INFLUENCE OF BUMPER DESIGN TO LOWER LEG IMPACT RESPONSE

Transcription

1 F2006SC05 INFLUENCE OF BUMPER DESIGN TO LOWER LEG IMPACT RESPONSE Svoboda Jiri*, Kuklik Martin Czech Technical University in Prague, Faculty of Mechanical Engineering, Department of Automotive and Aerospace Engineering, Czech Republic KEYWORDS Pedestrian protection, legform impact, bumper design, impact energy absorption, FEM simulation ABSTRACT The paper presented here concerns with the bumper system design optimization for legform impact. Not only lower leg impact but also vehicle to dummy collision were simulate to answer the question whether the optimized body structure for the sub-system impact is also optimal for real pedestrian impact. Parametric analysis of bumper system under lower leg impact was performed. Modifications in bumper system, which required minimum package space to meet lower leg impact limits were designed. Besides the bumper absorber, the spoiler stiffener was designed not only for keeping the bend angle and the share displacement under limit value, but also for tibia acceleration reduction in this study. It was proved that properly designed stiffness of the lower stiffener and its relative position to the rest of the bumper significantly influenced both distribution of legform impact energy absorbed by bumper itself and spoiler (lower stiffener) and tibia acceleration. The effect of design changes, leading to improvement in the lower leg impact, on the total kinematics and loading on the pedestrian was investigated. Vehicle-to-dummy collisions ware simulated to investigate the dummy responses to impact with all bumper modifications examined in previous stage. Based of simulation results, it was possible to conclude, that the optimization objective for the front panel defined as: the lower legform under limit plus reduction of package space were not in contrary to requirements on pedestrian kinematics to reduce its injury risk. TECHICAL PAPER - Progress in pedestrian safety has been rapid over past decade. The vehicle performance requirements, proposed by European Experimental Vehicles Committee [1,2], have been playing an important role in this process. The intention of the test method is to improve a pedestrian's chances of survival in collision with a car. The accident research showed three the most injured areas of the pedestrian s body, which are associated with different area of the car, therefore the proposed method uses of three different impactors to evaluate the pedestrian friendliness of a vehicle. They are: headform to bonnet tests, upper leg to bonnet leading edge tests and lower leg to bumper tests. This paper deals with the bumper system design and its performance under lower leg impact, which was simulated and studied by finite element method. The sensitivity analyses results of the lower leg impact responses to design variables of bumper system are presented. Moreower, vehicle to dummy collision were simulate to answer the nowadays often discussed question whether the optimized body structure for sub-system impact is also optimal for real pedestrian impact.

2 BUMPER SYSTEM LEGFORM IMPACT MECHANICS Bumper system, which is normally used for preventing failure of components during low speed collision, must also satisfy the pedestrian protective requirements. This leads to more complex bumper system design with different types of energy absorbing structures. In the test procedure, a legform impactor is propelled at a stationary vehicle at a velocity of 40 km/h. The velocity is parallel to the longitudinal axis of the vehicle and a minimum of three separate tests is proposed at different points within a zone bounded by the corners of the bumper. The base of the impactor must be in a height similar to the ground in real position. The legform impactor basically consists in two parts connected by a translational and rotational joint. The mass of the impactor is located in a way that the dynamical behavior of the complete system represents as good as possible real life situations. The acceleration sensor is located in the upper part of the tibia, close to the knee. Here the mass is lower than in the femur and so the possibility of obtaining higher values increases. Shear displacement and bending angle correspond to the relative positions between the femur and the tibia. Three variables are measured during the legform impact: acceleration, shear displacement and bending angle. For this impact event, the proposed requirements are: Tibia Acceleration (near knee) < 150 g Lateral Knee Bend Angle < 15 degrees Lateral Knee Shear Displacement < 6 mm From the kinematics point of view the legform is a two-mass system connected by the knee joint which usually hits the bumper of most passenger cars by upper part of the tibia in the area around the knee.the front of a car must be soft enough because otherwise it could be impossible to stay below the defined acceleration limit. The tibia intrusion into the bumper effect force reward which together with the inertia forces applied in the center of mass both the tibia and the femur produce bending moment. These two bending moments are in opposite direction in case unsupported tibias lower part and cause extensive bending and share displacement. From this point of view the shear displacement and the bending are geometrical limits depending on the shape of the front of vehicle. Especially in respect to bending angle a flat wall the same height as the legform as a car frontal shape could be the best but it is not appropriate solution from the other reasons. Therefore the minimal bending angle and share displacement must be ensured otherwise. Being most of the legform mass in the upper part (femur), it is beneficial just to smoothly stop the lower part (tibia) giving it a quick rebound in order to follow the upper one, without increasing too much the bending. This solution needs reinforcement in the lower bumper (spoiler), with certain stiffness, which is difficult issue in the pedestrian protection for the legform. Furthermore the third level of support is made by the bonnet leading edge, which position related to the bumper and spoiler can vary bending results. IMPACT ENERGY MANAGEMENT From kinematics point of view, the target is to stop body (impactor) within as short as distance while acceleration must not exceed limit value (150g). To utilize package space as efficiently as possible and meet the requirements summarized in previous sentence the combination of design and materials used in bumper system should provide a deceleration pulse on the legform close to square wave [3,4,5]. Many works concerning bumper system design for legform impact improvement and many different technical solutions and new design concepts providing energy management for vehicle bumper systems to meet the proposed lower leg test criteria were published over past years [6-12]. They all shared a basic idea: to use the available package space in the most efficient manner. It means: the fast response of the energy absorbing structure to the impact event (shape force intrusion curve

3 close to rectangular shape), the more efficient the energy management and, therefore, the smaller the thickness of space needed to absorb the energy from the event. Energy absorber (foam block, honeycomb based plastic structures etc.) between bumper fascia and bumper beam was used to decelerate the lower leg in those technical solutions. The absorber stiffness and corresponding thickness was found to be significant for the level of tibia acceleration. The crash distance (thickness of energy absorber) calculated from the balance of the legform kinetic energy and deformation energy expressed by equation 1 was the commonly used method. 1 2 E kinetic = mδv = F( x) dx [1] 2 Force F(x) in equation 1 could be substitute with product of legform mass m and acceleration A(x) E = kinetic mδv = m A( x) dx Δv 2 = A( x) dx [2] 2 2 Integral in the right side of equation 2 was possible to substitute by maximum of crash distance Δx, peak acceleration A and efficiency of waveform η. Efficiency of foam absorbers varies from 45% to 65% [6] Δv E kinetic = Δv = AΔxη Δx = [3] 2 2Aη The equation 3 is well known relation of crash distance Δx, velocity change Δv and acceleration waveform expressed by its efficiency η and peak acceleration A, which says the higher efficiency the shorter crash distance therefore typical approach was to increase the efficiency of the energy absorber [6]. The previous publications concluded, to keep deceleration of the lower leg under the 150 G limit the package space of 60 ~ 80 mm was necessary and to reduce the knee bend angle the stiffener of the lower area of the bumper below the air dam (spoiler) was necessary. However, the design of energy absorber according above mentioned principles can work for many applications. Following that methodology for legform impact application results in too conservative bumper design. The difference lies in: The base structure beneath the energy absorbing layer in this application is not rigid but flexible. Issue of the energy absorbing layer structure on flexible based structures is explained in [13]. Not only the single energy absorber (the bumper itself) is capable to absorb the legform impact energy. Not the total initial legform kinetic energy must be absorbed by bumper system, some is transformed into the rotational movement of the legform and the rest is absorbed. Assuming 25% of the leg impact energy is dissipated by the fascia and leg rebound, the required energy absorption is reduced to 75%. The differences were the reason that neither equation like no. 3 nor simple models based on multibody techniques (Figure 3) were able to simulate the impact behavior of the bumper system in each detail. They can only discover basic design relations for the bumper system structure. That was the reason why finite element modeling technique was used in this study. This analysis reveals that the relative position of the bumper and spoiler was significant not only for the maximum knee bend angle as published in literature, but also tibia acceleration was influenced by the spoiler position and it s necessary stiffness thus. So, it was not possible to design an absorber only according equation 3. It was necessary to count with the spoiler contribution which one can maintain while modifying the width of the absorber layer. In this

4 study, not only the bumper itself also lower area (spoiler) were provided by energy absorber to absorb lower leg impact energy and thus control the level of deceleration. IMPACT ANALYSIS PARAMETRIC STUDY The transient response of the legform during impact with different design configuration of the vehicle front end structure was evaluated. Objective for this study was the reduction of the package space needed for the legform energy absorption without making pedestrian protection variables get worse. To find out which factors were the most important to pedestrian legform impact performance and good understanding of the legform-to-bumper impact issue, a parametric study based on the multibody model (Figure 1), which was designed with respect to previous analysis of problem, preceded the detail FE study. The car front end was described by a set of design parameters in the multibody system (MBS) to evaluate many configurations. The components taken into account in the MBS model were the bonnet leading edge, the bumper system (including the bumper fascia, bumper stiffener, energy absorber located between the fascia and the stiffener and crash boxes), the spoiler (fascia, stiffener and energy absorber located between). Some studies on this topic were carried out and detail results of such MBS parametric studies have been published by other authors and can be found in literature (for example [7]). The important outcome was that four parameters from the total number and limits in which each parameter could be varied were found to be most critical and adopted for the FE analysis. They were thickness of energy absorber given by the relative position of the bumper fascia and the bumper stiffener a, relative position of bumper and spoiler fascias b (both distances were measured in vehicle middle plane defined by longitudinal and vertical axis of vehicle) (Figure 2), energy absorber stiffness, and spoiler stiffness. Bonnet leading edge Cash box Bumper Spoiler Energy absorbers Original a b Modification Figure 1: MBS model Figure 2. Middle section of car parameters optimized FINITE ELEMENT ANALYSIS OF LOWER LEG IMPACT The geometry of the primary finite element vehicle model was based on specific vehicle geometry. But, it was assumed that the most observations made during this study would be valid for the majority of vehicles in the same class. The finite element simulation model of

5 vehicle included those structures and its supports that were relevant for a legform impact and collision with a full dummy. Figure 3: Simulation model Figure 4. Spoiler stiffener To perform sensitivity analyses a set of simulation models were derived from the primary model. In those models the four parameters defined above were varied in defined ranges. The distance between bumper fascia and bumper stiffener, which defined the energy absorber package space and thus stiffness, varied form 10mm (distance of basic model without energy absorption layer) to 85mm. Spoiler position related to the bumper fascia varied from 15mm backward (through 0 which is original position) to 30mm forward. To follow the idea described in paragraph Bumper system legform impact mechanics the spoiler stiffener was designed and implemented into the simulation models to provide support for lower tibia part (figure 4) during impact into bumper system. It consisted of two parts: spoiler beam and absorber with 40mm thickness which was included between the spoiler beam and the spoiler fascia. Material of both energy absorbers was the low density foam, which has become widely used in automobile interiors and bumper systems and provides the excellent energy absorption capabilities for a certain stress level. Low density foam materials can deform up to 90% strain in compression, while their porosity permits very large volumetric changes [12]. Figure 5 shows a compressive stress-strain curve of polypropylene foam material which is divided into three distinct stages. The first stage (strain < 5 %), the foam deforms in a mainly linear elastic manner. The second stage is a plateau deformation at almost constant stress which follows the final region characterize by a rapid increase of compressive stress. The stress-strain curves, which defined foam stiffness, used for foam definition in the FE models were experimental based. Different stiffness was achieved by the curve second stage shift so that the package space of each variant was utilized as efficiently as possible. stress strain 1 st 2 nd 3 rd Figure 5. Stress-strain curve of foam material

6 DISCUSSION OF RESULTS Legform-to-vehicle impact Appendix A shows the legform responses for distance between bumper stiffener fascia 25mm, 40mm and 55mm and related spoiler positions in contrast with the original design. The curves were labeled by two numbers, the distance between the bumper stiffener and its fascia represented the first one while the second one indicated the spoiler position relating to the bumper fascia. All configurations of bumper and spoiler, where the distance between the bumper fascia and the bumper stiffener filled with the energy absorber was equal and longer than 55mm, kept observed variables staying bellow their limits. Legform impact responses under its limit were obtained also for combinations with shorter distance between the bumper fascia and the bumper stiffener, namely 40_15, 40_30 and 25_30. Those results verified the basic idea presented in previous paragraphs: it was possible to reduce package space necessary for bumper energy absorber if the lower part of bumper system (spoiler) was utilized for legform impact energy absorption together with bumper energy absorber. The variants in which the relative position of bumper fascia and spoiler was equal 0 (the same like in the original design) excluding variant 40_0 and 25_0 satisfied the legform impact requirements. However, taking the spoiler slightly forwards reduced not only bending angle and share displacement in the knee since the change in kinematics of legform, but also tibia acceleration. In case of spoiler forward shift, the total package space fitted for legform impact energy absorption extended which allowed reduction the package space between bumper fascia and bumper stiffener itself (variants 40_15, 40_30 and 25_30). Vehicle-to-dummy collision Primary topic of presented work was lower leg under limit. But the proposed tests look at the impact response of separated impactors, which represents the different areas of the pedestrian in isolation, without feedback on behavior of the whole pedestrian body. Therefore the effect of design changes, leading to improvement in the lower leg impact, on the total kinematics and loading on the pedestrian is widely discussed. Simulations of vehicle-dummy collision for all configurations of the bumper system considered during optimization of the legform impact were carried out at velocity 40km/h were performed to clarify that issue. Model of the 50% Hybrid III dummy whose structure was adapted to the pedestrian impact was used in the vehicle-dummy simulation. The knee joint of the dummy model which was originally designed to simulate occupant response during frontal crash was modified to behave the same manner as legform knee joint to obtain comparable behavior of legform and dummy leg when being impacted laterally. An injury risk, mostly assessed on the base of acceleration measured on various dummy parts which is highly dependent on the vehicle structure impacted and impact velocity was found to be irrelevant for the potential injury risk assessment of a dummy in collision with various configurations of the car front panel. The reason was in the fact that the variation in the front panel design leaded to variation in the impact point of the head and other body parts for the same dummy size and same collision velocity. Therefore, the velocity of right femur, thorax and head were investigated and compared when risk of injury was assessed. It is not clear how close the velocity values are to actual injury risk, but this study indicates the relative effects and trends, which can be considered when designing for the pedestrian safety. Appendix B shoves a comparison of the impact velocities of selected dummy parts relative to vehicle during dummy-to-vehicle collision. Generally was possible to conclude that the increase in distances between the bumper fascia and the bumper stiffener at upper limit of the

7 investigated range resulted in a slightly higher angular velocity of the dummy. The dummy s upper leg was less supported by the bonnet leading edge in those cases which leaded to the higher angular velocity of the dummy. Shift the spoiler forward (for the same bumper fascia position) resulted in mild increase of the total velocity of the head and time shift of head velocity peak, but relative velocity of the right femur and the thorax decreased as distance between bumper stiffener and fascia increased (appendix B). The results of this study did not confirm the opinion that when the bumper modified to improve legform impact the impact response of the dummy will deteriorate. The optimization objective for the front panel which could be simply defined as: the lower legform under limit plus reduction of package space were not in contrary to requirements on pedestrian kinematics to reduce its injury risk. CONCLUSION The paper presented here concerns with the bumper system design optimization for legform impact. When the simulations models once created, not only lower leg impact but also vehicle to dummy collision were simulate to answer the nowadays often discussed question whether the optimized body structure for the sub-system impact also optimal for real pedestrian impact. The typical approach how to cope with the legform impact test requirements was insert an energy absorber 60 ~ 80 mm of package space between the bumper fascia and the bumper beam to decelerate the lower leg to satisfy the 150 G target together with reinforcement in the lower area of the bumper. The lower stiffener was designed not only for keeping the bend angle and the share displacement under limit value, but also for tibia acceleration in this study. Properly designed stiffness of the lower stiffener and its relative position to the rest of the bumper significantly influenced both distribution of legform impact energy absorbed by bumper itself and spoiler (lower stiffener) and tibia acceleration. The sensitivity analysis to parameters which characterized the shape and stiffness of the bumper system was performed using finite element analysis. The aim of the analysis was to reduce package space together with lower legform under limits. The results verified the basic idea that it was possible to reduce bumper absorber package space if the spoiler was used for legform impact energy absorption together with bumper energy absorber. The effect of design changes, leading to improvement in the lower leg impact, on the total kinematics and loading on the pedestrian was investigated. Simulations of vehicle-dummy collision for all configurations of the bumper system considered during optimization of the legform impact were carried out at velocity 40km/h. The relative velocity of right femur, thorax and head were used for assessment of severity of each collision. On the base of simulation results was possible to conclude, that the optimization objectives for the front panel defined as: the lower legform under limit together with reduction of package space were not in contrary to requirements on pedestrian kinematics to reduce its injury risk. REFERENCES (1) European Commission Document III/5021/96 EN, Draft Proposal for a European Parliament and Council Directive Relation to the Protection of Pedestrians and Other Road Users in the Event of a Collision with a Motor Vehicle, 1996 (2) EEVC WG17, Improved test methods to evaluate pedestrian protection afforded by passenger cars, December 1998 (3) C C Chou and G W Nyquist, Analytical Study of the Head Injury Criterion (HIC), Automotive Engineering Congress, Detroit, Michigan, 1974

8 (4) G G. Lim and C C. Chou, :Estimating the minimum space to meet Federal interior head impact, SAE 1995 World Congress, Detroit, Michigan, 1995 (5) M Huang, Vehicle Crash Mechanics, Boce Raton, CRC Press LLC, 2002 (6) S Schuler and F Mooijman, Bumper Systems Designed for Both Pedestrian Protection and FMVSS Requirements, SAE 2003 World Congress, Detroit, Michigan, 2003 (7) Y H Han and Y W Lee, Optimization of Bumper Structure for Pedestrian Lower Leg Impact:, SAE 2002 World Congress, Detroit, Michigan, 2002 (8) P Naughton and P Cate, An Approach To Front-End System Design for Pedestrian Safety, SAE 2001 World Congress, Detroit, Michigan, 2001 (9) E Glasson, V Maistre and C Laurent, Car Front End Module Structure Development Regarding Pedestrian Protection and other Mechanical Constraints, SAE 2001 World Congress, Detroit, Michigan, 2001 (10) S Shuler and S Santhanam, Predicting the Bumper System Response of Engineering Thermoplastic Energy Absorbers with Steel Beams, SAE 2002 World Congress, Detroit, Michigan, 2002 (11) D Svand and T Morgan, Engineering Thermoplastic Energy Absorbers for Bumpers, SAE 1999 World Congress, Detroit, Michigan, 1999 (12) J Hassan, P Schuster and G Frederick, Use of Polyurethane Material Models for Simulating Leg-Form Impact in Different Explicit Finite Element Codes, SAE 1998 World Congress, Detroit, Michigan, 1998 (13) S X He, T Devilbiss and R Angamuthu, A Design Methodology for Interior Components to Comply with FMVSS 201 Head Impact Requirement, SAE 2000 World Congress, Detroit, Michigan, 2000

9 Appendix A. Legform responses for distance between bumper stiffener and bumper fascia 25mm, 40mm and 55mm and related spoiler positions in contrast with the original design.

10 Appendix B. Comparison of the impact velocity on the dummy relative to vehicle during collision with the original car panel and front panel optimized for legform impact.