In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.
Fracture surface of an aluminium crank arm from a bicycle. The dark area (due to oil, dirt and fretting) is a slow growth fatigue crack and may contain striations. The bright area is caused by sudden fracture.
Micrographs showing how surface fatigue cracks grow as material is further cycled. From Ewing & Humfrey, 1903
Fracture surface in a glass rod showing beach marks surrounding the initiation site.
Example of a HFMI treated steel highway bridge to avoid fatigue along the weld transition.
In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. A component's thickness affects the constraint conditions at the tip of a crack with thin components having plane stress conditions and thick components having plane strain conditions. Plane strain conditions give the lowest fracture toughness value which is a material property. The critical value of stress intensity factor in mode I loading measured under plane strain conditions is known as the plane strain fracture toughness, denoted . When a test fails to meet the thickness and other test requirements that are in place to ensure plane strain conditions, the fracture toughness value produced is given the designation . Fracture toughness is a quantitative way of expressing a material's resistance to crack propagation and standard values for a given material are generally available.
Morphology of fracture surfaces in materials that display ductile crack growth is influenced by changes in specimen thickness.