A New Model for Initiation and Growth of Fatigue Cracks

Abstract

Abstract The initiation and growth of fatigue cracks are often associated with localized fatigue-induced or pre-existing soft layers (∼0·1 μm thick). The deformation of specimens, consisting of soft layers hundreds of μm thick sandwiched between harder regions, simulated many features of fatigue on a larger scale and demonstrated that slip-band extrusions were produced when soft layers were ‘squeezed out’ at the surface. It is concluded that fatigue cracks initiate and grow (in stage I and stage II) by a mechanism of intrusion which occurs when soft layers are ‘sucked in’ as a result of preferential tensile deformation in the soft layers. This deformation is discussed in terms of macroplasticity theory and dislocation theory. It is concluded that the production of fine sub-grain structures in soft layers during fatigue greatly reduces the constraint normally experienced by thin soft layers and permits extensive deformation in these layers at relatively low stresses. The proposed model accounts for most of...