Effect of fatigue precracking on crack engagement during sonic IR testing

A series of sonic IR tests were performed on modified eccentrically-loaded single edge tension [ESE(T)] specimens in which crack lengths were nominally identical, but the precracking load history used to develop the sharp cracks used in the tests was varied from specimen to specimen. A measure for crack engagement length, defined as the length of crack that develops heat during sonic IR inspection was established for these tests. The result suggest that, in a certain regime, crack engagement length correlates with fatigue opening load, which is the stress intensity factor at which the crack front is fully open during the fatigue precracking. The implication of this is that laboratory test results for sonic IR should be performed with test specimens that have undergone careful precracking procedures, and that these procedures should be reported with test results.