Striation (fatigue)

Striations, in the context of fatigue, refer to visual markings or alterations observed on fractured surfaces of materials, particularly metals, that have undergone fatigue failure. These striations are typically microscopic and represent the incremental crack growth during each stress cycle. They are a key characteristic feature used in fractography to diagnose fatigue as the failure mechanism.

Striations appear as a series of parallel ridges or bands on the fracture surface, perpendicular to the direction of crack propagation. The spacing between striations is related to the crack growth rate – wider spacing generally indicates a faster crack growth rate, while narrower spacing indicates a slower rate. However, the correlation between striation spacing and crack growth rate is not always straightforward and can be influenced by factors such as material properties, stress level, and environmental conditions.

The formation of striations is generally attributed to the repeated plastic deformation at the crack tip during each loading cycle. During the tensile portion of the cycle, the crack tip blunts and advances slightly. During the compressive portion, the crack tip sharpens. This cyclical process leaves behind the characteristic striation marking.

It is important to note that not all fatigue fractures exhibit clear striations. Their presence can depend on factors like the material's ductility, the stress intensity factor range, and the testing environment. In some cases, other features such as beach marks (macroscopic markings) or a more granular or featureless fracture surface may be observed in addition to, or instead of, striations. Therefore, careful fractographic analysis, often involving techniques like scanning electron microscopy (SEM), is essential to accurately interpret the fracture surface and determine the underlying cause of failure.