Abstract
In order to clarify the crack propagation behavior in the transition state from microstructurally small cracks to mechanically large cracks under creep-fatigue conditions, crack propagation tests were carried out using three kinds of specimens of a Type 304 stainless steel; (1) smooth specimens, (2) smooth specimens with small pre-cracks (about 100μm) introduced by fatigue at room temperature, and (3) through notched specimens. It was found that the scatter in the propagation rate due to microstructural inhomogeneity was emphasized for small cracks of shorter than 100μm in half length being equal to two grain diameters. Most of small cracks arrested and the maximum value of their propagation rates showed little dependence on the crack length. On the other hand, this scatter converged upon the mechanically large crack propagation rate when the half crack length on the specimen surface reached about 500μm. Numerical simulation was carried out based on a model of random fracture resistance of grain boundaries which had been used only for the simulation of small cracks. The model was also applicable to the transition state from microstructurally small cracks to mechanically large cracks.
