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With an aim to establish a probabilistic model for fatigue crack growth rate da/dN from a reliability engineering point of view, a DC electrical potential system was developed to measure the crack length automatically as a function of the number of repeated cycles during fatigue tests. The crack length measured at an arbitrary interval of the number of stress cycles was fed into a personal computer, and processed in real time. This personal computer was also used to control the applied load precisely. To examine the performance of the system developed, preliminary tension-tension fatigue tests were carried out on the center-cracked specimens of 2024-T3 Al alloy under the condition of the constant stress range and the constant stress ratio. The relation between the electrical potential and the crack length was first determined using four specimens, and a sixth order polynomial was obtained for this relation. Then, statistical data of da/dN were obtained using eighteen specimens. The scatter band of the log da/dN vs. logΔK plot of the whole data was narrower than that in our previous tests in which a travelling microscope was used for crack length measurement. This result shows that the electrical potential system developed works well. The maximum error of the system in crack length measurement was about 50μm. The coefficient of variation of the crack propagation life was also smaller in the present test than in the previous test. This is considered due to that the applied load and the temperature were controlled more precisely than in the previous test. Thus, the system developed enables to obtain statistical data of the fatigue crack growth rate accurately and efficiently.

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Reliability engineering approach to fatigue crack growth rate using DC electrical potential method. Development of DC electrical potential system and preliminary experiments.