The 3 rd TSME International Conference on Mechanical Engineering October 2012, Chiang Rai

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1 P including Paper ID Crush Response of Polyurethane Foam-Filled Aluminium Tube Subjected to Axial Loading Nirut Onsalung*, Chawalit Thinvongpituk, Visit Junchuan and Kulachate Pianthong Department of Mechanical Engineering, Faculty of Engineering, Ubon Ratchathani University, Warinchamrab, Ubonratchathani, *Corresponding Author: Tel , Fax Abstract In this paper, the experimental investigation of polyurethane (PU) foam-filled into circular aluminum tubes subjected to axial crushing was presented. The purpose of this study is to improve the energy absorption of aluminium tube under axial quasi-static load. The aluminium tube was made from the AA6063-T5 aluminium alloy tubes. Each tube was filled with polyurethane foam. The density of 3 foam was varied from 100, 150 and 200 kg/mp with empty tube. The range of diameter/thickness (D/t) ratio of tube was varied from The specimen were tested by quasi-static axial load with crush speed of 50 mm/min using the 2,000 kn universal testing machine. The loaddisplacement curves while testing were recorded for calculation. The mode of collapse of each specimen was analyzed concerning on foam density and the influence of D/t ratio. The results revealed that the tube with foam-filled provided significantly increment of the energy absorption than that of the empty tube. While the density of foam and D/t ratios increase, the tendency of collapse mode is transformed from asymmetric mode to concertina mode. Keywords: polyurethane foam, energy absorption, axial load, foam density, aluminium tube. 1. Introduction Impact safety and weight saving are two important design target for energy absorbing structures in the automotive industry. When using aluminium in the body structure of a vehicle, weight savings of as much as 25% may be possible compared with conventional steel structures. Automotive structure should be having extreme strong component for protect the passenger room when accident occurs. In general, the vehicle structures will be used steel or aluminium tube as different cross-sectional for main structures because it can easily find out by local and transform in various shapes application. The most of experimental and numerical investigations have been conducted aiming at determining and understanding the crushing behaviour of metallic tube to increasing efficiency of the energy absorption. To meet that requirement, new material for structural applications such as fiberreinforced composite was introduced and investigated [1]. In another approach, the use of lightweight materials fill in automotive structures may result in many potential economic and functional benefits due to their improved properties. The filling of structural member with foam material for increasing energy absorption capacity has also been taken considerable interest. The previous article on crushing behavior of foam-filled metallic tubes are conducted by Hanssen et al. [2]. He studies the influence of foam density of aluminum foam-filled into square aluminium tubes under quasi-static axial loading. A year later, he developed an empirical model for the mean crushing load of foam filled circular and square tube by using experimental investigation under quasi-static and dynamic loading condition [3]. The numerical and experimental studies on the crush response of foam-filled sections under axial loading are continuously studied by researchers [4, 5]. Recent study of Mamalis et al. [6] revealed that the use of aluminium foam and polyurethane (PU) foam may enhance the structural crashworthy of rectangular tube. Some investigators have conducted studies concerning on other type of foam-filled into structures with circular and rectangular tubes [7]. Polyurethane foam are one of the low density or light weight material that often used for reduce the weight of structure which investigating by Niknejad [8]. The influence of foam density of polyurethane foam-filled steel tubes under quasi-static loading was also study by Onsalung et al. [9, 10]. The earliest investigations on the crushing behavior of thin-walled metallic tube filled with polyurethane foam was conducted by Guillow et al. [11], who study the average crushing loads of polyurethane foam-filled thin-walled aluminium tubes under quasi-static and dynamic condition. The results show that the crushing load of foam-filled tube

2 are greater than the sum of the average crushing loads of empty tube (alone) and foam (alone). This result is contrasted with those of Reid et al. [12]. Tu et al. [13] studied the mechanical response of rigid polyurethane foams under axial compression in the rise and transverse direction. Shim et al. [14] study the effects of velocity and geometry of rigid polyurethane foam blocks subjected to axial impact loading by free fall drop weight. In addition, the use of foam-filled in other geometry sections has also investigated such as S- shape [15], hat-sections [16-18], and conical tubes [19]. The aim of this paper is investigated the influence of polyurethane foam-filled circular aluminium tubes. The influence of foam density, the D/t ratio, and collapse mode are focused base on the energy absorption capacity. 2. Methodology 2.1 Definition of parameters Generally, there are various kind interested characterization parameters that used to evaluate the performance of the energy absorbing device. In this paper the energy absorption and the load efficiency are used as key indicators. The energy absorption (E a ) is defined as an integration of the area under load-displacement curve in Fig.1 which can be calculated and approximated by Eq. (1). Where S is the maximum crush distance, P is the instantaneous crushing load, P e is the load efficiency and P max is maximum point of load on load-displacement curve. 2.2 Polyurethane foam Polyurethane (PU) foam is one of the commonly known low density foam material. It is being used as filling material in automotive structure. The rigid polyurethane foam is a mixture of two chemical substances. They are polyester polyol and isocyanate, both substances are mixed in a ratio of 1:1 in liquid form. The mixture is injected into the specimens with different density. The mixture is allowed to expand inside the tube for 5 minutes. Finally the rigid polyurethane foam with certain density was achieved inside each tube. The mechanical property of polyurethane foam was achieved from compression test. The PU foam specimens were produced in cubic shape with about mm 3 width and tested by uniaxial compressive tests at speed of 5 mm/min. Figure 2 shows engineering stress-strain curve of PU foam in compressive test. Fig. 2 The stress-strain curve of PU foam. Fig. 1 Schematic load-displacement curve for explains the parameter in study. The load efficiency (P e ) is defined as the ratio of mean load to maximum load. This parameter illustrates to the uniformity of energy absorption behaviour of structure. Lower load efficiency implies the rapidness of deceleration or acceleration which may harmful to human. Equation (2) is the equation to calculate P e. 2.3 Specimen and preparation The test tubes in this study are made from aluminium alloy tubes (AA6063-T5). The specimen is cut from tube wall along the axial direction of a tube for tension testing. The standard test method (ASTM E8M) of metallic materials is applied using universal testing machine. a S mean (1) 0 E = PdS P S Pe P mean /Pmax (2) Fig. 3 The stress-strain curve of AA6063-T5.

3 Stress and strain are recorded digitally using PC controller unit. Figure 3 show the engineering stress-strain curve of aluminium tubes obtain from tensile test. The mechanical properties of aluminium are the density about 2,700 kg/m 3, Young s modulus 69 GPa, ultimate tensile strength 245 MPa, yield strength 187 MPa and Poisson s ratio 0.3. The compositions in weight of aluminium tube are of Mg = 0.5, Si = 0.44, Cu = 0.03, Fe = 0.17, Mn = 0.07 and Zn = An experimental programme consisting of total 60 tests was carried out to study. The reaction force and displacement were recorded to calculate for energy absorption. 3. Results and Discussion 3.1 Collapse mode Example deformation histories of some specimens are shown in Fig. 6. Similar progressive pattern was also observed in other specimens. Fig. 4 The specimen before and after foam-filled tube with its dimension. The circular tubes with five different value of D/t ratio were applied in the present study. The PU foam is injected into that test tube. The original test tube specimens before filling PU foam are 200 mm long. Then, it will be cutting at the both end to 150 mm long. Each D/t ratios are consisting of the empty and foam-filled tube which varies the density of foam, e.g. 100, 150, and 200 kg/m 3. Typical specimens before and after filling foam are shown in Fig Test procedure The empty and foam-filled tubes were crushed by a 2000 kn ESH testing machine shown in Fig.5. (a) Concertina mode (b) Mix mode Fig.5 The universal testing machine and experimental setup. The specimen was tested under simply supported using a speed test of 50 mm/min. The crush stroke was 60% of original height until fully collapse. Three specimens were tested for each case in order to receive more accurate results. (c) Diamond mode Fig. 6 Deformation histories of some specimens corresponding to load-displacement curve.

4 When the specimen was axially crushed, it normally deforms in some mode. The progressive collapse of foam-filled tubes will form folding lobe from one end. Then, as the previous lobe is complete, the adjacent folding lobe is formed on top and stack on each other. This process continues in series until the test is terminated. Figure 6(a) shows the typical collapse mode of crush tube in concertina (axisymmetric) mode which corresponding to the pattern of loaddisplacement curve. In Fig. 6(b) is the collapse mode of mix mode (concertina with diamond) and Fig. 6(c) is the diamond (asymmetric) mode. The final collapses of empty and foam-filled tube are summarized in Table 1. Table 1 Typical collapsed mode in the study. D/t ratio and the curve is called load-displacement curve. The load displacement curves of the specimen are used to determine mean compressive forces and energy absorption characteristics. Figure 7 (a)-(e) shows the typical load-displacement curves of the empty and foam-filled tube. (a) D/t Foam density (kg/m 3 ) (b) D/t Considering the empty tube, it found that D/t ratio and deforms in diamond mode. If the D/t ratios increase to 33.87, 42.33, and 50.8 the test tube becomes collapse in mix mode. In case of foam-filled tube with density 100 and 150 kg/m 3 with D/t ratio and 21.17, the tube is collapse in mix mode and collapse from the both end. Then, at D/t ratio 33.87, 42.33, and 50.8, the specimen is transformed into concertina mode and also increasing the number of folds. In case of foam-filled tube with 200 kg/m 3, every D/t ratios are deformed in concertina mode. The deformation mode in Table 1 indicates that while the density of foam and D/t ratios increase, the tendency of collapse mode is transformed from asymmetric mode to concertina mode. The results suggest that the number of folds of deformed tubes tends to increase as the density of foam increased. It should be noted that the concertina mode is mainly expected mode in this study. 3.2 Load-displacement history During the empty and foam-filled tubes are being crushed, the displacement and reaction force are recorded. These data are, then, plotted (c) D/t (d) D/t 42.33

5 (e) D/t 50.8 In order to compare, the energy absorption of tubes with different D/t ratio are plotted and shown in Fig. 8. The result shows that the energy absorption of each D/t ratios is increase when the density of foam increases. Table 2 shows the calculating result of energy absorption and the load efficiency obtained from load-displacement curve. The last column in Table 2 show the mode of collapse of test tubes i.e. C is concertina mode, M is the mix mode, and D is diamond mode. Fig.7 Load-displacement curve of test tubes with different D/t ratios and densities. Considering foam-filled tube each D/t ratio, while density of foam increase the load values is also increased but the folds length in the loaddisplacement curve will be reduce when compare with folds of empty tube. When the D/t ratios increase the number of folds on loaddisplacement curve tends to increasing too. Mostly the load-displacement curves of each D/t ratio will have peak value at the first lobe and then it is observed that the curves are normally fluctuating in wavy shape. In general, the load-displacement curves of foam-filled tubes lie above the empty tube s curves. Each loop of curve is corresponding to the lobe in the deformed tubes. This indicates that the foam-filled tube may absorb more energy than the empty one. 3.3 Energy absorption Energy absorption of each tube is defined as the energy required to cause the collapse mode observed. This energy is converted firstly into elastic strain energy in the deformed tube and the remaining is dissipated in plastic deformation during collapse. The energy absorption and load efficiency of empty and foam-filled tubes are calculated by Eqs. (1)-(2). Fig. 8 Calculation result of the energy absorption Table 2 The important results of tested tubes obtain from load-displacement curve. Foam D/t E density a P e Collapse ratio mode kg/m 3 kj % D M M C D C M C M C C C M C C C M C C C Considering between the empty tube and foam-filled tube with 200 kg/m 3, the results is found that the D/t ratio of can be absorb energy more than the empty tube about % while the tube with D/t ratio of 21.17, 33.87, 42.33, and 50.8 can absorb energy more than empty tube about %, %, %, and % respectively. However, the specimen of D/t ratio and can absorb highest energy than other tube. This is the influence of diameter and tube thickness, in another word the specimen is thicken. It could be seen that the foam filled tubes with D/t ratio of absorb almost equal energy as the D/t ratio When increase the density of foam each foam-filled tube can be

6 absorb more and more energy. The experiment results indicate that the increment of energy absorption is increased because the influence of D/t ratios rather than the foam density. 3.4 Load efficiency Fig. 9 Calculation result of the load efficiency The load efficiency of each tube is plotted in Fig. 9. It is observed that the specimens of D/t ratios of are more efficiency than other D/t ratios. The specimens with lower D/t ratio tends to absorb energy more efficiency. The empty and foam-filled tubes of D/t ratio have load efficiency higher than D/t and about 14.85% and 36% respectively. In case of D/t ratio and 50.8 of are almost the same the load efficiency. When considering the empty and foam-filled tube each case based on the load efficiency, it is found that the efficiency will be increased as the foam density increases. 4. Conclusion and Remarks This paper has investigated and described the crushing load and energy absorption responses of empty and PU foam-filled circular aluminium tubes under quasi-static axial loading. It is found that the number of folds in deformed tube is increasing as D/t ratios increase. When the D/t ratios increase, the empty tube tends to change their mode from diamond mode to mix mode while the foam-filled tube changes from mix mode to concertina mode. The results show that the foam filled tubes tends to fail into concertina mode because the influence of foam density and D/t ratio. In addition, when the foam densities increase it helps lifting the whole loaddisplacement curve while the initial peak is about the same. Consequently, the load efficiency of foam-filled tube is higher. 5. Acknowledgement The authors would like to thank the office of the higher education commission, Thailand for supporting grant under the program strategic scholarships for frontier research network for the Ph.D. program Thai doctoral degree for this research. 6. References [1] Savage, G., Bomphray, I., and Oxley, M. (2004). Exploiting the fracture properties of carbon fibre composites to design lightweight energy absorbing structures, Engineering Failure Analysis, vol. 11(5), 2004, pp [2] Hanssen, A.G., Langseth, M., and Hopperstad, O.S. (1999). Static crushing of square aluminium extrusions with aluminium foam filler, International Journal of Mechanical Sciences, vol. 41(8), 1999, pp [3] Hanssen, A.G., Langseth, M., and Hopperstad, O.S. (2000). Static and dynamic crushing of square aluminium extrusions with aluminium foam filler, International Journal of Impact Engineering, vol. 24(4), 2000, pp [4] Meguid S. A., Attia M. S., and Monfort A. (2004). On the crush behaviour of ultralight foam-filled structures, Materials & Design, vol. 25(3), 2004, pp [5] Zhang, C.J., Feng, Y., and Zhang, X.B. (2010). Mechanical properties and energy absorption properties of aluminum foam-filled square tubes, Transactions of Nonferrous Metals Society of China, vol. 20(8), 2010, pp [6] Mamalis, A.G., et al. (2009). On the crashworthiness of composite rectangular thinwalled tubes internally reinforced with aluminium or polymeric foams: Experimental and numerical simulation, Composite Structures, vol. 89(3), 2009, pp [7] Hanssen, A.G., Langseth, M., and Hopperstad, O.S. (2000). Static and dynamic crushing of circular aluminium extrusions with aluminium foam filler, International Journal of Impact Engineering, vol. 24(5), 2000, pp [8] Abbas Niknejad, et al. (2011). Theoretical and experimental studies of the instantaneous folding force of the polyurethane foam-filled square honeycombs, Materials and Design vol. 32, 2011, pp [9] Onsalung, N., Thinvongpituk, C., and Pianthong, K. (2010). The influence of foam density on specific energy absorption of rectangular steel tubes, Energy Research Journal, vol. 1(2), 2010, pp [10] Onsalung, N., Thinvongpituk, C., and Pianthong, K. Crush characteristic of foam-filled

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