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Inertia relief approach is often used to predict fatigue life of automotive structures. This approach, which completely ignores resonance effects, can provide reliable results, if the natural frequencies of the system are higher than loading frequencies by more than four times. In this paper, a method is proposed to augment the inertia relief results by including modal resonance effects. This new method has the same accuracy as that of the direct transient dynamic analysis with the efficiency of inertia relief approach. Here, inertia relief and normal mode results are obtained using commercial finite element codes and the modal resonance parameters are calculated in a manner suitable to handle long time histories of proving ground loads.

Fatigue life estimates of automotive structures are often obtained using stress estimated from static solution methods such as inertia relief [1, 2]. This approach can provide reliable results if the natural frequencies of the system are at least twice that of the highest loading frequency. Direct and modal transient fatigue approaches would include dynamic effects but are not practical due to increased computer resource requirements. This paper outlines a modified modal superposition based transient method. In this method, modal stress is obtained using a commercial FEA code. The transient solution is then obtained externally to significantly reduce computer requirements. Additionally, a numerical procedure is outlined which is capable of incorporating non-zero initial conditions for increased accuracy.