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Design of a Conceptual Bumper Energy Absorber Coupling Pedestrian Safety and Low-Speed Impact Requirements

Design of a Conceptual Bumper Energy Absorber Coupling Pedestrian Safety and Low-Speed Impact... Hindawi Applied Bionics and Biomechanics Volume 2018, Article ID 9293454, 8 pages https://doi.org/10.1155/2018/9293454 Research Article Design of a Conceptual Bumper Energy Absorber Coupling Pedestrian Safety and Low-Speed Impact Requirements 1 1 2 1 3 Fuhao Mo, Siqi Zhao, Chuanhui Yu, Zhi Xiao , and Shuyong Duan State Key Laboratory of Advanced Design and Manufacture for Vehicle Body, Hunan University, Changsha, Hunan 410082, China Safety Engineering and Virtual Technology Department, SAIC Motor Technical Center, Jiading District, Shanghai 201804, China School of Mechanical Engineering, Hebei University of Technology, Beichen District, Tianjin 300401, China Correspondence should be addressed to Zhi Xiao; hnuxiao@163.com Received 3 August 2017; Accepted 29 October 2017; Published 14 January 2018 Academic Editor: Jun Xu Copyright © 2018 Fuhao Mo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The car front bumper system needs to meet the requirements of both pedestrian safety and low-speed impact which are somewhat contradicting. This study aims to design a new kind of modular self-adaptive energy absorber of the front bumper system which can balance the two performances. The X-shaped energy-absorbing structure was proposed which can enhance the energy absorption capacity during impact by changing its deformation mode based on the amount of external collision energy. Then, finite element simulations with a realistic vehicle bumper system are performed to demonstrate its crashworthiness in comparison with the traditional foam energy absorber, which presents a significant improvement of the two performances. Furthermore, the structural parameters of the X-shaped energy-absorbing structure including thickness (t ), side arc radius (R), and clamping boost beam thickness (t ) are analyzed using a full factorial method, and a multiobjective optimization is implemented regarding evaluation indexes of both pedestrian safety and low-speed impact. The optimal parameters are then verified, and the feasibility of the optimal results is confirmed. In conclusion, the new X-shaped energy absorber can meet both pedestrian safety and low-speed impact requirements well by altering the main deformation modes according to different impact energy levels. In previous studies, several attempts considering pedes- 1. Introduction trian safety and low-speed impact have been tried [8]. Yao et al. designed a car-front structure on the purpose of pedes- The front car bumper system is a complex energy-absorbing system in a car design [1] which must meet both the require- trian safety. The structure includes a mechanical cushion in the car bumper for impact energy absorption and a bounce ments of pedestrian safety [2, 3] and low-speed impact [4]. device of hood cover triggered by outer force, and the An energy absorber is often set between the bumper beam and the bumper skin to absorb impact energy [5–7]. How- bumper performance was verified [9]. Wang et al. analyzed the low-speed impact based on dynamic load strength tests ever, the bumper system design requirements of pedestrian safety and low-speed impact are somewhat contradicting of three typical standards of bumper system [10]. Some new bumper systems were designed using new materials regarding force and impact energy levels. Taking the foam [11–14] or structures [15, 16] to achieve the purpose of bumper energy absorber as an example, the absorber satisfy- ing the low-speed impact well can be generally too stiff when improving the crashworthiness under the two collision circumstances. In study of Lv et al., a systematic method considering the impact with pedestrian lower extremities due to the high force level. On the contrary, the situation is had been performed to design and optimize the car front- end structure in order to reduce pedestrian injury risks similar. Besides, the traditional energy absorbers are usually [17]. Shuler designed a new bumper energy absorber using an integrated structure made of thermoplastic polymer or foamed polypropylene (EPP) which could need an overall engineering plastics, which included a body and the upper and lower crushable members which would absorb more replacement due to a local damage. 2 Applied Bionics and Biomechanics A-A w h 4 : 1 0 10203040 Displacement (mm) Experiment Stimulation (a) Structural features (b) Xenoy composite properties Figure 1: Structural features and material properties of a single X-shaped energy-absorbing unit. energy during impact [18]. Mohapatra designed a tunable simulation parameters of Mat 24 in LS-DYNA codes are pre- energy absorber which consists of a frame and a body includ- sented and validated through the implemented experimental ing a mount of tunable crush lobes to absorb the energy tests using Instron 5984. Initial geometric parameters of this unit are then during pedestrian and low-speed impacts [19]. But they featured a complex structure, difficult to manufacture, and determined regarding the vehicle bumper system that would still used an integrated structure. Davoodi et al. made a be applied on, with the depth l = 80 mm, the width w = conceptual design and a simulation verification analysis on 40 mm, R = 180 mm, r =10mm, t = 2.5 mm, and the height the bumper energy absorber with fibre-reinforced epoxy h = 56 mm. The compression test is performed on the polymer composite material [20]. But the energy absorber X-shaped energy absorber with a U shape impactor at a was mainly in consideration of pedestrian safety without speed of 4 km/h. The compression force and energy- detailed design description for low-speed impact. There- absorbing curves are shown in Figure 2. fore, it is expected to design a bumper energy absorber During the entire compression process, the X-shaped which can well consider the requirements of both pedestrian unit shows different deformation modes with various force safety and low-speed impact with evidently different impact levels and energy-absorbing rates. In the deformation stage energy levels. from 0 to 12 mm, the unit begins to deform to an elastic limit Composite material with resin matrix which performs with low force level and low energy-absorbing ability. In light-weighted, safe, and flexible performance in design and 12~40 mm deformation, the two sides of the unit arc get into manufacturing is being more and more widely used in vehicle contact and begin to perform a self-locking status. This leads bumper system [21–25]. The present study aims to design an to a rapid increase of energy-absorbing ability and force energy-absorbing structure of the bumper system with levels of the X-shaped unit. In the phase of the deformation composite materials which can adaptively adopt different higher than 40 mm, the energy absorption unit totally kinks deformation modes according to the amount of impact together and is continuously compressed to a deformation energy to benefit both pedestrian and low-speed impact. limit. Thus, a proper structure design with a number of Multiobjective optimization has also been implemented to X-shaped units can be expected to meet different safety optimize the conceptual design of this energy-absorbing requirements under various impact force and energy levels. structure in a realistic family car model, and its results are compared with the original foam absorbing structure. 2.2. Design of Modular Bumper Energy Absorber. With regard to impact energy levels and installation space in the realistic car model, a modular energy absorber is designed as shown 2. Methods and Materials in Figure 3(a). It includes fifteen X-shaped units and two 2.1. Conceptual Design of the X-Shaped Energy-Absorbing clamping boost beams to lock the units between them. The Unit. To create a single structure with different energy absorber is installed between the bumper skin and bumper absorption phases, an X-shaped absorber made of Xenoy beam as the location could be seen in Figure 3(b). Based on the present car model and energy absorber composite is proposed as shown in Figure 1. The Xenoy composite (PC/PBT 1103) with a density of 1145 kg/m , design, the finite element models of pedestrian lower legform elastic modulus of 2317.48 MPa, Poisson’s ratio of 0.3, and and low-speed impact are established using Hypermesh yield strength of 33.19 MPa is adopted. Its validated software as shown in Figure 4 according to the 631/2009/ Load (N) Applied Bionics and Biomechanics 3 0 10 20304050 0 10 20304050 Deformation (mm) Displacement (mm) (a) (b) Figure 2: Energy deformation and load deformation curves of X-shaped absorber unit under compression. Energy absorber Rear clamping boost beam Front clamping boost beam Energy-absorbing units Bumper beam Skin (a) (b) Figure 3: Schematic diagram of the (a) energy absorber and (b) installation position. Energy absorber 8 km/h Energy absorber 40 km/h Mass Constraint Shank impactor Collision impactor Bumper beam Bumper beam (a) (b) Figure 4: Finite element models of (a) pedestrian lower extremity impact and (b) low-speed impact. EC regulation [26] and the CMVSS215 regulation, respec- optimization is adopted to determine the structural param- tively. The impact velocity of the legform is 40 km/h with eters of the modular energy absorber with X-shaped units. impact energy at 827.16 J. The low-speed impactor is set at Tests are designed using the full factorial method, input 8 km/h with impact energy at 3207.01 J. Then, impact factors are defined as X-shaped unit thickness (t ), X- simulations are initially performed. shaped unit side arc radius (R), and clamping boost beam thickness (t ) in three levels (Table 1). Output indexes 2.3. Structural Optimization. To further improve the include maximum tibial acceleration (MTA), maximum performance of the new bumper system, multiobjective knee bending angle (MKBA), maximum knee shear Energy (J) Force (N) 4 Applied Bionics and Biomechanics Table 1: Levels of structural parameters. Table 2: Design of experiments with experimental conditions. Number Factor Case1 Case2 Case3 MTA MKBA MKSD CI BD Run A B C (g) ( ) (mm) (mm) (mm) A t 3 mm 4 mm 5 mm 1 1 1 1 139.1 4.46 2.40 113.47 49.70 B R 60 mm 120 mm 180 mm 2 1 2 2 149.3 4.05 1.40 102.02 44.72 C t 1 mm 2 mm 3 mm 3 1 3 3 124.3 4.11 1.60 101.35 44.22 4 2 1 2 138.5 4.71 2.09 85.69 64.79 displacement (MKSD), collider intrusion (CI), and bumper 5 2 2 3 148.3 7.11 3.97 82.08 65.26 deformation (BP). 6 2 3 1 140.9 4.56 2.50 83.44 54.66 Tests are performed adopting the Hypermesh software, 7 3 1 3 171.5 6.86 3.24 80.56 72.83 the full factorial experiments are detailedly made then. 8 3 2 1 162.8 6.02 2.97 79.04 67.56 9 3 3 2 179.0 6.42 3.27 78.76 68.43 3. Results and Discussions 10 1 1 2 156.4 4.01 1.48 106.99 47.42 The overall results of low-speed impact and pedestrian safety 11 1 1 3 130.8 4.14 1.66 101.77 50.24 tests are listed in Table 2. The correlation of output index 12 1 2 1 127.3 3.93 1.54 110.68 46.73 values to input structural parameters is shown in Figure 5. 13 1 2 3 124.9 3.99 1.59 95.75 49.35 As can be visualized in Figure 5, t is the most influential 14 1 3 1 148.6 4.42 1.61 110.17 46.62 parameter of all these factors. MTA is also greatly influenced 15 1 3 2 143.6 4.00 1.44 101.55 44.37 by R, while the effect of t is less. MKBA, MKSD, BD, and CI 16 2 1 1 138.1 4.87 2.33 90.84 42.60 are affected by t a lot and the influence of R is slight. Regarding pedestrian safety tests, Figure 5(a) reveals the 17 2 1 3 143.4 5.28 2.38 84.15 67.08 interaction effect between t and R on MTA. The MTA value u 18 2 2 1 133.5 4.33 2.06 84.34 64.84 considerably increases with the increase of t at high levels t u u 19 2 2 2 144.6 4.64 2.14 83.30 65.90 from approximately 4.2 mm to 5 mm. On the contrary, the 20 2 3 2 135.9 4.55 2.13 82.43 64.30 decline of t leads to the decrease of the MTA at low t values. u u 21 2 3 3 141.7 5.15 2.39 80.90 64.01 The influence of R on the MTA is less. For the values of R 22 3 1 1 164.3 6.06 2.86 87.40 68.89 from 80 mm to 180 mm, the MTA increases initially and then 23 3 1 2 170.9 6.32 2.93 81.03 69.03 decreases. The minimum MTA of 130 g is obtained at approximately 3.8 mm t and 180 mm R. The changes of 24 3 2 2 159.4 6.39 3.14 78.10 68.31 the MKBA value on t and t are presented in Figure 5(b). b u 25 3 2 3 161.2 6.89 3.58 77.35 68.04 It presents that increasing t leads to decrease of the MKBA. 26 3 3 1 183.8 6.02 2.96 79.59 68.43 Similarly, the MKBA slightly increases with the decline of t . 27 3 3 3 194.9 6.89 3.68 77.90 68.70 The minimum MKBA of approximately 4 is obtained at 3mm t and 2 mm t . The dependence of MKSD on t and u b b t is presented in Figure 5(c). It is observed that the MKSD selected among the results, and the optimization results are notably increases with the increase in t and is slightly shown in Table 3. Since the above results are based on the influenced by t . optimization results of the algorithm, analyses are performed For low-speed impact tests, Figure 5(d) plots the influ- to verify the obtained structural parameters. The three opti- ences of t mal structural parameters are substituted to the original finite and t on CI. The CI decreases from 95 mm to b u 78 mm with the increase of t from 2 mm to 5 mm while element model of pedestrian safety and low-speed impact. the effects of t on CI are less. The effect of t and t on BD Two contrast simulation models are established and the eval- b u b values can be visualized in Figure 5(e). It is revealed that uation results are shown in Table 3. BD increases to a maximum point and then decreases with As shown in Table 3, all damage index values of the opti- mized structure are superior to the initial solution while sat- t from 3 mm to 5 mm. BD has a gentle increase with the increase of t . The maximum BD of approximately 70 mm isfying the requirements of the regulations. The error of the is obtained at 4.8 mm t . value between the final verification and the optimal solution After this, we adopt a set of samples to ensure that the is controlled within 15%. This indicates that the optimization accuracy of the Kriging model is accepted. We use four cri- method used in this study is reliable. teria to judge the accuracy of the model: R-squared (R The performances of pedestrian safety and low-speed ), root mean square error (RMSE), relative average absolute error impact protection based on the traditional foam absorber, (RAAE), and relative maximum absolute error (RMAE). the original X-shaped energy absorber model, and the The values are 0.999, 0.131, 0.492, and 0.009, respectively. It optimal verification model are compared and shown in can be observed that this model is relatively accurate and Figure 6. It should be noted that most risk index values of the impact simulations with X-shaped energy absorbers are can be used for the subsequent optimization model. Then, the multiobjective particle swarm optimization reduced including all below the corresponding thresholds algorithm including 511 iterations is selected to optimize compared to those of the impact simulations with the tradi- the design variables. Then, a relatively good result was tional foam absorber. One of the most important reasons t (mm) R (mm) t (mm) t (mm) t (mm) Applied Bionics and Biomechanics 5 180 7.5 6.5 5.5 4.5 180 0 5 5 2.8 4.8 4.8 4.8 160 160 2.6 4.6 4.6 4.6 44 4.4 2.4 4.4 140 140 4.2 2.2 4.2 120 4 4 1.8 3.8 3.8 1.6 3.6 3.6 3.4 1.4 3.4 3.2 1.2 3.2 3 3 (a) (b) 4 115 5 3 3 5 4.8 2.8 4.6 2.6 2.6 4.6 4.4 2.4 4.2 2.2 2.2 4.2 4 2 3.8 1.8 1.8 3.8 3.6 1.6 3.4 1.4 1.4 3.4 3.2 1.2 1 3 (c) (d) 4.8 2.8 4.6 2.6 4.4 2.4 4.2 2.2 3.8 1.8 3.6 1.6 3.4 1.4 3.2 1.2 (e) Figure 5: Response surfaces showing simultaneous effects of (a) t and R on MTA, (b) t and t on MKBA, (c) t and t on MKSD, (d) t and t u b u b u b u on CI, and (e) t and t on BD. b u Table 3: Multiobjective optimization results and verification. Variables A B C MTA (g) MKBA ( ) MKSD (mm) CI (mm) BD (mm) Regular value —— — 150.0 15.00 6.00 165.00 64.00 Foam absorber —— — 221.58 7.80 3.21 77.78 67.56 Original results 2.5 180.0 2 143.6 6.41 3.29 111.93 53.91 Optimal results 3.2 146.4 3 127.0 4.50 1.93 93.55 51.55 Verification 3.2 146.4 3 134.5 3.88 1.76 94.81 47.09 Deviation —— — 5.91% 13.78% 8.81% 1.35% 8.65% t (mm) t (mm) t (mm) t (mm) t (mm) MTA (g) MKSD (mm) BD (mm) CI (mm) MKBA (º) 6 Applied Bionics and Biomechanics Pedestrian safety Low-speed impact 30 0 5 10 15 20 25 30 35 40 Time (ms) Foam absorber Optimal results Original results Regular value (a) Deformation modes (b) Tibial acceleration 20 7 −5 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Time (ms) Time (ms) Foam absorber Optimal results Foam absorber Optimal results Original results Regular value Original results Regular value (c) Knee bending angle (d) Knee shear displacement 70 200 0 -20 0 20 40 60 80 100 120 140 160 180 04 20 0 60 80 100 120 140 160 180 Time (ms) Time (ms) Foam absorber Optimal results Foam absorber Optimal results Original results Regular value Original results Regular value (e) Bumper deformation (f) Collider intrusion Figure 6: Comparison of evaluation index values regarding pedestrian safety and low-speed impact. Bumper deformation (mm) Knee bending angle (º) Collider intrusion (mm) Knee shear displacement (mm) Tibial acceleration (g) Applied Bionics and Biomechanics 7 system compared to the traditional foam absorber, in partic- can be due to dual deformation modes of the X-shaped energy-absorbing unit during various impacts with different ular to provide an effective force and energy-absorbing con- amounts of energy. In the pedestrian safety test, the units trol through different deformation modes. Meanwhile, due absorb energy mainly before forming the self-locking struc- to the modular design, only the damaged bumper energy- ture and effectively decline the peak value of the impact force. absorbing units during the impact need to be replaced and In the low-speed impact test, the X-shaped units absorb the other units remaining intact can be used again which energy mainly by the self-locking mode with higher energy- means that the new energy absorbers are easy to repair in absorbing efficiency. an economical way. It can be observed in Figure 6(b) that at 4 ms, the leg In addition, the parameters of pedestrian safety and impactor gets into contact with the bumper skin which low-speed impact are greatly improved after applying the leads to an elastic deformation of the X-shaped energy structural parameters obtained by the optimization algo- absorber; the first peak is obtained. At about 7 ms, the rithm in this study. For pedestrian safety, the maximum X-shaped energy absorber reaches the elastic limits after MTA decreases from 143.6 mm to 134.5 mm, the maximum ° ° compressing and forms the second peaks. Further, when MKBA decreases from 6.41 to 3.88 with a reduction of the X-shaped energy absorption unit exceeds the elastic 39.47%, and the maximum MKSD decreased from 3.29 mm limit to 13 ms, the two arc sides get into contact with each to 1.76 mm with a reduction of 46.50%. For low-speed other to form a third peak. At 40 ms, the energy of the X- impact, the maximum CI decreases from 111.93 mm to shaped energy absorption unit is gradually released, resulting 94.81 mm with a reduction of 15.30%. The maximum in a certain rebound. value of BD reduces from 53.91 mm to 47.09 mm with a Figure 6(b) shows that the X-shaped energy absorber reduction of 12.65%. All these indicates the efficiency and shows an evidently better energy absorption performance contributions of the multiobjective optimization method when compared with the foam absorber. After using the used in the design of the new energy absorber with the new energy absorber with the X-shaped units, the maximum X-shaped unit. tibial acceleration related to pedestrian protection decreases notably to 127 g. As shown in Figure 6(a), the impact load 4. Conclusions is distributed to different compression stages to achieve the purpose of reducing damage with multiple peaks instead This paper proposes and designs a new conceptual type of of a large acceleration peak of the traditional foam energy bumper energy absorber in a multioptimization method absorber. When the leg impactor comes into contact with considering the requirements of both pedestrian safety and the bumper skin and the X-shaped energy absorption unit low-speed impact, which adopts a modular design in the begins to compress, the tibial acceleration curve obtains form of assembling with an X-shaped unit. This unit type the first peak. Then, the energy absorber is continuously presents grading deformation modes with different energy- compressed until its elasticity limit and until the second absorbing rates and force levels. The results reveals that the acceleration peak is formed. Further, the elastic limit is new bumper energy absorber proposed in this paper adap- exceeded and a self-locking status of the X-shaped unit tively uses different energy absorption modes in different is formed; the third peak is obtained. The maximum knee collision forms based on the structural characteristics of its bending angle and shear displacement are also significantly own X-shaped unit and rapidly increases the energy reduced by 50% (Figures 6(c) and 6(d)) to 4.5 and absorption capacity after self-locking. So, it performs a better 1.93 mm, respectively. All these indicate that the X-shaped comprehensive performance compared to the traditional bumper energy absorber adaptively adopts the small defor- foam-type energy absorber by effectively controlling the force mation mode in the pedestrian safety test due to the low level and energy-absorbing rate. The modular design also impact energy. indicates its easy changing and fixing. In the low-speed impact test as shown in Figure 6(e), the Besides, the multiobjective optimization of the structural maximum deformation of the bumper has a significant parameters is performed for the detailed design of the new decline when comparing the new X-shaped energy absorber bumper energy absorber. The pedestrian protection and with the traditional foam absorber. At the initial stages of low-speed impact performance of the new energy absorber 0~30 ms, the X-shaped units are in the deformation phase with optimized structural parameters are greatly improved, before two arcs are in contact and the two sides of the arc and the requirements of pedestrian safety and low-speed are in contact with each other to form a self-locking struc- impact are better balanced. ture, reaching a peak of 90 ms while the energy absorption capacity rapidly increases. It is revealed that the new bumper Conflicts of Interest energy absorber adaptively adopts the large deformation mode in the low-speed collision test, which absorbs more The authors declare that they have no conflicts of interest. energy and significantly reduces the bumper deformation peak, as shown in Figure 6(a). In Figure 6(f), the maximum value of the collider intrusion has also been largely reduced Acknowledgments due to the structure optimization. All of the above indicates that the new X-shaped energy This work was supported by the National Natural Science absorber shows a better performance in the present bumper Foundation of China (Grant nos. 51405150 and 51475154). 8 Applied Bionics and Biomechanics [15] M. K. Shin, S. I. Yi, O. T. Kwon, and G. J. Park, “Structural References optimization of the automobile frontal structure for pedestrian [1] M. M. Davoodi, S. M. Sapuan, A. Aidy, N. A. Abu Osman, protection and the low-speed impact test,” Proceedings of the A. A. 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Design of a Conceptual Bumper Energy Absorber Coupling Pedestrian Safety and Low-Speed Impact Requirements

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Hindawi Publishing Corporation
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Copyright © 2018 Fuhao Mo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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1754-2103
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10.1155/2018/9293454
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Hindawi Applied Bionics and Biomechanics Volume 2018, Article ID 9293454, 8 pages https://doi.org/10.1155/2018/9293454 Research Article Design of a Conceptual Bumper Energy Absorber Coupling Pedestrian Safety and Low-Speed Impact Requirements 1 1 2 1 3 Fuhao Mo, Siqi Zhao, Chuanhui Yu, Zhi Xiao , and Shuyong Duan State Key Laboratory of Advanced Design and Manufacture for Vehicle Body, Hunan University, Changsha, Hunan 410082, China Safety Engineering and Virtual Technology Department, SAIC Motor Technical Center, Jiading District, Shanghai 201804, China School of Mechanical Engineering, Hebei University of Technology, Beichen District, Tianjin 300401, China Correspondence should be addressed to Zhi Xiao; hnuxiao@163.com Received 3 August 2017; Accepted 29 October 2017; Published 14 January 2018 Academic Editor: Jun Xu Copyright © 2018 Fuhao Mo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The car front bumper system needs to meet the requirements of both pedestrian safety and low-speed impact which are somewhat contradicting. This study aims to design a new kind of modular self-adaptive energy absorber of the front bumper system which can balance the two performances. The X-shaped energy-absorbing structure was proposed which can enhance the energy absorption capacity during impact by changing its deformation mode based on the amount of external collision energy. Then, finite element simulations with a realistic vehicle bumper system are performed to demonstrate its crashworthiness in comparison with the traditional foam energy absorber, which presents a significant improvement of the two performances. Furthermore, the structural parameters of the X-shaped energy-absorbing structure including thickness (t ), side arc radius (R), and clamping boost beam thickness (t ) are analyzed using a full factorial method, and a multiobjective optimization is implemented regarding evaluation indexes of both pedestrian safety and low-speed impact. The optimal parameters are then verified, and the feasibility of the optimal results is confirmed. In conclusion, the new X-shaped energy absorber can meet both pedestrian safety and low-speed impact requirements well by altering the main deformation modes according to different impact energy levels. In previous studies, several attempts considering pedes- 1. Introduction trian safety and low-speed impact have been tried [8]. Yao et al. designed a car-front structure on the purpose of pedes- The front car bumper system is a complex energy-absorbing system in a car design [1] which must meet both the require- trian safety. The structure includes a mechanical cushion in the car bumper for impact energy absorption and a bounce ments of pedestrian safety [2, 3] and low-speed impact [4]. device of hood cover triggered by outer force, and the An energy absorber is often set between the bumper beam and the bumper skin to absorb impact energy [5–7]. How- bumper performance was verified [9]. Wang et al. analyzed the low-speed impact based on dynamic load strength tests ever, the bumper system design requirements of pedestrian safety and low-speed impact are somewhat contradicting of three typical standards of bumper system [10]. Some new bumper systems were designed using new materials regarding force and impact energy levels. Taking the foam [11–14] or structures [15, 16] to achieve the purpose of bumper energy absorber as an example, the absorber satisfy- ing the low-speed impact well can be generally too stiff when improving the crashworthiness under the two collision circumstances. In study of Lv et al., a systematic method considering the impact with pedestrian lower extremities due to the high force level. On the contrary, the situation is had been performed to design and optimize the car front- end structure in order to reduce pedestrian injury risks similar. Besides, the traditional energy absorbers are usually [17]. Shuler designed a new bumper energy absorber using an integrated structure made of thermoplastic polymer or foamed polypropylene (EPP) which could need an overall engineering plastics, which included a body and the upper and lower crushable members which would absorb more replacement due to a local damage. 2 Applied Bionics and Biomechanics A-A w h 4 : 1 0 10203040 Displacement (mm) Experiment Stimulation (a) Structural features (b) Xenoy composite properties Figure 1: Structural features and material properties of a single X-shaped energy-absorbing unit. energy during impact [18]. Mohapatra designed a tunable simulation parameters of Mat 24 in LS-DYNA codes are pre- energy absorber which consists of a frame and a body includ- sented and validated through the implemented experimental ing a mount of tunable crush lobes to absorb the energy tests using Instron 5984. Initial geometric parameters of this unit are then during pedestrian and low-speed impacts [19]. But they featured a complex structure, difficult to manufacture, and determined regarding the vehicle bumper system that would still used an integrated structure. Davoodi et al. made a be applied on, with the depth l = 80 mm, the width w = conceptual design and a simulation verification analysis on 40 mm, R = 180 mm, r =10mm, t = 2.5 mm, and the height the bumper energy absorber with fibre-reinforced epoxy h = 56 mm. The compression test is performed on the polymer composite material [20]. But the energy absorber X-shaped energy absorber with a U shape impactor at a was mainly in consideration of pedestrian safety without speed of 4 km/h. The compression force and energy- detailed design description for low-speed impact. There- absorbing curves are shown in Figure 2. fore, it is expected to design a bumper energy absorber During the entire compression process, the X-shaped which can well consider the requirements of both pedestrian unit shows different deformation modes with various force safety and low-speed impact with evidently different impact levels and energy-absorbing rates. In the deformation stage energy levels. from 0 to 12 mm, the unit begins to deform to an elastic limit Composite material with resin matrix which performs with low force level and low energy-absorbing ability. In light-weighted, safe, and flexible performance in design and 12~40 mm deformation, the two sides of the unit arc get into manufacturing is being more and more widely used in vehicle contact and begin to perform a self-locking status. This leads bumper system [21–25]. The present study aims to design an to a rapid increase of energy-absorbing ability and force energy-absorbing structure of the bumper system with levels of the X-shaped unit. In the phase of the deformation composite materials which can adaptively adopt different higher than 40 mm, the energy absorption unit totally kinks deformation modes according to the amount of impact together and is continuously compressed to a deformation energy to benefit both pedestrian and low-speed impact. limit. Thus, a proper structure design with a number of Multiobjective optimization has also been implemented to X-shaped units can be expected to meet different safety optimize the conceptual design of this energy-absorbing requirements under various impact force and energy levels. structure in a realistic family car model, and its results are compared with the original foam absorbing structure. 2.2. Design of Modular Bumper Energy Absorber. With regard to impact energy levels and installation space in the realistic car model, a modular energy absorber is designed as shown 2. Methods and Materials in Figure 3(a). It includes fifteen X-shaped units and two 2.1. Conceptual Design of the X-Shaped Energy-Absorbing clamping boost beams to lock the units between them. The Unit. To create a single structure with different energy absorber is installed between the bumper skin and bumper absorption phases, an X-shaped absorber made of Xenoy beam as the location could be seen in Figure 3(b). Based on the present car model and energy absorber composite is proposed as shown in Figure 1. The Xenoy composite (PC/PBT 1103) with a density of 1145 kg/m , design, the finite element models of pedestrian lower legform elastic modulus of 2317.48 MPa, Poisson’s ratio of 0.3, and and low-speed impact are established using Hypermesh yield strength of 33.19 MPa is adopted. Its validated software as shown in Figure 4 according to the 631/2009/ Load (N) Applied Bionics and Biomechanics 3 0 10 20304050 0 10 20304050 Deformation (mm) Displacement (mm) (a) (b) Figure 2: Energy deformation and load deformation curves of X-shaped absorber unit under compression. Energy absorber Rear clamping boost beam Front clamping boost beam Energy-absorbing units Bumper beam Skin (a) (b) Figure 3: Schematic diagram of the (a) energy absorber and (b) installation position. Energy absorber 8 km/h Energy absorber 40 km/h Mass Constraint Shank impactor Collision impactor Bumper beam Bumper beam (a) (b) Figure 4: Finite element models of (a) pedestrian lower extremity impact and (b) low-speed impact. EC regulation [26] and the CMVSS215 regulation, respec- optimization is adopted to determine the structural param- tively. The impact velocity of the legform is 40 km/h with eters of the modular energy absorber with X-shaped units. impact energy at 827.16 J. The low-speed impactor is set at Tests are designed using the full factorial method, input 8 km/h with impact energy at 3207.01 J. Then, impact factors are defined as X-shaped unit thickness (t ), X- simulations are initially performed. shaped unit side arc radius (R), and clamping boost beam thickness (t ) in three levels (Table 1). Output indexes 2.3. Structural Optimization. To further improve the include maximum tibial acceleration (MTA), maximum performance of the new bumper system, multiobjective knee bending angle (MKBA), maximum knee shear Energy (J) Force (N) 4 Applied Bionics and Biomechanics Table 1: Levels of structural parameters. Table 2: Design of experiments with experimental conditions. Number Factor Case1 Case2 Case3 MTA MKBA MKSD CI BD Run A B C (g) ( ) (mm) (mm) (mm) A t 3 mm 4 mm 5 mm 1 1 1 1 139.1 4.46 2.40 113.47 49.70 B R 60 mm 120 mm 180 mm 2 1 2 2 149.3 4.05 1.40 102.02 44.72 C t 1 mm 2 mm 3 mm 3 1 3 3 124.3 4.11 1.60 101.35 44.22 4 2 1 2 138.5 4.71 2.09 85.69 64.79 displacement (MKSD), collider intrusion (CI), and bumper 5 2 2 3 148.3 7.11 3.97 82.08 65.26 deformation (BP). 6 2 3 1 140.9 4.56 2.50 83.44 54.66 Tests are performed adopting the Hypermesh software, 7 3 1 3 171.5 6.86 3.24 80.56 72.83 the full factorial experiments are detailedly made then. 8 3 2 1 162.8 6.02 2.97 79.04 67.56 9 3 3 2 179.0 6.42 3.27 78.76 68.43 3. Results and Discussions 10 1 1 2 156.4 4.01 1.48 106.99 47.42 The overall results of low-speed impact and pedestrian safety 11 1 1 3 130.8 4.14 1.66 101.77 50.24 tests are listed in Table 2. The correlation of output index 12 1 2 1 127.3 3.93 1.54 110.68 46.73 values to input structural parameters is shown in Figure 5. 13 1 2 3 124.9 3.99 1.59 95.75 49.35 As can be visualized in Figure 5, t is the most influential 14 1 3 1 148.6 4.42 1.61 110.17 46.62 parameter of all these factors. MTA is also greatly influenced 15 1 3 2 143.6 4.00 1.44 101.55 44.37 by R, while the effect of t is less. MKBA, MKSD, BD, and CI 16 2 1 1 138.1 4.87 2.33 90.84 42.60 are affected by t a lot and the influence of R is slight. Regarding pedestrian safety tests, Figure 5(a) reveals the 17 2 1 3 143.4 5.28 2.38 84.15 67.08 interaction effect between t and R on MTA. The MTA value u 18 2 2 1 133.5 4.33 2.06 84.34 64.84 considerably increases with the increase of t at high levels t u u 19 2 2 2 144.6 4.64 2.14 83.30 65.90 from approximately 4.2 mm to 5 mm. On the contrary, the 20 2 3 2 135.9 4.55 2.13 82.43 64.30 decline of t leads to the decrease of the MTA at low t values. u u 21 2 3 3 141.7 5.15 2.39 80.90 64.01 The influence of R on the MTA is less. For the values of R 22 3 1 1 164.3 6.06 2.86 87.40 68.89 from 80 mm to 180 mm, the MTA increases initially and then 23 3 1 2 170.9 6.32 2.93 81.03 69.03 decreases. The minimum MTA of 130 g is obtained at approximately 3.8 mm t and 180 mm R. The changes of 24 3 2 2 159.4 6.39 3.14 78.10 68.31 the MKBA value on t and t are presented in Figure 5(b). b u 25 3 2 3 161.2 6.89 3.58 77.35 68.04 It presents that increasing t leads to decrease of the MKBA. 26 3 3 1 183.8 6.02 2.96 79.59 68.43 Similarly, the MKBA slightly increases with the decline of t . 27 3 3 3 194.9 6.89 3.68 77.90 68.70 The minimum MKBA of approximately 4 is obtained at 3mm t and 2 mm t . The dependence of MKSD on t and u b b t is presented in Figure 5(c). It is observed that the MKSD selected among the results, and the optimization results are notably increases with the increase in t and is slightly shown in Table 3. Since the above results are based on the influenced by t . optimization results of the algorithm, analyses are performed For low-speed impact tests, Figure 5(d) plots the influ- to verify the obtained structural parameters. The three opti- ences of t mal structural parameters are substituted to the original finite and t on CI. The CI decreases from 95 mm to b u 78 mm with the increase of t from 2 mm to 5 mm while element model of pedestrian safety and low-speed impact. the effects of t on CI are less. The effect of t and t on BD Two contrast simulation models are established and the eval- b u b values can be visualized in Figure 5(e). It is revealed that uation results are shown in Table 3. BD increases to a maximum point and then decreases with As shown in Table 3, all damage index values of the opti- mized structure are superior to the initial solution while sat- t from 3 mm to 5 mm. BD has a gentle increase with the increase of t . The maximum BD of approximately 70 mm isfying the requirements of the regulations. The error of the is obtained at 4.8 mm t . value between the final verification and the optimal solution After this, we adopt a set of samples to ensure that the is controlled within 15%. This indicates that the optimization accuracy of the Kriging model is accepted. We use four cri- method used in this study is reliable. teria to judge the accuracy of the model: R-squared (R The performances of pedestrian safety and low-speed ), root mean square error (RMSE), relative average absolute error impact protection based on the traditional foam absorber, (RAAE), and relative maximum absolute error (RMAE). the original X-shaped energy absorber model, and the The values are 0.999, 0.131, 0.492, and 0.009, respectively. It optimal verification model are compared and shown in can be observed that this model is relatively accurate and Figure 6. It should be noted that most risk index values of the impact simulations with X-shaped energy absorbers are can be used for the subsequent optimization model. Then, the multiobjective particle swarm optimization reduced including all below the corresponding thresholds algorithm including 511 iterations is selected to optimize compared to those of the impact simulations with the tradi- the design variables. Then, a relatively good result was tional foam absorber. One of the most important reasons t (mm) R (mm) t (mm) t (mm) t (mm) Applied Bionics and Biomechanics 5 180 7.5 6.5 5.5 4.5 180 0 5 5 2.8 4.8 4.8 4.8 160 160 2.6 4.6 4.6 4.6 44 4.4 2.4 4.4 140 140 4.2 2.2 4.2 120 4 4 1.8 3.8 3.8 1.6 3.6 3.6 3.4 1.4 3.4 3.2 1.2 3.2 3 3 (a) (b) 4 115 5 3 3 5 4.8 2.8 4.6 2.6 2.6 4.6 4.4 2.4 4.2 2.2 2.2 4.2 4 2 3.8 1.8 1.8 3.8 3.6 1.6 3.4 1.4 1.4 3.4 3.2 1.2 1 3 (c) (d) 4.8 2.8 4.6 2.6 4.4 2.4 4.2 2.2 3.8 1.8 3.6 1.6 3.4 1.4 3.2 1.2 (e) Figure 5: Response surfaces showing simultaneous effects of (a) t and R on MTA, (b) t and t on MKBA, (c) t and t on MKSD, (d) t and t u b u b u b u on CI, and (e) t and t on BD. b u Table 3: Multiobjective optimization results and verification. Variables A B C MTA (g) MKBA ( ) MKSD (mm) CI (mm) BD (mm) Regular value —— — 150.0 15.00 6.00 165.00 64.00 Foam absorber —— — 221.58 7.80 3.21 77.78 67.56 Original results 2.5 180.0 2 143.6 6.41 3.29 111.93 53.91 Optimal results 3.2 146.4 3 127.0 4.50 1.93 93.55 51.55 Verification 3.2 146.4 3 134.5 3.88 1.76 94.81 47.09 Deviation —— — 5.91% 13.78% 8.81% 1.35% 8.65% t (mm) t (mm) t (mm) t (mm) t (mm) MTA (g) MKSD (mm) BD (mm) CI (mm) MKBA (º) 6 Applied Bionics and Biomechanics Pedestrian safety Low-speed impact 30 0 5 10 15 20 25 30 35 40 Time (ms) Foam absorber Optimal results Original results Regular value (a) Deformation modes (b) Tibial acceleration 20 7 −5 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Time (ms) Time (ms) Foam absorber Optimal results Foam absorber Optimal results Original results Regular value Original results Regular value (c) Knee bending angle (d) Knee shear displacement 70 200 0 -20 0 20 40 60 80 100 120 140 160 180 04 20 0 60 80 100 120 140 160 180 Time (ms) Time (ms) Foam absorber Optimal results Foam absorber Optimal results Original results Regular value Original results Regular value (e) Bumper deformation (f) Collider intrusion Figure 6: Comparison of evaluation index values regarding pedestrian safety and low-speed impact. Bumper deformation (mm) Knee bending angle (º) Collider intrusion (mm) Knee shear displacement (mm) Tibial acceleration (g) Applied Bionics and Biomechanics 7 system compared to the traditional foam absorber, in partic- can be due to dual deformation modes of the X-shaped energy-absorbing unit during various impacts with different ular to provide an effective force and energy-absorbing con- amounts of energy. In the pedestrian safety test, the units trol through different deformation modes. Meanwhile, due absorb energy mainly before forming the self-locking struc- to the modular design, only the damaged bumper energy- ture and effectively decline the peak value of the impact force. absorbing units during the impact need to be replaced and In the low-speed impact test, the X-shaped units absorb the other units remaining intact can be used again which energy mainly by the self-locking mode with higher energy- means that the new energy absorbers are easy to repair in absorbing efficiency. an economical way. It can be observed in Figure 6(b) that at 4 ms, the leg In addition, the parameters of pedestrian safety and impactor gets into contact with the bumper skin which low-speed impact are greatly improved after applying the leads to an elastic deformation of the X-shaped energy structural parameters obtained by the optimization algo- absorber; the first peak is obtained. At about 7 ms, the rithm in this study. For pedestrian safety, the maximum X-shaped energy absorber reaches the elastic limits after MTA decreases from 143.6 mm to 134.5 mm, the maximum ° ° compressing and forms the second peaks. Further, when MKBA decreases from 6.41 to 3.88 with a reduction of the X-shaped energy absorption unit exceeds the elastic 39.47%, and the maximum MKSD decreased from 3.29 mm limit to 13 ms, the two arc sides get into contact with each to 1.76 mm with a reduction of 46.50%. For low-speed other to form a third peak. At 40 ms, the energy of the X- impact, the maximum CI decreases from 111.93 mm to shaped energy absorption unit is gradually released, resulting 94.81 mm with a reduction of 15.30%. The maximum in a certain rebound. value of BD reduces from 53.91 mm to 47.09 mm with a Figure 6(b) shows that the X-shaped energy absorber reduction of 12.65%. All these indicates the efficiency and shows an evidently better energy absorption performance contributions of the multiobjective optimization method when compared with the foam absorber. After using the used in the design of the new energy absorber with the new energy absorber with the X-shaped units, the maximum X-shaped unit. tibial acceleration related to pedestrian protection decreases notably to 127 g. As shown in Figure 6(a), the impact load 4. Conclusions is distributed to different compression stages to achieve the purpose of reducing damage with multiple peaks instead This paper proposes and designs a new conceptual type of of a large acceleration peak of the traditional foam energy bumper energy absorber in a multioptimization method absorber. When the leg impactor comes into contact with considering the requirements of both pedestrian safety and the bumper skin and the X-shaped energy absorption unit low-speed impact, which adopts a modular design in the begins to compress, the tibial acceleration curve obtains form of assembling with an X-shaped unit. This unit type the first peak. Then, the energy absorber is continuously presents grading deformation modes with different energy- compressed until its elasticity limit and until the second absorbing rates and force levels. The results reveals that the acceleration peak is formed. Further, the elastic limit is new bumper energy absorber proposed in this paper adap- exceeded and a self-locking status of the X-shaped unit tively uses different energy absorption modes in different is formed; the third peak is obtained. The maximum knee collision forms based on the structural characteristics of its bending angle and shear displacement are also significantly own X-shaped unit and rapidly increases the energy reduced by 50% (Figures 6(c) and 6(d)) to 4.5 and absorption capacity after self-locking. So, it performs a better 1.93 mm, respectively. All these indicate that the X-shaped comprehensive performance compared to the traditional bumper energy absorber adaptively adopts the small defor- foam-type energy absorber by effectively controlling the force mation mode in the pedestrian safety test due to the low level and energy-absorbing rate. The modular design also impact energy. indicates its easy changing and fixing. In the low-speed impact test as shown in Figure 6(e), the Besides, the multiobjective optimization of the structural maximum deformation of the bumper has a significant parameters is performed for the detailed design of the new decline when comparing the new X-shaped energy absorber bumper energy absorber. The pedestrian protection and with the traditional foam absorber. At the initial stages of low-speed impact performance of the new energy absorber 0~30 ms, the X-shaped units are in the deformation phase with optimized structural parameters are greatly improved, before two arcs are in contact and the two sides of the arc and the requirements of pedestrian safety and low-speed are in contact with each other to form a self-locking struc- impact are better balanced. ture, reaching a peak of 90 ms while the energy absorption capacity rapidly increases. It is revealed that the new bumper Conflicts of Interest energy absorber adaptively adopts the large deformation mode in the low-speed collision test, which absorbs more The authors declare that they have no conflicts of interest. energy and significantly reduces the bumper deformation peak, as shown in Figure 6(a). In Figure 6(f), the maximum value of the collider intrusion has also been largely reduced Acknowledgments due to the structure optimization. All of the above indicates that the new X-shaped energy This work was supported by the National Natural Science absorber shows a better performance in the present bumper Foundation of China (Grant nos. 51405150 and 51475154). 8 Applied Bionics and Biomechanics [15] M. K. Shin, S. I. Yi, O. T. Kwon, and G. J. Park, “Structural References optimization of the automobile frontal structure for pedestrian [1] M. M. Davoodi, S. M. Sapuan, A. Aidy, N. A. Abu Osman, protection and the low-speed impact test,” Proceedings of the A. A. 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Applied Bionics and BiomechanicsHindawi Publishing Corporation

Published: Jan 14, 2018

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