CRASHWORTHINESS OF AUTOMOBILE IN A VEHICLE-TO-POLE CRASH SIMULATION

Transcription

1 International Journal of Automobile Engineering Research and Development (IJAuERD) ISSN(P): ; ISSN(E): Vol. 4, Issue 2, Apr 2014, 1-10 TJPRC Pvt. Ltd. CRASHWORTHINESS OF AUTOMOBILE IN A VEHICLE-TO-POLE CRASH SIMULATION H. D. GOPALAKRISHNA, PARIMESH PANDA & PAVAN JOIS & NIKHIL P N Department of Mechanical Engineering, R.V. College of Engineering, Bengaluru, Karnataka, India ABSTRACT Crash Simulation is a virtual recreation of a destructive crash test of a car by using computer simulation software in order to examine the level of safety of the car and its occupants. This paper was an effort to analyze the crashworthiness of the vehicle for different feasible materials, which are not only light weight but also have high energy absorption capacity while undergoing plastic deformation. Finite element model of a vehicle was developed in UG NX 6.0 and it was solved for full frontal impact in ANSYS Workbench 14.5 by using explicit dynamics code. Automobile s crashworthiness was realized by plotting the graph of impact energy against time and sensitivity was realized by plotting the graph of reaction force against time. The analysis was limited to full frontal impact of an automobile at a speed of 35 mph with rigid pole, in accordance with NHTSA (National Highway Traffic Safety Administration). KEYWORDS: Computer Simulation, Crashworthiness, Explicit Dynamics Code, Impact Energy, Sensitivity INTRODUCTION A vehicle is expected to provide adequate protection to driver and passengers in a catastrophe. To protect the occupants of a car, there are many new perceptible safety features such as airbags, anti-lock braking system and traction control. A less tangible feature that cannot easily be seen by drivers and passengers is the crash response behavior. In a well-designed automobile, the car body and various components are the protective layer for the occupants of the vehicle. They serve as the crumpling zone to absorb the energy of impact (Vikas Sharma et al. 2013). Car body light weight and crashworthiness are the two important aspects of auto-design (Tony Punnoose Valayil et al. 2013). With the development of technology, people have more and more stringent demands for automobile passive safety and fuel economy, which requires the improvement of automobile structure crashworthiness and light weighting degree (Yuxuan Li et al. 2003). A major concern of both the industry and government is the development of vehicles that would consume less fossil fuel, thus compromising the safety of occupant resulting from the reduced weight of the automobile(z.q. Cheng et al. 2001). Crashworthiness is an engineering term used to define the ability of a vehicle s structure to protect its occupants during an impact. The structure must fulfill the following requirements to be crashworthy- The structure must absorb as much impact energy as possible by plastically deforming in a controllable manner to minimize the remaining impact energy which can then be handled by the restraint system. The structure must preserve at least the minimum survival space to keep injury and fatality levels as low as possible. Crashworthiness improvement can be approached in two ways: (1) Geometry optimization for crashworthiness and (2) Material optimization for crashworthiness (Hesham Kamel Ibrahim 2009). This paper focuses on material

2 2 H. D. Gopalakrishna, Parimesh Panda & Pavan Jois & Nikhil P N optimization for crashworthiness.today s automobile manufacturers are increasingly using lightweight materials to reduce weight; these include plastics, composites, aluminium, magnesium and new types of high strength steels. A light car will always experience a higher deceleration level in a crash test as compared to its heavier opponent, as deceleration is the ratio of crush load and mass. So, only those lightweight materials which possess high stiffness values can replace heavy weight materials. Many of these materials have limited strength or ductility. Rupture is a serious possibility during the crash event. Furthermore, joining of these materials presents another source of potential failure. Both material and joining failure will have serious consequences on vehicle crashworthiness and must be predicted (Waseem Sarwar et al. 2007). In this paper, (1) Steel V-250 (2) Aluminium NL (Non-Linear) (3) Aluminium 6061-T6 (4) Polycarbonate are considered for crashworthiness. GEOMETRIC MODELING AND MESHING Body panel is designed using the modeling software UG NX 6.0. A set of equally spaced cross sections are generated along the entire length of the body panel. The body panel is developed using synchronous modeling feature of UG NX 6.0. The model has dimensions of 748mm*2902mm*672mm as shown in the Figure 1.The body panel acts as a protective layer for the occupants of the vehicle. The frontal body panel structure behaves as the crumpling zone which absorbs the energy of impact during a crash. So, the body panel is the prime concern. The projected area of the body panel on X-Z plane is 50.27*10 4 mm 2 (748mm*672mm). The model is imported to ANSYS Workbench 14.5 Design Modeler. The pole against which the shell is subjected to a crash is modeled in ANSYS Workbench 14.5 Design Modeler. Figure 2 shows the pole model with the body panel. The pole is modeled at a distance of 2.25mm from the tip of the body panel. The pole diameter and height is considered to be 216mm and 600mm respectively. The projected area of the pole on X-Z plane is 12.96*10 4 mm 2 (216mm*600mm), which accounts for 26% of the body panel s projected area. Figure 1: Top View & Side View of Body Panel Figure 2: Crash Model Impact Factor(JCC): Index Copernicus Value(ICV): 3.0

3 Crashworthiness of Automobile in A Vehicle-to-Pole Crash Simulation 3 The meshing of model is done by using ANSYS mesh feature in ANSYS Workbench 14.5 Mechanical. SHELL163 is a 4-node element with both bending and membrane capabilities. Both in-plane and normal loads are permitted for such elements. This element has 12 degrees of freedom at each node: translations, accelerations and velocities in the nodal x, y and z directions and rotations about the nodal x, y and z-axes. Thus, the element type of body panel is chosen as SHELL163. This element is used in explicit dynamic analyzes only. SOLID186 is a higher order 3-D 20-node solid element that exhibits quadratic displacement behavior. The element is defined by 20 nodes having three degrees of freedom per node: translations in the nodal x, y and z directions. Thus, the element type of the pole is chosen as SOLID186. While the mesh pattern on the body panel consisted of 5372 nodes and elements, the mesh pattern on the pole consists of 936 nodes and 697 elements. ANALYSIS SETTINGS The type preference is kept as high velocity because the objective is to simulate a high velocity crash. Since, most of the explicit dynamics problems require more than 10 5 cycles, thus, the number of computational cycles for this crash analysis is set as 10 7 cycles. Due to computational capability and computational time limitations, all the crash simulations are done for an impact time of 1 ms. Maximum energy error for the solver is restricted to 0.1. BOUNDARY CONDITIONS The stiffness behavior of the pole is set as RIGID.The stiffness behavior of the model is set as FLEXIBLE. The simulation is done to analyze a high velocity crash test. So, the body panel is given an initial velocity of 15*10 3 mm/s i.e. 54kmph or 35mph in negative Y-direction as per the NHTSA (National Highway Traffic Safety Administration) standards. According to the assumptions stated above, the body panel is also given an acceleration of 1400mm/s 2 in negative Y-direction (considering the body accelerates from 0 to 54kmph in 10sec). MATERIAL PROPERTIES Table 1 and Table 2 list the important material properties of Steel V250, Aluminium 6061-T6, Polycarbonate, Aluminium NL (Non-Linear) and Structural Steel NL (Non-Linear). Table 1: Material Properties of Steel V250, Aluminium 6061-T6 and Polycarbonate Material Name Steel V250 Aluminium 6061-T6 Polycarbonate Density (kgm -3 ) pecific eat g C) Shear Modulus (Pa) 7.18 E E+10 1 E+09 Gruneisen Coefficient Parameter C1(ms^-1) Parameter S Parameter S2(sm^-1) Table 2: Material Properties of Aluminium NL (Non-Linear) and Structural Steel NL (Non-Linear) Material Name Density Young s Poisson s Yield Tangent (kgm -3 ) Modulus(Pa) Ratio Strength(Pa) Modulus(Pa) Aluminium NL E E E+08 Structural Steel NL E E E+09

4 4 H. D. Gopalakrishna, Parimesh Panda & Pavan Jois & Nikhil P N ASSUMPTIONS The following assumptions are made in the simulation of a crash The body panel is approximated to be the whole vehicle. The pole is assumed to be a rigid support. No friction is considered at the base of body panel. Effect of braking and deceleration just before the crash is neglected. The body panel is subjected to an initial velocity and constant acceleration. Air drag on the body panel s surface is neglected. RESULTS AND DISCUSSIONS Crashworthiness In an impact, the crash structure must dissipate the energy of the impact whilst ensuring that the occupants of the vehicle are not subjected to excessive acceleration. It should also ensure that the survivable zone within the car remains intact (i.e. the crash structure does not ingress too far into the vehicle). This energy dissipation is achieved through plastic work done in deforming the material in the crash structure. Therefore, by comparing the capacity for energy dissipation of different materials and considering their mass, it is possible to assess which material would provide optimal energy dissipation per unit mass. The ratio of internal energy absorbed and mass is referred to as Specific Energy Absorption (SEA). In this paper, SEA is treated as the figure of merit which is used to analyze the crashworthiness of different materials. Figure 3, 4, 5 and 6 depict the plot of Internal Energy against time for Steel V250, Aluminium NL (Non-Linear), Aluminium 6061-T6 and Polycarbonate respectively. The body panel does not absorb any energy till the time it comes in contact with the pole. The rate of energy absorption is low till the material continues to deform elastically. The rate of energy absorption increases once the body panel begins to deform plastically. Energy absorbed by body panel is nothing but the kinetic energy lost by the body panel during the crash. This is in accordance with the Conservation of Energy principle. Figure 3: Energy Absorbed by Steel V250 Impact Factor(JCC): Index Copernicus Value(ICV): 3.0

5 Crashworthiness of Automobile in A Vehicle-to-Pole Crash Simulation 5 Figure 4: Energy Absorbed by Aluminium NL (Non-Linear) Figure 5: Energy Absorbed by Aluminium 6061-T6 Figure 6: Energy Absorbed by Polycarbonate Table 3 lists the energy absorbed, mass and SEA of the respective materials. From Table 3, it is evident that Polycarbonate has lowest mass (due to very low density of 1200 kg/m 3 ) but absorbs a considerable amount of energy, thus possessing a very high SEA value. On the other hand, Steel V250 absorbs highest amount of energy but it is the heaviest material (due to very high density of 8129 kg/m 3 ). Aluminium NL (Non-Linear) and Aluminium 6061-T6 have both mass and energy absorption capacity in the same range. It can also be seen that Steel V250, Aluminium NL (Non-Linear) and Aluminium 6061-T6 have their SEA values in the same range. It is very clear that composite structures have dramatically higher SEA than metallic structures.

6 6 H. D. Gopalakrishna, Parimesh Panda & Pavan Jois & Nikhil P N Table 3: Specific Energy Absorption for Different Materials Material Maximum Energy Absorbed (kj) Mass (kg) SEA(J/kg) Steel V Aluminium NL(Non-Linear) Aluminium 6061-T Polycarbonate Sensitivity Figure 7, 8, 9 and 10 depict the plots of Reaction Force (suffered by pole) against time when a body panel made up of Steel V250, Aluminium NL (Non-Linear), Aluminium 6061-T6 and Polycarbonate hits the pole respectively. The pole does not experience any reaction forces when there is no physical contact between the pole and the body panel. The moment when the body panel just hits the pole, the pole suffers a very high reaction force suddenly. Initial sudden abruptions in the reaction force can be attributed to the shocks induced in the pole during continuous crushing of the body panel against the pole. The reaction force suffered by the pole gradually keeps on decreasing as the impact time increases. The shocks induced in the pole gradually die down. Since, the body is having a constant acceleration; it continues to exert a constant but comparatively less force on the pole even after it is plastically deformed. Thus, the pole suffers a continuous non-zero reaction force. Reaction Force is induced in the pole as an opposition to the applied force by the body panel. This is in accordance with Newton s Third Law of Motion. It is very well understood fact that amongst different materials, material having higher density would exert more force on the pole as compared to other materials. Thus, the pole suffers very high reaction forces when the body panel is made up of Steel V250. However, the pole suffers quite low reaction forces in case of a body panel made up of Aluminium NL (Non-Linear). Figure 7: Reaction Forces for Steel V250 Figure 8: Reaction Forces for Aluminium NL (Non-Linear) Impact Factor(JCC): Index Copernicus Value(ICV): 3.0

7 Crashworthiness of Automobile in A Vehicle-to-Pole Crash Simulation 7 Figure 9: Reaction Forces for Aluminium 6061-T6 Figure 10: Reaction Forces for Polycarbonate Simulation Figure 11, 12, 13and 14 depict the pattern of deformation that the body panel made up of Steel V250, Aluminium NL (Non-Linear), Aluminium 6061-T6 and Polycarbonate undergoes when it crashes with the pole at 35mph. Figure 11: Total Deformation of Steel V250 Body Panel

8 8 H. D. Gopalakrishna, Parimesh Panda & Pavan Jois & Nikhil P N Figure 12: Total Deformation of Aluminium NL (Non-Linear) Body Panel Figure 13: Total Deformation of Aluminium 6061-T6 Body Panel Figure 14: Total Deformation of Polycarbonate Body Panel CONCLUSIONS The overall objective of the paper was to simulate a full frontal crash of a vehicle with a pole. Simulation was performed using the AUTODYN software package from ANSYS Workbench The frontal part of the body panel absorbs most of the energy as it is subjected to maximum plastic deformation. Generally, the minimum deformation was seen at the points near the contact region of the shell with the pole. Among the metallic structures, Aluminium 6061-T6 is the best crashworthy material. However, polycarbonate proved to be a better crashworthy material than any other metallic structure. Specific Energy Absorption was the Impact Factor(JCC): Index Copernicus Value(ICV): 3.0

9 Crashworthiness of Automobile in A Vehicle-to-Pole Crash Simulation 9 common figure of merit for comparing crashworthiness of different materials. Figure 15 played a very instrumental role in comparing crashworthiness of different materials. Figure 15: Comparison of Crashworthiness of Different Materials REFERENCES 1. Vikas Sharma, Ram Bansal, R. B. Sharma, & Y P Upadhyay (2013). Simulation of Generic Sports Utility Vehicle-to-Pole Front Crash Analysis using a CAE based Methodology. International Journal of Automobile Engineering Research & Development, 1(3), Tony PunnooseValayil, & Dr. Jason Cherian Issac (2013). Crash simulation in ANSYS LS-DYNA to explore the crash performance of composite and metallic materials. International Journal of Scientific & Engineering Research, 4(8). 3. Yuxuan Li, Zhongqin Lin, Aiqin Jiang,&Guanlong Chen (2003). Use of high strength steel sheet for lightweight and crashworthy car body. Materials and Design, 24, Z.Q. Cheng, J.G. Thacker, W.D. Pilkeya, W.T. Hollowellb, S.W. Reagana,& E.M. Sievekaa (2001). Experiences in reverse-engineering of a finite element automobile crash model. Finite Elements in Analysis and Design, 37, HeshamKamel Ibrahim (2009). Design Optimization of Vehicle Structures for Crashworthiness Improvement Unpublished master s thesis). Department of Mechanical and Industrial Engineering, Concordia University Montreal, Quebec, Canada. 6. WaseemSarwar,& Nasir Hayat (2007). Crash Simulation and Analysis of a Car Body Using ANSYS LS-DYNA.Failure of Engineering Materials & Structures, ThanaponChotika, Jan-WelmBiermann, &SaiprasitKoetniyom (2011, October). Energy Absorption Analysis of Various Vehicles under Crash Test Simulation. Paper presented at the 2nd TSME International Conference on Mechanical Engineering,Krabi, Thailand. 8. Zhida Shen, Xin Qiao, &Haishu Chen (2013). BIW Safety Performance Research Based on Vehicle Frontal Crash.Springer-Verlag Berlin Heidelberg, XIII(9),

10