Effects of defects, inclusions and inhomogeneities on fatigue strength

Models predicting the effects of defects, inclusions and inhomogeneities on the fatigue strength of metals are reviewed. Their historical development is discussed, from the Isibasi model and the Frost model to recent models based on fracture mechanics. The method of application of existing models and the data necessary for prediction are explained. Fracture mechanics models mostly employ the value of threshold stress intensity factor range ΔKth 1c for a long crack as the basic data but usually apply only to two-dimensional cracks or two-dimensional defects. Furthermore, a careless assumption of the value of ΔKth 1c will result in a large prediction error of fatigue limit σw. Models that do not use ΔKth 1c are also introduced and discussed. Several models that consider the effects of non-metallic inclusions and inhomogeneities are also introduced, and the difficulties of prediction are indicated. The importance of understanding the fatigue limit phenomenon for the accurate prediction and choice of relevant models is emphasized, with regard to the existence of small non-propagating cracks and the statistical nature of the behaviour of small cracks and microstructures.