Abstract
Striations are observed on the fracture surface of the broken metal material, and the working load has been estimated from its interval. However, striation is not always observed, and in that case, the fatigue crack propagation rate cannot be estimated by the conventional method. At the fatigue crack tip, the plastic zone formed during tensile loading is compressed into the surrounding elastic zone during unloading. From this fact, it is considered that the crack propagation rate is closely related to plastic strain distribution. In this study, we used a Fe-3Al bcc single crystalline alloy and the plastic strain just under the fracture surface was investigated. Using ECCI method and EBSD method in the different stress intensity factor range, the change of the inclination of dislocation wall near the fracture surface and the crystal orientation gradient is measured, and the possibility of the estimation of the applied stress using the microstructure near the fatigue fracture surface is examined. As for the dislocation structure, it is shown that there is a boundary where the dislocation wall tilts when a new slip system starts to work as the observation region approaches the fracture surface. And, it was proven that the distance from the fracture surface to the boundary changed with the ΔK value. And, this paper indicates the possibility of obtaining useful information for fatigue fracture analysis such as ΔK by investigating the region in which dislocation wall exists. As for the crystal orientation gradient, there was a decrease of GROD value near the fracture surface, and this seemed to be the effect of the cyclic plasticity region. It is also shown that the GROD values can be qualitatively evaluated by the difference of ΔK values.
