A Critical Evaluation of Mathematical Equations for Fatigue Crack Growth with Special Reference to Ferrite Grain Size and Monotonic Yield Strength Dependence

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Abstract
In order to select the correct mathematical equation among many. for fatigue crack growth or to develop a new criterion, some critical assessments are carried out by compiling the experimental data on the effect of ferrite grain diameter on fatigue crack growth rate for two low-carbon steels and a 3 weight percent silicon iron. The effect of monotonic yield strength is also studied. Emphasis is on fatigue crack growth in the second region of fatigue crack growth rate versus ΔK curve. Results suggest that, if fatigue crack growth rate is expressed as da/dN=B(ΔKsσc)′​′ where σc is some constant with the dimension of stress and s is an appropriate measure of length, then σc and B are practically independent of monotonic yield strength, but n has the trend of a slowly increasing function of the ferrite grain diameter and, thus, indirectly a decreasing function of the monotonic yield strength. Moreover, it is suggested that n is also affected with some other microstructural factor.