Initiation and growth of micro-cracks under cyclic loading

The early stages of fatigue damage causing nucleation of micro-cracks, followed by their growth and coalescence to macro-cracks have been studied in single and multi-phase steels. The effect of microstructure on initiation and growth of micro-cracks has been examined. The inter-relation between mechanics, mechanism, microstructure, and the phenomenology associated with the early stages of anomalous crack growth is the primary content of this investigation. The initiation of micro-cracks in single and multi-phase materials during cyclic loading, the growth of the short cracks, and the transition from short to long cracks in relation to the microstructures of a few commercial (304LN stainless, SA333, 0.003, 0.25 and 0.47% plain carbon) steels are discussed. It is found that formation of irregular voids inside slip bands, initiation and growth of small voids at the grain boundary and subsequent joining of these with other voids and splits, slip band impingement at grain boundary and elastic-plastic incompatibility synergistically influence the process of crack initiation. The fatigue threshold values obtained from short crack growth experiments are found to lie in a narrow range and are closer to long crack thresholds. The cracks were found to have affinity to grow through interfaces, but the length of a crack passing through a specific phase was found to be approximately proportional to its amount.