Meaning
Nucleation of a fatigue fracture along the plane of maximum shear stress marks the first stage of material failure under cyclic loading.
Repeated Process
Repeated sliding of atomic planes creates small steps on the surface known as extrusions and intrusions. These features act as microscopic stress concentrators that facilitate the shear crack initiation process. In many ductile metals the crack will grow along the shear plane for a distance equal to several grain diameters before turning to follow the principal tensile stress.
Oxidation Factors
Oxidation and humidity can accelerate shear crack initiation by weakening the atomic bonds at the tip of the slip band. Corrosive environments prevent the slip steps from re-bonding during the compression half of the cycle. This irreversible damage leads to a faster accumulation of plastic strain and a shorter time to the appearance of the first crack.
Improving Prevention
Improving the surface quality through polishing or shot peening can delay shear crack initiation by removing existing defects or introducing compressive residual stresses. Engineers also select alloys with specific alloying elements that inhibit the formation of persistent slip bands. These strategies extend the fatigue life of components like valve springs and transmission shafts that operate under high cycle counts.
