Development of Nano-scale Dynamic Observation System on Fatigue Crack Growth Behavior using Image Processing Technique

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In order to investigate fatigue crack growth mechanism, fatigue testing machine which can be installed the stage of An atomic force microscope (AFM) was designed and manufactured. Fatigue crack growth test under constant amplitude loading was conducted on an cc-brass using this testing apparatus. The knowledge obtained in this project is as follows: Nano-scale on-line observation of fatigue crack growth behavior becomes possible through the development of new designed testing system installed on AFM. In the case that the fatigue crack driving force is relatively small, one slip system, Schmitt factor of which is relatively high, operated preferentially and fatigue crack grew along this slip line. However, cyclic strain hardening occurs on that slip line, resulting in the decrease of growth rate or crack arrest. At this point, the effect of secondary slip system become larger and fatigue crack deflected to the secondary slip direction. After fatigue crack grew a short distance, the primary slip system become operates, and fatigue crack branched and grew along that. It was found that fatigue crack grew a zig-zag manner in low crack growth rate region. On the other hand, in the high crack driving force region, discrete slip lines which emanated from crack edges were observed clearly along the two preferred slip directions. It implies that two preferential slips operated to almost identical extent simultaneously and fatigue crack grew in near mode I. The slip deformation during loading and unloading was observed successively and it was found that the operation of slips behind the fatigue crack tip resulted in crack closure and that at the crack tip brought about crack growth.