Despite increasingly stringent crash requirements, the body structures of future mainstream production cars need to get lighter. Carbon fiber reinforced polymer (CFRP) composites with a density 1/5th of steel and very high specific energy absorption represent a material technology where substantial mass can be saved when compared to traditional steel applications. BMW have addressed the demanding challenges of producing several hundred composite Body-in-White (BIW) assemblies a day and are committed to significant adoption of composites in future vehicle platforms, as demonstrated in the upcoming i3 and i8 models. A next step to further integrate composites into passenger cars is for primary structural members, which also perform critical roles in passive safety by absorbing large amounts of energy during a crash event.
In order to move forward the integration of CFRP materials in primary structures, the same high level of confidence in passive safety design simulations achieved by BMW for metallic structures needs to be achieved with composite designs. Following encouraging results from early component level studies using the CZone extension for Abaqus, BMW undertook the challenge to predict the performance of a large car front end BIW made entirely from composites. Low and high speed load cases for both full frontal and offset frontal impact against a rigid barrier were considered. Data from the three crash tests of the full system configurations compared well against simulation results obtained prior to the crash tests.
The CZone approach to composite impact is based on the principle of applying the forces generated through the fragmentation of the composite at the crushing interfaces to the adjacent finite elements. These forces are transferred back into the rest of the structure away from the crush zone and can potentially instigate non-crushing failure of the composite structure, an important design consideration. The “crush stress” which is assigned to the finite elements is considered as a macroscopic material property and can be measured via laboratory coupon testing.