A Study on Applicability of Microscopic Approach to clear the Fracture Behavior of Materials

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We studied on essential aspects of fatigue crack propagation behavior from a microscopic view by means of Scanning Electoron Microscope (SEM) and Molecular Dynamics (MD). Fatigue tests of Cu single cristals (fcc material) and 3%Si steels (bcc material) were conducted using CT specimens. Combinations of crystallographic orientation and pre-crack direction are varied systematically for each specimens. It is shown that fatigue crack of both materials propagates microscopically on the active slip plane which is characteristic of each crystal system, but not on the mechanical principal stress plane. This mechanism can not be explained by the Laird model, which is thought to be appropriate in continuum mechanics, and can be explained by the modefied Neumann model. In these materials an active slip system interferes intricately with the others, and fatigue crack propagation rates depend much on this mechanism, therfore Paris law are not useful in these cases. In MD simulations corresponding to test conditions, it is shown that the propagation behavior of fatigue crack of both materials is similar to tests resuls and SEM observations of fractured surfaces. Changes of atomcs under cyclic loading are also simulated and represent the irreversible changes of atoms (fatigue damages). In case of 3%Si steels, we investigated effects of grain boundary on fracture behavior and comperared with test resuls. When a crack across the grain boundary, fracture behavior will be changed largely upon the crystallographic direction of two adjoining grains. It can be said that MD method may be a useful tool for analyzing a microscopical mechanism of fracture in metals.