In situ characterization of the fatigue crack-tip stress fields

A neutron diffraction technique was used to directly measure residual stress fields around a fatigue-crack tip at the mid-thickness and near the surface of the compact-tension specimen, where the fatigue crack was grown with a R-ratio of 0.1 under a load control mode. The stress mapping was conducted as a function of distance from the crack tip with 1-mm spatial resolution of neutron beam along the crack-propagation direction. The residual strain fields near the surface were much more significant than those at the mid-thickness. The evolution of the residual stress fields was examined in situ under applied load, in order to investigate the distribution of stresses in the vicinity of the crack tip. The three orthogonal stress components (i.e., crack growth, crack opening, and through thickness) of the specimen under loading were calculated from the directly-measured three orthogonal strain components without any assumptions of a plane strain or plane stress condition. A clear distinction for the evolution of the crack-tip stress field was observed along the direction of the through-thickness of the sample, which might account for the intrinsic differences of a crack-opening process and resultant crack-tip driving force between at the mid-thickness and near the surface.