Fatigue striation spacing analysis

Abstract

Fatigue crack growth in ductile materials produces features, called striations, on a fracture surface, which can aid greatly in understanding conditions associated with the fracture, including the number of stress variations that produced the fracture. In addition, striations can help to determine the directionality of fracture propagation as well as the location of the origin. Striations can be observed and counted using images from the scanning electron microscope or the transmission electron microscope. Many authors have demonstrated a 1:1 correlation between striations and stress cycles. Fracture surfaces typically contain thousands of striations, so that the counting of individual striation counting, some striations become obscured by rubbing of the fracture surfaces or are so finely spaced as to be very difficult to resolve. Therefore, normal practice consist of measuring striation spacings at a few locations and using statistical methods to estimate the total number of striations. Procedures used for estimating the total number of cycles can be as simple as dividing the crack interval distance of concern by the measured (or average) striation spacing. For a variety of reasons, this approach is not always satisfactory. There are other simple techniques that can be applied, without resorting to fracture mechanics, which enhance both the accuracy of the measurements and aid in understanding the results.