Transient stress effects on fatigue crack growth thresholds due to testing methodology

It is observed that various testing methodologies used to determine ΔKth introduce transient stress effects. These effects give the impression that the measured values of ΔKth are sensitive to the test variables. However, when the same results are collectively plotted as ΔK vs. Kmax a single curve results. This is because each test method introduces its own “internal stresses” that affects the individual ΔKth that pertains to a segment of the total ΔK–Kmax curve. The implication is that any test method used to measure threshold will have history effects that cannot be avoided. Attempts can only be made to minimize such effects. It is important to recognize that threshold is not a single value, but a unique curve represented in a 2D-space by a ΔK–Kmax plot, that is independent of testing methods.