Abstract
The fatigue crack propagation properties were investigated in six types of steels for welded structures, those yield strengths range from 163 to 888 MPa. The effect of stress ratio R and material properties, such as yield strength or grain size, on the fatigue crack growth behavior can be traced on the crack clousre, and when the growth rates are plotted in terms of effective stress intensity factor range ΔKeff, the crack growth behavior of all steels at various R can be expressed by one curve. Then the resultant curve (da/dN-ΔKeff for all steels) can be available in the most conservative estimation of fatigue crack propagation in welded steel structures even under high tensile residual stress or high R. The crack closure is dependent on both R and ΔK, and the crack opening ratio for all steels can be expressed by the function of ΔK and R which has previously proposed by the authors. Namely, crack closure is remarkable only near threshold ΔKth for R≈O. Considering this dependence of crack closure on ΔK, the well-known relationship between Paris' coefficients C and m can be explained quantitatively. Moreover, the predicted variation in ΔKth with R using above-mentioned U equation shows good agreement with the experimental results. The ΔKth at R=0.1 decreased with increasing yield strength or decreasing grain size except for the steel with the lowest yield strength.
