THRESHOLD STRESS INTENSITY FACTOR OF FATIGUE CRACKS

Based on a micromechanical model, the threshold stress intensity factor is predicted. It is shown that the threshold stress intensity factor is given by M root bE, where E is the Young's modulus; b is the Burgers vector; M is a microstructure parameter depending on the crack tip geometry and its surrounding microstructure. Although M is structure sensitive, the values of M for most polycrystalline metals are found to lie within a quite narrow range. The model leads to an explanation of the dependence of the threshold stress intensity factor on the stress ratio. It is concluded that the random stress variation due to the stress sources other that the applied cyclic stress is responsible for its nonlinearity. The grain size effect on the threshold stress intensity factor is also investigated. It is shown that M = a + bD(c) where a, b and c are material constants and D is the grain size. The theoretical predictions are in agreement with the experimental data available.