Neutron diffraction measurements of residual stresses around a crack tip developed under variable-amplitude fatigue loadings

ABSTRACT The spatially resolved neutron‐diffraction residual stress mappings were performed on five compact‐tension (CT) specimens subjected to various variable‐amplitude fatigue loadings (e.g. overload, underload and their mixed loads) during fatigue crack propagation. Three principal residual‐stress components (i.e. longitudinal, transverse and normal stresses) were measured as a function of the distance from the crack tip along the crack‐propagation direction. The shape of respective crack tips on the five CT specimens was examined using scanning electron microscope. The results show the distinct residual‐stress fields near the crack tip and significant changes in the crack‐tip geometry for five different loading cases. It is thought that the combined effects of the changes in the residual‐stress state and crack‐tip geometry seem to be a key factor to account for the observed transient crack‐growth phenomena.