Investigation of Nonlinear Behavior of Vehicle Body Panels Under Shipping Conditions

The large deflection induced geometric nonlinear behavior of vehicle body panels under shipping conditions is investigated in this paper. The example structure presented is the door outer panel of a passenger car. The studies include the nonlinear finite element analysis for static displacements, stresses, and normal modes, and the fracture mechanics analysis for fatigue crack growth rate and fatigue life prediction involving panel large deflections. The finite element code NASTRAN is employed for the nonlinear analysis and the iteration strategy used is the modified Newton-Raphson method. The fracture mechanics analysis is based on the finite element method of computing the energy release rate, G, using the virtual crack closure technique. The stress intensity factor, K, is derived from the G values, and the experimental-based Paris Law is used for the panel fatigue life prediction. Although the transverse deflection of the panel is large, i.e., close to the panel thickness, at the container design loading levels, the difference between the linear and nonlinear solutions is not significant. One possible reason is that the midplane of the panel is not stretched excessively due to the nature of the constraints provided by the container dunnage supports on panel edges. This situation exists for most vehicle body panels under shipping conditions so that the linear analysis strategy currently used in the automotive industry for the shipping container design is feasible. The results of the fracture mechanics analysis of the panel indicate that the dominant failure mode for the assumed panel fatigue cracking is the opening mode or Mode-I, and the linear analysis for the fatigue life prediction of vehicle body panels under shipping conditions provides reliable solutions. The geometric nonlinearities of the panel in critical areas can be improved significantly by properly designing the shipping containers.