A two-stage micromechanics model for short fatigue cracks

The influence of microstructure on the propagation behavior of short surface cracks is examined in a simulated HAZ microstructure in C Mn steel. The detection of crack initiation and crack growth measurement is carried out by the replication of hourglass specimens under torsional and push-pull loading. The length of ferrite at the prior austenite grain boundaries is characterized as the effective microstructural parameter. Propagation rate is affected by the microstructure; it decreases when the crack approaches the grain boundary and increases again when it crosses the grain boundary. The short crack growth models proposed by other researchers are critically examined and a modified model based on micromechanics is presented to incorporate the effect of microstructure on the rate of crack growth. The crack growth behavior predicted by the model matches closely with the experimental results.