The Study of Crack Closure Phenomenon Following One Tensile Overload

During the load-controlled high-cycle fatigue test, when the overload was applied, it is shown that from the crack-growth rate (da/dN) versus stress-intensity-factor range (∆K) curve, the crack-growth rate decreased, following the overload, which indicated the crack-closure phenomenon. The crack-growth-retardation period was observed after the overload. The goal of this study is to investigate the deformation evolution during tensile loading and unloading cycles using neutron diffraction. Neutron diffraction is used to investigate the crack-closure phenomenon by measuring the changes in the elastic-lattice-strain profiles around the fatigue-crack tip in a compact-tension (CT) specimen during tensile loading and unloading cycles. Spatially-resolved-strain measurements were performed to determine the in-plane and through-thickness lattice-strain profiles ahead of the crack tip under a constant tensile load. The strain scanning was repeated under various applied loads ranging from 667 to 6,667 N. Subsequently, an overload at 8,889N was applied. The strain scans repeated. After the overload, large compressive strain fields were observed close to the crack tip, indicative of the crack-closure phenomena.