Abstract
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: dc/dN = AΔK 3ƒ −λ and striation spacing, s by the following formula: s = BΔK 5sƒ −λ , where ƒ= 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.
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