Numerical Simulation of Side Impact Crashes

INTRODUCTION

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Statistics of the NHTSA (National Highway Traffic Safety Administration) have indicated that lateral and oblique side impact collisions are the second most frequent cause of serious motor vehicle accidents after frontal impact. In the US, nearly 10,000 people die in side impact crashes every year (http://www.nhtsa.gov). Side impact collisions differ from frontal impacts, in that the occupant interacts directly with the vehicle structure. The occupant has very little protection from the striking vehicle in such collisions. Neither bumpers nor engines help absorb the energy of the impact. Traffic accidents are a danger to human safety and health, causing significant loss of human life. Injury prevention is therefore necessary to improve the happiness of individuals and families. Hence, side impact airbags, sidebars and other protection equipment have been developed. To confirm the effectiveness of protection equipment installed in vehicles, studying the degree of impact is fundamental to understanding the effect of automobile collisions on the human body. Therefore, the dynamic response of the human body to traffic accidents should be analyzed to reduce the level of occupant injuries.

Crash testing is a necessity. Generally, the dynamic response and injury to human bodies involved in crash tests can be analyzed in two ways by experimental and numerical simulation. For the vehicle sample, the experimental method can be further divided into real car collision tests (Berg et al., 2001) and sled experiments (Watanabe et al., 2000; Yoganandan and Pintar, 2005; Dehner et al., 2007). Although real car collision tests can achieve results closely resembling a real accident, this method is complex and expensive. However, even a sled experiments are costly. Additionally, practical and ethical concerns limit the use real occupants in collision tests. Research and development tests using dummies can now simulate human responses in a car collision (Bostrom et al., 2000; Yoshida and Tsutsumi, 2001; Croft and Philippens, 2007; Kapoor et al., 2008). Recently, rapid advances in computer technology have allowed applied mathematicians, engineers and scientists to solve previously intractable problems. The simulation tools for predicting occupant kinematics and calculate injury criteria include MADYMO, Pam Crash and LS-DYNA (Teng et al., 2010; Kapoor et al., 2006; Gong et al., 2008; Teng et al., 2007; Zaouk and Marzougui, 2002; Croft et al., 2002; Kirkpatrick, 2000; Deshpande et al., 1999). Numerical crash simulations provide a valuable tool for automotive engineers. To optimize the protection for various situations, finite element modeling is necessary for investigating dynamic behavior of occupants and also in injury analysis.

To reduce the severity of human injury through improved automobile design and designing protection devices, the dynamic response and injury of occupants in collisions must be analyzed. This study investigates the development and validation of a vehicle, movable barrier and dummy FEM model for studying interaction. Based on the Federal Motor Vehicle Safety Standard No. 214 (FMVSS214), the numerical crash test model was developed to simulate a side impact accident. The crash simulations were conducted using the LS-DYNA3D finite element code. The dynamic response of the human body to crashes is discussed herein. Additionally, the injuries of occupants were measured. The simulated models obtained could help evaluate vehicle crash safety and guide the future development of safety technologies.

DYNAMIC SIDE IMPACT REGULATION

Regulation: The safety standards are regulations written in terms of minimum safety performance requirements for motor vehicles or items of motor vehicle equipment. These standards are specified such that the public is protected against unreasonable risk of crashes occurring as a result of the design, construction or performance of motor vehicles and is also protected against unreasonable risk of death or injury in the event crashes do occur. Side impact protection standards have been adopted in both the United States and Europe. FMVSS 214 (U.S.) and ECE R95 (European) dynamic side impact regulations are quite different, especially with respect to their concerns about occupant injury. In this study, a side impact test was performed according to the FMVSS 214 specification.

FMVSS 214 specifies performance requirements for protection of occupants in side impact crashes. The aim of this standard is to reduce the risk of serious and fatal injury to occupants of motor vehicles in side impact crashes by setting vehicle crashworthiness requirements in terms of accelerations measured on anthropomorphic dummies in test crashes. For the physical test, the procedure was simplified with the struck vehicle stationary and the Moving Deformable Barrier (MDB) striking the target vehicle. The wheels of the deformable barrier were crabbed at an angle of 27 degrees from the longitudinal direction of the barrier. Figure 1 shows the test setup. Dummies in vehicle must satisfy requirements of FMVSS 214 when stationary vehicle is impacted by MDB at 54 km h^-1 (33.5 mph). This test was intended to simulate an intersection crash involving two moving vehicles. Dummy injuries to the thorax and pelvic area were assessed along with vehicle structural damage.

Injury criterion: The thorax and the pelvis are mainly injured by side impact. The SID dummy response measurements consisted of the thorax and pelvic accelerations. The protection criterion based on the tolerance of the human body are given as follows:

The thoracic trauma index: The Thoracic Trauma Index (TTI) was measured as an indicator of side impact injury risk. The TTI is the average peak acceleration of the thorax and takes considers equally the acceleration in the ribs and in the spine.

Fig. 1:	The test setup of FMVSS 214

The SID thorax response data included accelerations measured at the upper (T1) and lower spine (T12) spines and the upper (UR) and lower (LR) ribs on the impacted side of the dummy rib cage. The TTI was calculated by the formula:

where, Max (LU,UR) denotes the larger of the peak accelerations of either the upper or lower rib, expressed in g and Peak (T12) denotes the lower spine (T12) peak acceleration, expressed in g. The FMVSS No. 214 specification stipulates that the TTI shall not exceed 85 and 90 g for a passenger car with four side doors and with two side doors, respectively.

The pelvic acceleration: The FMVSS No. 214 specification requires that the peak lateral acceleration of the pelvis shall not exceed 130 g for all vehicles.

THE NUMERICAL MODEL OFDYNAMIC SIDE IMPACT

The finite element side impact test model was conducted according to the FMVSS 214 specification and procedure. The model was consisted of three systems combined into one FE model: (a) the side-impact vehicle model; (b) the MDB model and (c) the SID model. The overall models of side impact test are validated according to the FMVSS No. 214. The validations are described by Teng et al. (2003).

Full-vehicle model: In this study, a Ford Taurus model was analyzed in a dynamic side impact test. EASi Engineering for the National Highway Traffic Safety Administration (NHTSA) developed the full-vehicle FE model. The FE model of the Ford Taurus has 171 parts, represent the vehicle components. Of the 171 parts, 149 were used with shell elements to model the sheet metal components, 21 were assigned beam elements to represent the steel bars and one part was modeled with brick elements to denote the radiator. The full-vehicle FE model for the side impact simulation, consisting of 49453 nodes and 5327 elements was used.

MDB model: The MDB, weighing 1367 kg, is designed to represent an average midsize vehicle in the US market. The FE model of the MDB was originally developed by NHTSA. The model is composed of seven components, 8908 nodes and 5848 elements.

SID model: The SID FE model used in the simulation is based on the Hybrid III50% dummy.

Fig. 2:	Finite element model of SID (Side Impact Dummy)

The model includes the head, neck, upper spine, lumbar spine, pelvis, upper legs, lower legs, feet, jacket and ribcage. The geometry of the different components of the dummy was obtained from design drawings of the Hybrid III50% dummy. The model is shown in Fig. 2. The model is composed of 69 components, 43874 nodes and 57032 elements. The overall mass and the mass and inertia of each component of the dummy, match those of the Hybrid III50% dummy. All parts in the SID were modeled using different materials, including (1) Elastic, for simulating soft tissue and skin; (2) Rigid, for simulating the large parts of human body; (3) Fu-Chang Form, for simulating the muscles and (4) Mooney-Rivlin rubber, for simulating body parts characterized by extension, contraction and bending. The joints at the knees, hips and thorax were modeled with revolving, spherical and cylindrical joints. Each joint was assigned properties that simulating those of the real dummy.