The effect of frequency on fatigue crack propagation rate and striation spacing in 2024-T3 aluminium alloy and SM-50 steel

The effect of loading frequency on fatigue crack propagation rate and striation spacing in 2024-T3 aluminium alloy and SM-50 steel was studied at room temperature. The fatigue crack propagation rate is expressed experimentally by the equation: $\text{dc/dN = AΔK}{}^3\text{?}{}^{\mathrm{?\lambda }}$ and striation spacing, s by the following formula: $\text{s = BΔK}{}^5\text{s?}{}^{\mathrm{?\lambda }}$, where ?= loading frequency, λ = 0.08~0.14 and γ = 0'06~0?12.Fractographical studies were made.The ln dc/dN?lnΔK straight line and the ln s?lnΔK straight line intersect each other. That is, dc/dN < s below the stress intensity factor at the intersecting point, and dc/dN >s above the critical stress intensity factor. It is to be noted that dc/dN and s will not coincide except the very narrow region near the intersecting point.The formula of dc/dN experimentally obtained in this article has quite the same type as indicated by the dislocation dynamic theory of fatigue crack propagation.