Abstract
Study on fatigue crack growth in steels was carried out from energetic point of view, i.e. taking account of plastic work around the fatigue crack. Based on the examination of the relation between fatigue crack growth rate (d a/d N) and the plastic work around the fatigue crack tip ( W 0.02 in SUS304, Fe-3Si and HT 60 steels, a material constant-fatigue crack propagation work coefficient- Q 0.02 is proposed. It is the ratio of W 0.02 to d a/d N and means the degree of the resistance to fatigue crack growth. Numerical expression of Q 0.02 by mechanical properties was derived, which is given by Q 0.02= 9.3x10 1(σ y+σ 0.2) σ y 1.3 Comparison of Q 0.02 of various steels showed that Q 0.02 of high strength steels is very small compared with that of low strength steels. Graphical representation of the relation between Q 0.02 and d a/d N at various values of ΔK/ σ y for steels revealed that d a/d N at given value of ΔK/ σ y increase with decreasing Q 0.02. It is shown that fatigue crack growth behaviour of a steel (d a/d N- ΔK relation) can be obtained from the Q 0.02-d a/d N diagram by knowing the mechanical properties. Discussion on design stress level of the steels is also given.
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