Direct, Real-time Observation of Fatigue Crack Growth Behavior under Hi-Lo Two-step Loadings and Quantitative Analysis of Deformation near Crack Tip Using Image Processing Technique.

Abstract

Fatigue crack growth tests under Hi-Lo two-step loadings were carried out on a 3% silicon iron in a scanning electron microscope, using the specially designed servo-hydraulic fatigue loading system. Fatigue crack growth behavior was discussed based on direct, real-time observation and microscopic quantitative analysis of principal shear strain distribution near the fatigue crack tip. which was obtained by a newly developed image processing technique. It was found that the delayed retardation behavior of crack growth occurred under Hi-Lo two-step loadings and the crack growth rate decreased as the frequency of crack branching increased. At the lowest growth rate, the interval of crack branching was found to be only about 4∼10μm. Principal shear strain distribution exhibited that the preferred slips occurred actively at not only the main crack tip but also the branched crack tips. Decentralization of the crack driving force may account for the retardation of crack growth rate.