Abstract
In modern design, it is an important task to estimate fatigue life. The growth law of a small crack must be known in order to estimate the fatigue life of machines and structures, because the fatigue life of members is controlled mainly by the behavior of a small crack. The growth rate of a small crack cannot usually be predicted by linear elastic Fracture mechanics, but is determined uniquely by the term σ n a l. In this paper, two fatigue crack growth laws, dl dN = C ΔK m and dl dN = C 1 σ n al , which hold in large and small cracks, respectively, are taken as the representative ones expressing the crack growth rate of ductile materials. Many experimental results indicate that the relation dl dN ∝ ΔK m holds under low nominal stresses and the relation dl dN ∝ σ n al holds under high nominal stresses. A unifying explanation for two growth laws is made based on an assumption that the crack growth rate is proportional to the reversible plastic zone size. Moreover, an effective and convenient method based on a small-crack growth law, dl dN = C 3 (σ a,/σ u) nl , in which the effect of mechanical properties is partly considered, is proposed for predicting the fatigue life of plain members, where σ u is the ultimate tensile strength. The validity of this method is confirmed through its application to other researchers' data.
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