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Thermoplastics and composites are increasingly becoming popular among automotive design engineers because of their high specific stiffness and flexibility in manufacturing. While plastics like composites are orthotropic, unfilled thermoplastics like ABS Cycolac may be considered isotropic as they show little variation in properties between the flow direction and the direction transverse to the flow. However, this assumption is not enough to treat the latter as metals in finite element analysis. Metals like mild steel, offer considerable ductility, while thermoplastics show limited ductility and begin to fracture with several cracks appearing on the surface. Therefore, in the case of such plastics, it is important to consider the degradation of material properties in nonlinear finite element analysis using Damage Mechanics material law.

Durability problems in the automotive body structures due to road loads, need to be predicted early in the design process, in order to reduce the design, engineering, manufacturing, tooling and prototype costs and timing. This paper describes a procedure which can be used for identifying potential durability problem areas in an automotive body structure and predict the fatigue life of the critical areas due to road loads. A fully trimmed body model and loads generated from ADAMS** dynamic code were used for the analysis. Measured loads at the suspension attachment areas were used to analyze the local body structure. The peak stress levels of the various critical areas along with their fatigue life for the load-time history were obtained and displayed for easy visualization.