Fatigue and fracture analyses of automotive and welded structures.

Abstract

The fatigue life of a Lower Control Arm (LCA) of a vehicle is investigated under chuckhole loads applied at the ball joint. Two simplified bushing models, a rigid bushing model and a nonlinear elastic bushing model, are considered. Existing multiaxial fatigue theories are adopted in consistent with the critical plane approach to assess the fatigue life of the LCA. The results indicate that the inclusion of the nonlinear elastic behavior of the bushings is necessary since the predicted fatigue life of the LCA based on the nonlinear elastic bushing model is about 40% of that based on the rigid bushing model. The stress and strain histories of two different designs of Lower Control Arms (LCAs) under chuckhole loads are obtained by elastic-plastic finite element analysis and two stress/strain estimation methods based on elastic finite element analysis. For the old LCA, the fatigue lives based on the estimated strains obtained from the two estimation methods are larger by one order of magnitude when compared with that based on the elastic-plastic finite element analysis. For the new LCA, the fatigue life based on the estimated strains obtained from Neuber's method is close to that based on the elastic-plastic finite element analysis. A finite element procedure to determine a possible new fracture parameter $J\sb{ld}$ is presented for welded structures with consideration of residual stresses. The method is based on the energy difference of two cracked solids with slightly different crack sizes. Our computational results show that $J\sb{ld}$ and the J integral agree well for a cracked plate without consideration of residual stresses. When the residual stresses are considered, the values of $J\sb{ld}$ for different contours close to the crack tip in the cracked plate subject to remote tensile stresses are in good agreement. The computational results also indicate that for the given residual stress distribution, the values of $J\sb{ld}$ with consideration of residual stresses are lower than those without consideration of residual stresses for the cracked plate subject to large remote tensile stresses.