Striations are marks produced on the fracture surface that show the incremental growth of a fatigue crack. A striation marks the position of the crack tip at the time it was made. The term striation generally refers to ductile striations which are rounded bands on the fracture surface separated by depressions or fissures and can have the same appearance on both sides of the mating surfaces of the fatigue crack. Although some research has suggested that many loading cycles are required to form a single striation, it is now generally thought that each striation is the result of a single loading cycle.

The presence of striations is used in failure analysis as an indication that a fatigue crack has been growing. Striations are generally not seen when a crack is small even though it is growing by fatigue, but will begin to appear as the crack becomes larger. Not all periodic marks on the fracture surface are striations. The size of a striation for a particular material is typically related to the magnitude of the loading characterised by stress intensity factor range, the mean stress and the environment. The width of a striation is indicative of the overall crack growth rate but can be locally faster or slower on the fracture surface.

The study of the fracture surface is known as fractography. Images of the crack can be used to reveal features and understand the mechanisms of crack growth. While striations are fairly straight, they tend to curve at the ends allowing the direction of crack growth to be determined from an image. Striations generally form at different levels in metals and are separated by a tear band between them. Tear bands are approximately parallel to the direction of crack growth and produce what is known as a river pattern, so called, because it looks like the diverging pattern seen with river flows. The source of the river pattern converges to a single point that is typically the origin of the fatigue failure.

Striations can appear on both sides of the mating fracture surface. There is some dispute as to whether striations produced on both sides of the fracture surface match peak-to-peak or peak-to-valley. The shape of striations may also be different on each side of the fracture surface. Striations do not occur uniformly over all of the fracture surface and many areas of a fatigue crack may be devoid of striations. Striations are most often observed in metals but also occur in plastics such as Poly(methyl_methacrylate).

Small striations can be seen with the aid of a scanning electron microscope. Once the size of a striation is over 500 nm (resolving wavelength of light), they can be seen with an optical microscope. The first image of striations was taken by Zapffe and Worden in 1951 using an optical microscope.

The width of a striation indicates the local rate of crack growth and is typical of the overall rate of growth over the fracture surface. The rate of growth can be predicted with a crack growth equation such as the Paris-Erdogan equation. Defects such as inclusions and grain boundaries may locally slow down the rate of growth.

Variable amplitude loads produce striations of different widths and the study of these striation patterns has been used to understand fatigue. Although various cycle counting methods can be used to extract the equivalent constant amplitude cycles from a variable amplitude sequence, the striation pattern differs from the cycles extracted using the rainflow counting method.

The height of a striation has been related to the stress ratio ${\displaystyle R}$ of the applied loading cycle, where ${\displaystyle R=K_{\text{min}}/K_{\text{max}}}$ and is thus a function of the minimum ${\displaystyle K_{\text{min}}}$ and maximum ${\displaystyle K_{\text{max}}}$ stress intensity of the applied loading cycle.