The ultrasonic interrogation of components for the detection and sizing of defects has advantages over other techniques in that surface as well as subsurface defects can be probed. However, the phenomenon of crack closure or crack surface contact can reduce the detectability of a crack and lead to erroneous crack sizing. The significance of crack closure is twofold: (1) It is important for the interpretation of fatigue data, in particular as affected by the stress ratio, by spike overloads/underloads, and by threshold effects. (2) It affects the detection probability of fatigue cracks, which influences strongly the capability for accurate life prediction. In the present paper, the interaction of ultrasound with fatigue cracks will be discussed with primary emphasis on the first objective.
The experimental configuration of our ultrasonic measurements is such that a fatigue crack is illuminated by a longitudinal wave incident perpendicular to the crack face and focussed in the plane of the crack. The longitudinal wave is partially transmitted by the closure zone and picked up by another focussed receiver transducer. By changing the angular orientation of this receiver, longitudinal or transverse polarized waves diffracted in the closure zone can be detected. By spectral analysis, translation of the sample with respect to the detection system and rotation of the receiver, one can monitor the frequency, spatial, and angular dependence of the crack transmissivity. These data are composed with a “spring” model, which replaces the topological description of the closure region, providing the spatial distribution of the average diameter, d, and average separation, C, of the contacts.
From this contact topology and the “flow pressure” (three times the ultimate tensile strength) of the material we deduced the residual stress field in the wake of the crack. These results agree well with X-ray diffraction measurements. Furthermore, we estimated the “shielding” stress intensity factor using these results for a number of fatigue cracks grown under a variety of loading conditions. If shielding is taken into account, the effects of load excursions on fatigue crack propagation can be estimated with reasonable accuracy.