Influence of Material Properties on Automobile Energy-Absorbing Components Crashworthiness

In the present work, the simulation analysis of automobile energy-absorbing components was carried out using Finite Element (FE) method. The numerical simulations were carried out using the software LS-DYNA. Automobile energy-absorbing components usually were made of a metal thin walled tube. In the paper, several types of material properties were studied and compared. Results show that the material properties have influence to automobile energy absorbing components crashworthiness.


INTRODUCTION
In cities traffic jam, automotive accident often occurs to low-speed and behind or angle collisions. No attention is paid to the low speed crashed for no personnel injury [1]. Therefore, it is quite necessary to study the technical problems of the car involved in low velocity impact.
In low velocity impact accident, the automobile energyabsorbing component is expected to be collapsed with absorbing crash energy prior to other body parts so that the damage of the main cabin frame is minimized and passengers may be saved [2].
The automobile energy-absorbing component equipped at the front end of car (see Fig. 1), is one of the most important automotive parts for crash energy absorption.
In the present work, the automobile energy-absorbing component (a kind of thin walled metal tube ) at lowvelocity impact was studied [3,4], and material properties on automobile energy -absorbing components crashworthiness were proposed.

Model Building
In case of frontal low-speed collision, the automobile energy-absorbing component absorbs impact energy and reduces the peak load of the impact mainly by the plastic deformation [5,6]. The automobile energy-absorbing component comprises a front longitudinal beam, a bumper beam, a crash-box and front and rear flange. The crash-box plays a key role in low-speed collisions, and the structure is given in Fig. (2).
In this paper, crash-box was made of thin walled tubes, and a finite element model was developed using the software Hypermesh [7]. The axial low velocity impact of the square cross-section tube (70mm width, 140mm long and1.65mm thick) is studied firstly. The tube finite element mesh is illustrated in Fig. (3). A rigid plate of 1000kg, placed on the top of the tube as shown in Fig. (3), axially impacted at the velocity of 4.44 m/h.

Main Crashworthiness Evaluation Parameters
In low-speed collision simulation, the most important crashworthiness evaluation parameters of the crash-box are the total impact energy absorption via plastic deformation Es, the peak impact force Fp, the compressed displacement of the tube e δ , and the average impact load Fm. The average impact force Fm is given by equation (1), where Fm is arithmetic average value, with Fm increasing, the absorbing energy of the tube will be increased. The greater the Fm, the more energy is absorbed, and the better the crew safety. Therefore, in precondition of not more than the permission peak value, the bigger the average impact load is, the better the absorption performance is, and the shorter the compressed displacement is, the better the absorption performance is [8].

MAIN INFLUENCING FACTORS TO CRASH-WORTHINESS
There are many factors that can influence crashworthiness characteristics of crash-box in collision simulation process, including crash-box shape, wall thickness, material and so on, in which material properties have great effects on device characteristics. In this paper, influence of material properties to crash-box is chiefly researched.

Material Strain Rate
Vehicle crash simulation, the effect of strain rate to automobile crashworthiness cannot be ignored.
The curve of impact load vs displacement is shown in Fig. (4). The curve of energy vs displacement is shown in Fig. (5). The curve of energy vs time is shown in Fig. (6). Table 1 provides a comparison of the main parameters (peak values of impact load for tubes and displacement) obtained by no strain rate and strain rate.
Comparing the simulation results, the energy of the impact is absorbed almost completely for the two conditions, the energy absorption considering strain rate is better than

Influence of Material Properties
The Open Mechanical Engineering Journal, 2014, Volume 8 115 not considering the effect of strain rate. But the energy absorbing distance of considering strain rate model is less than that not considering strain rate model, the energy absorption considering strain rate is better than not considering the effect of strain rate.

Material Yield strength
The tube was modeled using steel, in this paper, Density, ρ = 7.85 x 10-6kg/mm 3 , Poisson ratio, v= 0.3 and Young's modulus, E = 210GPa. yield strength, σ y = 430MPa and σ y = 516MPa, on the square cross section tubes were adopted as shown in Fig. (3). FE simulation results are shown in Fig.  (7). The peak values of impact load for tubes are shown in Table 2.
Results showed that the energy absorption characters of thin-walled tube of square cross section had been increased greatly when yield strength raised. Where the peak value of impact load was 370.1 kN, increased about 19% compared with σy=450MPa, and displacement was 73.25mm, decreased about 27.2% compared with σ y =450MPa. Fig. (7). Impact load vs time curve of crash-box.

Material Type
There are many types of metal materials, steel is the common type of automobile crash-box, which is used in a wide variety of impact loading applications, since it is relatively cheap, versatile and efficient for absorbing energy. With the rapid development of automobile industry, the lightweight materials have been applied in the automobile. Aluminum alloy material through the extrusion forming is subjected to the extensive concern [8].
The peak values of impact load for tubes with different material are shown in Table 3. Results showed that all the energy was almost absorbed by two materials, because the energy curve finally turned to a horizontal line. The peak impact of tubes is fundamentally different when different material is adopted. The peak impact of aluminum alloy tubes decreased 37.34kN compared with steel, and displacement increased 20mm compare with steel. Fig. (8). Energy curve of the tube for the two kind of materials.

CONCLUSION
The automobile low velocity impact is a very complex mechanical problem, and theory analytical solution is quite difficult. So computer simulation is an effective method. Through study models of material parameters, material properties on automobile energy -absorbing components crashworthiness have very important influence. Suitable results of the design are obtained by change material parameters. Material strain rate, material yield strength and material type have certain effect to crashworthiness of energy-absorbing component. Fig. (9). Impact load vs displacement curve of the tube.