Abstract

Parametric Stress Modulation Effect on Surface Wave Interaction with Cracks

S.I.Rokhlin, J.Y.Kim, V.A.Yakovlev (The Ohio State University, Edison Joining Technology Center, NDE Program, Columbus, Ohio, USA)

e-mail: rokhlin.2@osu.edu

This paper addresses experimental and theoretical aspects of parametric effects on a surface acoustic wave interacting with a partially closed fatigue crack emanating from a surface cavity. Such a crack may be considered as an imperfect interface in a solid under different clamping conditions modified by small external periodic loading. The loading results in change of interfacial conditions and/or crack length leading to wave mixing (modulation). In our case the ultrasonic pulse has a center frequency of 5MHz; the modulation frequency is 10Hz, thus the external dynamic action can be considered quasistatic. The measurements were performed for different crack lengths, variable static crack closure stresses, and different stress modulation levels. The reflected ultrasonic pulses were digitized and amplitudes of the first and higher modulation harmonics were extracted. The fatigue crack initiation and propagation and its opening-closure behavior are related to those harmonic amplitudes. It turns out that the application of low amplitude modulation load enhances the sensitivity of detection of partially closed fatigue cracks helping to discriminate the evolving crack from a volumetric flaw.

Section : 3