1989 年 38 巻 434 号 p. 1289-1295
Fatigue crack growth tests under constant amplitude and repeated two-step loadings were carried out on four kinds of materials of a high-strength low alloy Cr-Mo steel, which have different microstructures obtained by various heat treatments. Crack length and crack closure were measured by using the minicomputer-aided unloading elastic compliance method. The results obtained are summarized as follows.
The fatigue crack growth rates of as-rolled, and 600°C and 400°C tempered materials were quite similar. However, the behavior of effective stress intensity range ratio, U, was found to be different depending on the microstructures of the materials. For the 600°C and 400°C tempered materials, U showed the maximum value at the Kmax of about 20MPam1/2 and then gradually decreased with increasing Kmax because of cyclic softening.
It was found that for the 200°C tempered material, which has martensitic structure and high hardness, the dominant fracture appearance was cleavage facets at low ΔK region and intergranular facets at high ΔK region, and that the growth rate was higher than those of the other materials in the region of ΔK above 20MPam1/2. The U was found to be 0.5-0.7 in the whole region of ΔK tested. This is probably due to the plasticity-induced and roughness-induced crack closure, because the material used had enough ductility and showed rougher fracture surfaces.
Even for the materials which showed cyclic softening, the fatigue crack growth rates under repeated two-step loadings could be well predicted in terms of the measured ΔKeff, using the da/dn-ΔKeff relationship obtained from the constant amplitude loading tests. However, the predicted growth rates might be low and unconservative, when ΔKeff was estimated from the constant amplitude loading test results.