Observation of Fatigue Crack Propagation Process in Cold-Rolled Low-Carbon Steel by X-Ray Microbeam Technique

Abstract

The microstructural change around a fatigue crack in 15% cold-rolled low-carbon steel specimens was investigated by using the back-reflection X-ray microbeam diffraction technique. The results obtained are summarized as follows:(1) Dislocations introduced by predeformation rearrange themselves in the vicinity of a fatigue crack. The excess dislocation density in a subgrain, the subgrain size and the micro lattice strain in a grain are decreased by this rearrangement of dislocations while the excess dislocation density at subboundaries increases. In this way the preformed substructure developes near a fatigue crack. Micro cracks are expected to exist at subboundaries of fatigue-induced substructure and a main crack will propagate along subboundaries, joining these micro cracks.(2) The crack propagation rate has a great deal to do with the degree of development in substructure at crack tips, that is, the amount of decrement in micro lattice strain and subgrain size become larger as the propagation rate increases. It is considered that by using this relation, the estimation of the propagation rate of a crack whose rate is unknown will be possible, if the information about substructure at the crack tip is given in the observation by the X-ray microbeam.