Introduction
Composite materials are increasingly being utilized in automotive parts. Because of their high strength and stiffness to weight ratio, typical composite parts are about 30 to 40 percent lighter than steel components. SMC (Sheet Molding Compounds) composites and thermoplastics are being utilized as materials for body panels, hoods and bumpers of automobiles. High performance parts, such as drive shafts, transmission parts, flywheels, leaf springs and brake drums, are areas in which CFRP (Composite Fiber-Reinforced Polymer) have applications. Composites are also being considered as materials for the body and chassis. An example includes GM’s concept car, the Ultra-Light, whose body and chassis are composed of advanced composites and aluminum alloy.
However, crash as well as primary load bearing structural performance of composites have not been fully investigated. Until they are, composite utilization in vehicles will be confined to structures which are not in the main load bearing path statically or dynamically (i.e., body panels, hoods, etc.). Full integration of composites in the body, chassis, and power train components would be ideal for one apparent reason-a higher energy efficiency. The significance of integrating composites within automobile primary load bearing structures becomes more apparent in electric and hybrid vehicles, where their range and speed would be enhanced.
Utilizing DYNA3D, the crash performance of two composite structures was analyzed and compared to those of an aluminum alloy and steel structure.
Materials Models
The crash simulation was first performed with typical structural steel properties. Second an aluminum alloy was selected to replace the steel structure. Next, high strength SMC (Sheet Molding Compound) composite properties were chosen to replace the aluminum structure, and represented a typical composite bumper material. Currently, SMC composites are utilized in body panels, hoods, and bumpers. Typical SMC composites are made up of about 30 to 50% chopped glass fibers with about 25% resin (polyester, vinyl ester, phenolic, and epoxy) and the rest fillers (such as clay, alumina, calcium carbonate). The final simulation involved a carbon/epoxy composite bumper. Carbon composites have high strength and stiffness to weight ratio. This makes them an attractive alternative to replace steel and aluminum structural components in a vehicle. The main drawback of a carbon composite is its brittle nature. Unlike steel or aluminum, which can plastically deform, carbon composites display no plastic deformation.
The material properties utilized in the simulations are summarized in Table 1.
Crash Model
A simple front end frame crash was simulated utilizing DYNA3D. As shown in Figure 1, the structure consisted of a bumper and a front rail. The modeled structure was 4 feet wide, 2.3 feet long, and 0.33 feet high. At the end of the structure and along the chassis, a mass of 200 kg. was placed. The structure was modeled to crash into a solid barrier at a speed of 30 mph. Because of symmetry, it was possible to model only one half of the structure. This technique was employed to decrease the computational time needed for DYNA3D to run the simulation.
Results
The thickness for all four models was initially set at 0.1 cm. At this thickness, the steel model, was the only one able to absorb the total crash energy (i.e. kinetic energy). The aluminum, SMC, and carbon composite models showed lower energy absorption levels. However this lower energy absorption was not due to total failure of the structures, but was due to a redirectioning of the motion of the bumper. Thus in the simulations with the aluminum, SMC and carbon composite structures set at 0.1 cm thickness, there was some residual velocity left. In the simulations with thickness set at 0.1 cm , the aluminum, SMC and carbon composites structures would strike the rigid wall, the bumper would start crumpling and because of the inherent curvature in the bumper, the structures would proceed to rotate up the wall. Because of this the thickness of the aluminum, SMC and carbon composite structures were increased until full crash energy absorption was attained. The simulations were then stopped at this point. The final thickness of the aluminum, SMC and carbon composite structures were respectively 0.15 cm, 0.2 cm and 0.2 cm.
Figure 2 shows the deformation of the steel structure after full crash energy absorption was obtained. The X-displacement (centimeters) contour, shown in Figure 2, is given with respect to the original location of the structure. As can be seen in Figure 2, the bumper is crushed inwards after hitting the barrier. A crumple zone, at front, along the joint location connecting the chassis to the bumper is also noticeable. Slight buckling of the chassis can also be observed near the joint location. The crash performance of the aluminum structure, SMC, and carbon composite structures are summarized in Figure 3, Figure 4 and Figure 5 respectively. Like the steel model, they are shown after full energy absorption has been obtained.
The aluminum structure behaves similar to the steel structure. The bumper shows more pronounced crushing, and also there exists a larger crumple zone at the front of the chassis/bumper joint location. The buckling of the chassis is more pronounced. This was to be expected because aluminum alloys have lower stiffness and strength than steel.
The crash performance of the SMC composite structure was similar to the aluminum alloy and steel structure.
The carbon/epoxy composite bumper shows a total collapse of the bumper and is shown in Figure 5. There is no extensive plastic deformation as was experienced by the other three material. The front end of the bumper is entirely crushed on impact with the barrier.
Click here to see the steel simulation.
Click here to see the aluminum alloy simulation.
Click here to see the SMC simulation.
Click here to see the carbon composite simulation.
Conclusion
Although the structure modeled was a fairly simple one, the DYNA-3D crash simulations show that an SMC and carbon composite bumper/rail structure can absorb the same amount of energy as the steel or aluminum alloy. In addition, as can be seen from Table 2, the SMC composite structure is about 14% lighter than aluminum and more than 50% lighter than steel. The carbon composite structures is even lighter than the SMC. From Table 2, it can be seen that the carbon composite structure is almost 25% lighter than the aluminum structure and almost 60% lighter than the steel structure.
A good measure of crashworthiness is the specific energy absorbed by the structure. As shown in Table 3, the carbon composite structure has the highest specific energy absorption followed by SMC, aluminum alloy and steel. Hence, for this simulation, the crashworthiness of carbon composite and SMC composite, with a much lower weight, is superior to the other two materials.
