Surface Discontinuities and Fatigue Performance

Consider a typical scenario: a metallurgical analysis has shown that multiple fatigue cracks initiated along machining marks in a broken device manufactured by your company. A lawyer has hired a metallurgical expert that has provided high magnification scanning electron microscope images that exaggerate the depth of these marks and associated cracks. This expert proclaims: “But for these stress-concentrating manufacturing defects, the failure would not have occurred.” Your job is to figure out why the failure occurred, assess whether the expert is correct, and determine the extent to which these machine marks may have contributed to the failure.

We all know that surface conditions and discontinuities can affect fatigue performance. And certainly, not all surface discontinuities (or materials) are equal with respect to fatigue performance. However, in our experience, metallurgists and other failure analysts often ignore the big picture in a failure investigation and focus on the microscopic. This presentation reviews the fundamentals of fatigue, surface analysis, stress concentration and fatigue sensitivity, and fracture mechanics and stress analysis, and aims to provide failure analysts with tools by which to better evaluate the effects of varying surface conditions, discontinuities, and actual defects on the fatigue performance of engineering components as part of a failure analysis investigation. Lastly, we provide a case study that uses these tools to help evaluate the effect of machining marks on the fatigue fracture of an orthopedic implant.