Abstract
Fatigue crack growth tests under tension-tension loading were performed on center-notched specimens of a high strength steel HT60, and the statistical property of fatigue crack growth rate da/dN was studied by testing 36 specimens under the same condition. The fatigue crack length was measured automatically using the DC electrical potential method and a microcomputer. The experimental data were analyzed from the viewpoint of the composite variability model of the fatigue crack growth rate which had been proposed by the present authors. The model parameters were estimated by a new method proposed in this paper as well as by the one proposed previously. The result of data analysis was compared with the previous result of 2024-T3 Al alloy. When expressing C in the Paris equation da/dN=C(ΔK)m as C=C*gl where g and l correspond to inter-specimen variability and intra-specimen variability of C respectively, the coefficients of variation of g and l for HT60 were approximately of the same magnitude as those for 2024-T3 Al alloy. On the other hand, the correlation distance for intra-specimen variability was smaller for HT60 than for 2024-T3 Al alloy. When a straight line log(da/dN)=logCs+mslog(ΔK) was fitted to the data of each specimen, ms and logCs of each specimen followed normal distributions approximately and strong negative correlation was found between ms and logCs. This property was quite similar to that for 2024-T3 Al alloy.
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