Short fatigue crack growth under nonproportional multiaxial elastic–plastic strains

A short crack growth model is presented for calculating fatigue lives to technical crack initiation in materials and structures under nonproportional multiaxial loading. The model is based on an integration of the crack growth equation expressed in terms of the effective range of the cyclic J-integral. Crack closure, elastic–plastic deformation behaviour including cyclic and nonproportional hardening is considered explicitly. The deformation behaviour of the material is described using a newly developed plasticity model. The influence of microstructure and stage I crack growth is mirrored in the starter crack length. Crack coalescence and inhomogeneous strain fields are neglected as well as environmental influences on crack growth. A satisfying accuracy of calculated fatigue lives is achieved. This is verified by experiments on three materials under a variety of multiaxial nonproportional loading sequences. The correlation between the perception as stated in the model and real damage evolution is shown on calculated and experimentally determined short crack growth curves, initiation angles and strain life curves.