ABSTRACT
Fatigue causes more mechanical failure analyses than any other mechanism. It caused the bearings in our camper wheels to rumble and cracked the fan blades on our old car. Fatigue failures generally occur at loads less than that needed to cause plastic deformation and they usually require many stress cycles to initiate and then grow across a part. For fatigue to happen, the part has to undergo periodic tensile stress then relaxation, then stress, then relaxation, and this cycle has to be repeated many times. Stress concentrations, or stress risers as they are frequently called, are features of the part that multiply the local stress. Continuing our quest to better understand how and why high cycle fatigue failures occur, a basic understanding of the component metallurgy is extremely helpful. Progression marks tell us how the crack grew, and they are an incredibly valuable tool to understanding what happened during the life of the failure.
