Fatigue in the Aerospace Industry: Striations

In the aircraft engine industry, as in others, fracture surfaces are mined as a rich source of information. When present, fatigue striations can be used to judge the cracking mode, measure the crack growth rate, and estimate the number of propagation and initiation cycles. With training and practice, a failure analyst can learn to measure striations on fracture surfaces. Each individual striation is not counted, because there are thousands on a typical fracture and that would not be practical. Striations are measured from photographs taken in several locations across a fracture and their locations are recorded. It is best to start photographing selected areas at the deepest part of the fracture and work back toward the origin. Striations are usually much easier to see at the end of a fatigue zone due to several factors. The density of the striations (cycles per in. or lines per in.) is plotted as a function of crack depth. This data is fitted with a curve. The area under the striations/in. versus crack depth plot is an estimate of the total number of cycles the crack has propagated. Once the number of crack propagating cycles has been estimated, this data can be used to determine the number of cycles for crack initiation if the number of total cycles on the part is known.