SEDL / STP / STP924-EB / STP23224S

Use of Nondestructive Evaluation Techniques in Studies of Small Fatigue Cracks

Resch, MT
Research affiliateassociate professorresearch assistantresearch assistant, Stanford University, Stanford, CA

Nelson, DV
Research affiliateassociate professorresearch assistantresearch assistant, Stanford University, Stanford, CA

Yuce, HH
Research affiliateassociate professorresearch assistantresearch assistant, Stanford University, Stanford, CA

London, BD
Research affiliateassociate professorresearch assistantresearch assistant, Stanford University, Stanford, CA

Pages: 14    Published: Jan 1988

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Abstract

This paper reports progress to date in the development of an ultrasonic surface acoustic wave (SAW) technique to monitor in situ the depth of small cracks as they grow, as well as the stresses required to cause the cracks to begin to open and to fully open.

Two SAW transducers (transmitter and receiver) are used to monitor crack reflection signals. The surface length is measured by conventional methods. Based on new acoustic theory, the reflection signal and length data are used to infer crack depth below the surface. Acoustically determined depth is compared with that obtained by destructive examination of specimens, and agreement is good.

The crack opening stress determined acoustically is compared with opening stress determined by scanning electron microscope (SEM) measurements of crack mouth opening displacement versus applied stress. This comparison requires specimens small enough to fit inside an SEM chamber. Preliminary results demonstrate the ability of the acoustic measurements of crack opening to detect behavior below the surface not detected by SEM measurements. Acoustically determined changes in crack opening behavior as a function of constant-amplitude and variable-amplitude stress histories are also presented.