Element

Fatigue fracture is caused by repetitive loading of a material or component. A fractography analysis can evaluate this by checking for fatigue striations and by determining the crack propagation mode and direction. By determining whether the crack is in Stage I (initiation), Stage II (propagation), or Stage III (total separation), a fractography analysis can determine where the material began to fail, and where the fracture terminated. 

By contrast, cleavage is a low-energy fracture that occurred due to a rapid overload, under severe constraint, or in a metal’s brittle state. For this type of failure, a fractography analysis looks for macroscopic and microscopic features like river patterns, feather markings, and chevron patterns, to confirm transgranular cleavage as the fracture mode.

When materials are overloaded in their ductile state or with little constraint, a dimple rupture may occur. When evaluating this failure mode, fractography experts will look for tell-tale signs of dimple rupture, such as microvoid coalescence. 

Decohesive rupture is often an intergranular failure method that involves weakening of atomic bonds, and little plastic deformation. When combined with a full failure analysis, fractography is used to determine if these types of failures are caused by hydrogen or liquid metal embrittlement, stress corrosion cracking, creep, heat treatment embrittlement, or other causes.