CRASH INVESTIGATION OF SIDE INTRUSION BEAM DURING HIGH AND LOW PRESSURE TUBE HYDROFORMING

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1 4TH INTERNATIONAL CONFERENCE ON TUBE HYDROFORMING (TUBEHYDRO 2009) September 6-9, 2009, Kaohsiung, Taiwan. CRASH INVESTIGATION OF SIDE INTRUSION BEAM DURING HIGH AND LOW PRESSURE TUBE HYDROFORMING C. Nikhare 1, M. Weiss 1 and P. D. Hodgson 1 1 Centre for Material and Fibre Innovation, Faculty of Science and Technology, Deakin University, Waurn Ponds 3217, Australia Abstract: Advanced high strength steels (AHSS), in particular, are an attractive group materials, offering higher strength for improved energy absorption and the opportunity to reduce weight through the use of thinner gauges. High pressure tube hydroforming (HPTH) has been used to produce safety components for these steels, but it is expensive. Low pressure tube hydroforming (LPTH) is a lower cost alternative to form the safety components in the car. The side intrusion beam is the second most critical part after front rail in the car structure for passenger safety during crash. The forming as well as crash behaviour of a square side intrusion beam from both processes was investigated using numerical simulation. This paper investigated the interaction between the forming and crash response of these materials in order to evaluate their potential for use in vehicle design for crashworthiness. The energy absorption characteristics of the different tubes were calculated and the results from the numerical analyses compared for both hydroforming process. Keywords: Advanced High Strength Steels, Hydroforming, Crashworthiness, Low pressure 1. INTRODUCTION The most complicated system in the universe is the human body. Many of the system characteristics are not fully understand. This includes knowledge of the static, elastic and plastic behaviour of steel structures and their interaction during crash with the body [1]. Research continues to fully understand the reasons for body injuries during crash. An adequate relationship between increasing loading and increasing injury severity for all body regions is not yet fully established [2]. Thus to incorporate the human body into the vehicle design space, it is necessary to identify the load-carrying structures and the body regions and organs at risk of injury from impact loading. A side impact crash generates lateral loading on the body of the vehicle occupant. Therefore, the understanding of the side impact crash and the required characteristics for occupant protection is an important area of research. Another key area of research at present is to reduce the mass of vehicles for improved fuel consumption without compromising the better crashworthiness. Hydroforming is a metal forming process that is now widely used as it can achieve weight reduction of about 30% compared to conventionally manufactured components [3]. At the same time automakers are increasingly exploring the potential to use advanced high strength steels as they can also provide weight reduction without any reduction in other performance characteristics such as crash and durability. The tube hydroforming process can be categorised into three pressurization systems: 1) Low pressure hydroforming (P<83MPa) 2) Multi-pressure hydroforming (P = 69 to 173MPa) and 3) High pressure hydroforming (P = 83 to 414MPa) [4]. Most research to date has focussed on high pressure hydroforming particularly to improve the quality of product [5, 6, 7 and 8]. In comparison the research on low pressure hydroforming is limited. However, one of the attractions of this process is that it requires much lower pressures [9 and 10] and it is of note that the high pressures above were for simple low carbon structural steels. For the advanced high strength steels the stresses required to deform the metal are much higher and hence the pressure requirements are further increased. Although these technologies alter the potential to form advanced high strength steel to crash structures, here is little reported research and it has mostly been to frontal crash [11]. The objective of this paper was to develop a forming process that would optimise the protection of vehicle occupants involved in a side impact crash. High and low pressure tube hydroforming was used to generate the side intrusion beam. The process parameters for both processes were studied. Further the crash behaviour of side intrusion beam produced from both processes was analysed. Energy absorption during crash was critically studied. 107

2 2. MATERIAL AND METHODOLOGY 2.1 Material The steel types investigated in this study are commercial TRIP 780 and MART 900 grades. The mechanical properties of both steel grades are shown in Table 1. Table 1. Mechanical properties of TRIP 780 and MART Methodology In this study the high pressure tube hydroforming (HPTH) and the low pressure tube hydroforming (LPTH) process were applied to produce side intrusion beams of the same shape which were then numerically studied with regard to their crashworthiness. This included the investigation and comparison of the pressure and die closing forces and the determination of the stress and thickness distribution within the part wall after forming of TRIP 780. Using a numerical model of the side impact test the crashworthiness of tubes made of TRIP 780 and MART 900 was determined and compared High Pressure Tube Hydroforming (HPTH) The most commonly used tube hydroforming set up is shown in Fig. 1. The lower die is generally fixed with the tube placed in it, while the upper die moves down and closes the tool gap. The tube is then filled with an incompressible fluid and pressurized to form the tube into the desired shape. During this process the tube needs to be stretched to fill the die corners. To allow for a reproducible comparison between the high and the low pressure hydroforming process, the same part geometry was used for both processes (Fig. 4, with dimension). Figure 1 Start of High Pressure Tube Hydroforming (HPTH) Figure 2 Pre-form tube and start of Low Pressure Tube Hydroforming (LPTH) Low Pressure Tube Hydroforming (LPTH) In LPTH the desired shape is obtained using lower fluid pressures than in HPTH. The tube is located between the upper and lower die and pressurized. While the lower die is fixed, the upper die moves down and forces the tube into the required shape. In low pressure hydroforming, the hydroformed section length of line stays approximately the same as the circumference of the undeformed tube. Thus the perimeter of outer un-deformed tube must be the same as the inner perimeter of the die (Eq. 1). Therefore to obtain the same final part in this study the final wall thickness of the tube formed with high pressure hydroforming process was used as the initial wall thickness of the tube formed using low pressure hydroforming. Perimeter of Final Product = Perimeter of Initial tube for LPTH (1) 108

3 The LPTH process is illustrated in Fig. 2. In the first step the tube is pre-formed to make it fit into the lower die. In the second step the upper die is moved down Side impact test The crashworthiness of the produced tubes was numerically investigated by simulation of the side impact test (Figure 3). In this test the intrusion beam was placed on two fixed rollers having a diameter of 10mm which were situated 200mm apart. An Indenter with a diameter of 10mm was then accelerated and impacted on the intrusion beam and the load versus displacement relationship was measured on the indenter. Figure 3 Side Impact test Numerical Modeling In the numerical model the tube was assumed to be a cylinder and variations in wall thickness or material properties were neglected. For the HPTH process a tube with a material thickness of 2mm and an outside diameter of 50mm was studied while in the case of LPTH an overall material thickness of 1.75mm and a tube outside diameter of 57.12mm was used. In this way the same tube material volume was used in both processes. The tube length was limited to 400mm. Material input data based on the power law (equation (2)) and an isotropic plasticity model was applied to define the material properties. n σ = Kε (2) where, σ = True stress ε = True strain K = Strength coefficient n = Strain hardening exponent Figure 4 Required part with dimension The dies, indenter and rollers were considered as rigid bodies, while for the tube, deformable S4R 4-node double curved thin shell elements were applied. The interaction between the tube and the tooling was assumed to be frictionless. The internal fluid pressure was increased continuously in the high pressure hydroforming process while it was kept constant in the low pressure hydroforming process. In the side impact test the indenter was dropped onto the formed beam with a gravitational force of 17.6kN. The contact between the beam, the indenter and the roller was defined as frictionless. 109

4 3. RESULT AND DISCUSSION Fig. 5 shows the formed tubes after HPTH and LPTH of TRIP 780. While in HPTH an internal fluid pressure of 155MPa and a die closing force of 4000N/mm was necessary to form the tube, in LPTH an internal pressure of 10MPa and a die closing force of 2300N/mm were sufficient. Figure 5 Deformed tube made of TRIP 780 after HPTH and LPTH In Fig. 6(a) and (b) the thickness distributions and the relative thicknesses along the half tube circumference are shown for tubes formed of TRIP 780 using both hydroforming processes. In Fig. 6(b) it is clear that the HPTH forming process leads to severe thinning of the tube wall while in the LPTH the thickness variation is negligible. (a) (b) Figure 6 Thickness (a) and relative thickness (b) of the vertical deformed tube after HPTH and LPTH of TRIP 780 steel The average von-mises stress determined in the tube wall after forming is shown in Fig. 7. While positive wall stresses are generated during HPTH negative stresses are observed in the tube wall after LPTH. This indicates that in HPTH the tube elements are severely stretched while in LPTH the material is only sightly compressed. This result indicates that in HPTH the material is higly deformed and through that strain hardened while only a small amount of strain hardening is introduced in the LPTH process. HPTH, therefore, led to an average strength of the formed tube at ~900MPa. This formed the basis to the choice of a martensite grade with the same yield. 110

5 Figure 7 Stress in the vertical deformed tube after HPTH and LPTH of TRIP 780 steel (a) (b) Figure 8 TRIP 780 steel crash tubes (a) crash after HPTH and (b) crash after LPTH The impacted tubes are shown in Figure 8 while the load displacement responses measured on the indenter can be seen in Figure 9. In Figure 9a it is clear that the TRIP steel tube from the LPTH process shows lower reaction forces and higher deformation compared to tube that had been formed using the HPTH process, i.e., the LPTH formed tube shows less crashworthiness. By this it is mean that the energy absorbed in the initial stage of intrusion is greater to the HPTH to be and, therefore, the amount at intrusion is less. (a) (b) Figure 9 Load versus displacement (a) crash after HPTH and LPTH of TRIP steel and (b) crash after HPTH (TRIP steel) and LPTH (MART steel) 111

6 In an attempt to improve the crashworthiness of LPTH tubes, the use to form a LPTH of Martensite steel (MART 900) was simulated and the load displacement response during impact testing investigated. In Figure 9b the load displacement responses determined for the high pressure tube hydroformed TRIP steel tube and the low pressure hydroformed Martensite tube are compared. It is clear that tubes made of higher strength material (Martensite) that were formed with the LPTH process can show similar crashworthiness compared to tubes made of TRIP steel that were formed using the HPTH process. An interesting observation is that the work hardening behavior beyond yield (as considered by the n value) has little effect. Additionally to form the part out of a Martensite tube using the LPTH process requires significantly less pressure (10MPa) and a lower die closing force (2850N/mm) compared to the HPTH of the weaker TRIP steel tube. 4. CONCLUSION High and low pressure tube hydroforming processes were studied to form an identical part geometry for crash analysis using Finite Element Analysis (FEA). It was found that the die closing force as well as the internal fluid pressure needed to form the part can be significantly reduced by using the LPTH instead of the HPTH process. Whereas in the HPTH process significant strain hardening was observed which is good for crash of side intrusion beam and increases the load during the initial crash, while in LPTH the strain hardening is much less or negligible. However, using a stronger material with LPTH the same crash performance can be achieved while still reducing the input parameters (such as internal pressure and die closing force) to generate the part. Thus LPTH is a competitive process to produce the parts for side intrusion beam for better crash performance. ACKNOWLEDGEMENT This research was supported by Deakin University and AUTOCRC. The authors gratefully extend their gratitude to Professor John L Duncan from Auckland University, New Zealand. REFERENCES [1] Sparke, L.J., 2006, Vehicle design for minimum societal harm Improving side impact protection, Holden, pp. 43. [2] Teng, T.L., Chang, K.C., Nguyen, T.H., 2008, Crashworthiness evaluation of side-door beam of vehicle, TECHNISCHE MECHANIK 28, pp [3] Lucke, H.C., Hartl, C., Abbey, T., 2001, Hydroforming, Journal of Materials Processing Technology 115, pp [4] Singh, H., 2003, Fundamentals of Hydroforming, Association for Forming and Fabricating Technologies of the Society of Manufacturing Engineers, Michigan. [5] Mori, K., Maeno, T., Maki S., 2007, Mechanism of improvement of formability in pulsating hydroforming of tubes, International Journal of Machine Tools and Manufacture 47, pp [6] Asnafi, N., Skogsgardh, A., 2000, Theoretical and experimental analysis of stroke-controlled tube hydroforming, Materials Science and Engineering A 279, pp [7] Jain, N., Wang, J., 2005, Plastic instability in dual-pressure tube-hydroforming process, International Journal of Mechanical Sciences 47, pp [8] Smith, L.M., Ganeshmurthy, S., Alladi, K., 2003, Double-sided high-pressure tubular hydroforming, Journal of Materials Processing Technology 142, pp [9] Nikhare, C., Weiss, M., Hodgson, P.D., 2008, Numerical investigation of high and low pressure tube hydroforming, Numisheet, Conferrence proceedings. Switzerland, pp [10] Nikhare, C., Weiss, M., Hodgson, P.D., 2008, Experimental and Numerical investigation of low pressure tube hydroforming on stainless steel, Metal Forming, Conferrence proceedings. Poland, pp [11] Abedrabbo, N., Mayer, R., Worswick, M., Riemsdijk, I., 2008, Crash response of hydroformed high strength steel tubes, Numisheet, Conferrence proceedings. Switzerland, pp