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

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.