the pulse signal as shown in Figure 7a, then the sensor PZT (PZT2)  detected  the  response  signal  which  was  amplified  by  a  signal  amplifier  (MISTRAS  2/4/6‐C,  signal which was amplified by a signal amplifier (MISTRAS 2/4/6-C, PHYSICAL ACOUSTICS detected  the  response  signal  which  was  amplified  by  a  signal  amplifier  (MISTRAS  2/4/6‐C,  PHYSICAL ACOUSTICS CORPORATION, Princeton, NJ, USA), as shown in Figure 7b. Figure 7c  CORPORATION, Princeton, NJ, USA), as shown in Figure 7b. Figure 7c shows that the recorded PHYSICAL ACOUSTICS CORPORATION, Princeton, NJ, USA), as shown in Figure 7b. Figure 7c  shows that the recorded response signal was reversed in the time domain which reemitted by the  response signal was reversed in the time domain which reemitted by the PZT1. The focused signal, shows that the recorded response signal was reversed in the time domain which reemitted by the  PZT1. The focused signal, as shown in Figure 7d, is detected by PZT2 and was stored in the computer  as shown in Figure 7d, is detected by PZT2 and was stored in the computer for further analysis. PZT1. The focused signal, as shown in Figure 7d, is detected by PZT2 and was stored in the computer  for further analysis. The procedure was repeated under different bolt preloads to obtain the focused  The procedure was repeated under different bolt preloads to obtain the focused signals at different for further analysis. The procedure was repeated under different bolt preloads to obtain the focused  signals at different bolt preloads, and the relationship between the focused signal peak amplitudes  bolt preloads, and the relationship between the focused signal peak amplitudes and the bolt preloads signals at different bolt preloads, and the relationship between the focused signal peak amplitudes  and the bolt preloads was then derived.   was then derived. and the bolt preloads was then derived.   Figure 5. Experimental scheme.  Figure 5. Experimental scheme. Figure 5. Experimental scheme.  (a)  (b) (a)  (b) Figure 6. The experimental setup: (a) Smart washers on the test fixture; (b) Entire setup.  Figure 6. The experimental setup: (a) Smart washers on the test fixture; (b) Entire setup.  Figure 6. The experimental setup: (a) Smart washers on the test fixture; (b) Entire setup. Appl. Sci. 2016, 6, 320 7 of 10 Appl. Sci. 2016, 6, 320  7 of 10  (a)  (b) (c)  (d) Figure 7. Experimental signals. (a) Pulse signal; (b) Response signal; (c) Time reversed signal; (d)  Figure 7. Experimental signals. (a) Pulse signal; (b) Response signal; (c) Time reversed signal; Focused signal.  (d) Focused signal. 4. Experiment Results and Discussion   4. Experiment Results and Discussion The experiments were carried out based on the above‐mentioned procedure and the focused  The experiments were carried out based on the above-mentioned procedure and the focused signals of two specimens under the same conditions were obtained. The relationship between the bolt  signals of two specimens under the same conditions were obtained. The relationship between the preload and the peak of the focused signal extracted from the experimental data are shown in Figures  bolt preload and the peak of the focused signal extracted from the experimental data are shown in 8 and 9. According to the relevant theory of bolt connection, only a very small part of the external  Figures 8 and 9. According to the relevant theory of bolt connection, only a very small part of the load, which is applied on the bolt joint structure, works on the bolt when the bolt preload is enough,  external load, which is applied on the bolt joint structure, works on the bolt when the bolt preload is but  once  the  bolt  preload  is  insufficient,  the  external  load  that  works  on  the  bolt  will  be  greatly  enough, increased but and once will the most bolt lipr keeload ly lead is to insuf  the ficient, failure of the the external  bolt. For load  thethat  bolt works joint struct on the ure, bolt  it iswill  import be gr ant eatly   to ensure that the bolt preload is at a reasonable level. Therefore, for the bolt preload monitoring, the  increased and will most likely lead to the failure of the bolt. For the bolt joint structure, it is important prominent task is to monitor the axial force changes around its rated preload. Thus, it is unnecessary  to ensure that the bolt preload is at a reasonable level. Therefore, for the bolt preload monitoring, to monitor the force which is much smaller than the rated preload, and for this reason, the starting  the prominent task is to monitor the axial force changes around its rated preload. Thus, it is unnecessary value of the abscissa is 20 KN in the experiments.  to monitor the force which is much smaller than the rated preload, and for this reason, the starting Figure 8 shows the experimental result of the first specimen with its center distance of the convex  value of the abscissa is 20 KN in the experiments. and concave being 25 mm from the washer center. It can be found that, with the increase of the bolt  Figure 8 shows the experimental result of the first specimen with its center distance of the convex preload, the focused signal peak increases approximately linearly. This agrees with the analysis in  and concave being 25 mm from the washer center. It can be found that, with the increase of the bolt Section 2 which shows that the peak amplitude increases with the increase of the axial force. The  preload, the focused signal peak increases approximately linearly. This agrees with the analysis in slope of the curve is steep enough to interrogate the change of the bolt axial force. Even at the 60 KN  Section 2 which shows that the peak amplitude increases with the increase of the axial force. The preload, the focused signal peak still kept increasing, thus the designed smart washer can monitor  slope of the curve is steep enough to interrogate the change of the bolt axial force. Even at the 60 KN the  full  range  of  the  M16  bolt.  The  effectiveness  of  the  proposed  design  is  validated  by  this  preload, the focused signal peak still kept increasing, thus the designed smart washer can monitor the experiment.  full range of the M16 bolt. The effectiveness of the proposed design is validated by this experiment. Figure 9 shows the result of specimen 2 (the center distance is 29 mm); the focused signal peak  Figure 9 shows the result of specimen 2 (the center distance is 29 mm); the focused signal peak amplitude  keeps  increasing  and  does  not  reach  saturation.  Nevertheless,  the  curve  has  gently  amplitude keeps increasing and does not reach saturation. Nevertheless, the curve has gently changed changed between 30 and 40 kN, which may be caused by the machining errors of the specimen.  between 30 and 40 kN, which may be caused by the machining errors of the specimen. Appl. Sci. 2016, 6, 320 8 of 10 Appl. Sci. 2016, 6, 320  8 of 10  Appl. Sci. 2016, 6, 320  8 of 10  The experimental results show that the amplitude of the focused signal can indicate the bolt  The experimental results show that the amplitude of the focused signal can indicate the bolt The experimental results show that the amplitude of the focused signal can indicate the bolt  preload obviously, that is to say that the smart washers, based on the piezoelectric active sensing  pr preload eload obviously obviously, , that that isis to to say say that that the the smart smart washers, washers, based based on on the the piezoelectric piezoelectric active active sensing sensing  method and time reversal technique, can effectively monitor the bolt looseness in the related preload  method and time reversal technique, can effectively monitor the bolt looseness in the related preload method and time reversal technique, can effectively monitor the bolt looseness in the related preload  range. By comparing the experimental results of the two specimens, it is shown that a small change  range. range. By By compari comparing ng the the experimental experimental rresults esults of of the the two two specimens, specimens, it it is is shown shown that that aa small small change change  in the contact location will not obviously affect the contact area change under the applied preload  in the contact location will not obviously affect the contact area change under the applied preload in the contact location will not obviously affect the contact area change under the applied preload  range and thus will not affect the transmitted wave energy change. For the M16 bolt, the designed  range range and and thus thus will will not not af aff fect ect the the ttra ransmitted nsmitted wave wave ener energy gy change. change. For For the the M16 M16 bolt, bolt, the the designed designed  smart washer is sufficient to measure 20 kN~60 kN axial force which covers the rated preload range  smart smart washer washer isissuf  sufi ffic cient ientto tomeasur  measure e 20 20 kN~60  kN~60 kN  kN axial  axiafor l force ce which  whiccovers h covers the the rated  rated pr eload preloa range d range of  of the bolt, and the change of amplitude with obvious axial force. Hence, for the different types of  the of the bolt, bolt, and and the the change  chanof geamplitude  of amplitud with e with obvious  obvio axial us axi foral ce. force Hence, . Hence for,the  fordif  thfer e di ent ffetypes rent types of bolt,  of  bolt, by designing a washer that is different in size, the entire rated preload measurement of the bolt  by bolt, designing  by designing a washer  a washer that is tha diftfer  is ent differ in ent size, in the  size entir , the e entire rated pr raeload ted preload measur measurement ement of the bolt  of the can bolt be  can be fulfilled successfully.  fulfilled can be fu successfully lfilled success . fully.  Figure 8. The peak amplitude of the focused signal under different preloads of the first specimen.  Figure 8. The peak amplitude of the focused signal under different preloads of the first specimen. Figure 8. The peak amplitude of the focused signal under different preloads of the first specimen.  Figure 9. The peak amplitude of the focused signal under different preloads of the second specimen.  Figure 9. The peak amplitude of the focused signal under different preloads of the second specimen.  Figure 9. The peak amplitude of the focused signal under different preloads of the second specimen. 5. Conclusions  5. Conclusions  5. Conclusions As  previously  stated,  a  smart  washer  based  on  the  piezoelectric  active  sensing  method  was  As  previously  stated,  a  smart  washer  based  on  the  piezoelectric  active  sensing  method  was  As previously stated, a smart washer based on the piezoelectric active sensing method was proposed for bolt looseness monitoring. The applicability and effectiveness of the smart washer has  proposed for bolt looseness monitoring. The applicability and effectiveness of the smart washer has  proposed for bolt looseness monitoring. The applicability and effectiveness of the smart washer has been presented by experimental studies. Based on the theory that the change of the transmitted wave  been presented by experimental studies. Based on the theory that the change of the transmitted wave  been presented by experimental studies. Based on the theory that the change of the transmitted wave energy, through a bolted joint interface, depends on the bolt preload force, the bolt loosening status  energy, through a bolted joint interface, depends on the bolt preload force, the bolt loosening status  energy, through a bolted joint interface, depends on the bolt preload force, the bolt loosening status can be deduced by observing the change of the peak amplitude of focused signals. The experiment  can be deduced by observing the change of the peak amplitude of focused signals. The experiment  can be deduced by observing the change of the peak amplitude of focused signals. The experiment results show that the focused signal peak amplitude has an approximate linear correlation with the  results show that the focused signal peak amplitude has an approximate linear correlation with the  results show that the focused signal peak amplitude has an approximate linear correlation with the bolt preload.  bolt preload.  bolt preload. Further researches are needed to design the different curvature radii of the convex and concave  Further researches are needed to design the different curvature radii of the convex and concave  surfaces and to determine the sizes of the smart washer, in order to form a series of smart washers  surfaces and to determine the sizes of the smart washer, in order to form a series of smart washers  Appl. Sci. 2016, 6, 320 9 of 10 Further researches are needed to design the different curvature radii of the convex and concave surfaces and to determine the sizes of the smart washer, in order to form a series of smart washers that can measure the full range of the rated preload of different types of bolts. Lastly, through theoretical analysis and numerical simulation, the smart washer structure can be optimized. Acknowledgments: This research was supported by the National Natural Science Foundation of China under Grants No. 51375354 and No. 51278084, the Natural Science Foundation of Hubei Province under Grants No. 2015CFB306, and the Research Project of Hubei Education under Grants No. Q20131104 and No. Q20151103. 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Published: Oct 26, 2016

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