Failure analysis of mixed mode crack growth in heavy duty truck frame rail

A failure analysis investigation was performed on a fractured heavy duty truck frame rail obtained during endurance track testing. The fracture observed was on the frame web within the torque rod connection to the rear drive axle of the vehicle. This section of frame experiences multi-axial loading conditions including out-of-plane bending, twisting and shear under road loads. Metallographic examination revealed micro-cracks on the edges of an open hole located in an area of high stress concentration. This manufacturing defect acted as a stress raiser and resulted in fatigue crack initiation. Simulation of crack growth on frame rail using dynamic loads from a full vehicle model was completed. After careful analysis it was concluded that the failure occurred due to an aggressively drilled open hole which created small crack initiations in a high stress-state location of the frame. This resulted in extensive curvilinear crack growth under dynamic loads of the vehicle.