Small Defects and Inhomogeneities in Fatigue Strength: Experiments, Models and Statistical Implications

The method explained in this paper for quantitative evaluation of fatigue limit for materials containing defects is based on the experimental evidences that inhomogeneities and micro-notches can be treated like cracks. First, the basic concept of the √area parameter model is explained introducing the various data obtained by the first author's group for over last 15 years. Evidences are shown that small cracks, defects and nonmetallic inclusions having the same value of the square root of projection area, √area, have the identical influence on the fatigue limit regardless of different stress concentration factors. Various applications of these concepts to various defect types and microstructural inhomogeneities are shown. Since the estimation of fatigue strength is related to the estimation of the size of maximum defects occurring in a piece, the methods for searching the defects and the quality control of materials with respect to inclusion or defect rating as well as their statistical implications are discussed.