A Study on the Mechanism of Small Fatigue Crack Deflection Behavior in ƒ¿-Brass by Means of In-stu Atomic Force Microscopy and Crystallographic Orientation Analysis
Atsushi SUGETA, Yoshihiko UEMATSU, Ei-ichi KUMONAGA and Masahiro JONO
Abstract: Fatigue crack growth test under constant amplitude loading was carried out on ƒ¿-brass. Successive observation of transgranular small fatigue crack growth behavior was performed by means of an atomic force microscope (AFM) equipped with small in-plane bending fatigue testing machine. In the low growth rate region after crack initiation, the inclined fatigue crack grew along one slip plane in contrast with the alternating slip-off crack growth process in a long crack. Twin boundaries of ƒ¿-brass worked as a constraint against slip deformation, resulted in frequent crack deflection and crack branching. A large number of dislocations were piled up along the activated slip planes due to cyclic strain hardening, which changed the stress state around crack tip, resulted in the activation of slip deformation on the other slip plane. The fatigue crack deflection behavior was investigated by the crystallographic orientation analysis based on the Electron Backscatter Diffraction (EBSD). It was found that the direction of crack deflection did not decided only by Schmid factor. The slip factor considering the slip system and singular stress field at the crack tip was introduced in order to evaluate the easiness of slip deformation instead of Schmid factor. The direction of crack deflection was found to be explained well by the slip factor and the relative location between the preferential slip plane and crack front. Key Words:Fatigue crack growth, Atomic force microscopy, Dislocation, Slip deformation, Crystallographic orientation, Crack deflection, Slip factor