The study of dislocation structures at fatigue crack tips in polycrystalline copper under various crack propagation rates at stage II: crack propagation

The purpose of this study is to investigate the dislocation structures in f ront of fatigue crack tips embedded at various propagation rates in copper. Back electron images with the scanning electron microscope were used for t he observation. The results are, at a rate of 10(-6) mm per cycle, that the dislocation morphologies are distinguishable and can completely to evolve into various dislocation structures. At a rate of 10(-7) mm per cycle, the dislocation morphologies are the same as those of the 10(-6) mm per cycle r ate, but the ranges of the dislocation structures are too small to be disti nguished. Regardless of the plastic strain amplitudes, the average diameter of the dislocation cell in front of the crack tip is about 0.7 mu m. There fore, once the low-energy dislocation cell was formed, the crack initiates and/or propagates. At a 10(-8) mm per cycle rate, the crack tip is propagat ed toward the region of true strain localization. The dislocation structure threshold for propagating the crack tip is cell with an average diameter o f less than 0.7 mu m. (C) 2000 Elsevier Science S.A. All rights reserved.