Fatigue crack in a piece of steel. Fatigue occurs when a material is subjected to repeated loading and unloading.In this image, two different zones can be observed. At right, there are striations, related to the slow and progressive crack growth. At left, granular area due to sudden fracture, once the critical size of the crack has been reached. In this piece, the crack spread from right to left, so the concavity doesn't show the initial crack as it usually does..