Evaluation of a stress wave NDT technique for detecting skips in the gluelines of edge-glued red oak panels

A stress wave nondestructive testing technique was evaluated for 1) detecting skips or voids in the gluelines of edge-glued red oak panels; and 2) detecting differences in the thickness of solid gluelines. All tests were conducted on panels containing a single glueline with the direction of wave propagation perpendicular to the glueline. The test system measures transit time and amplitude of stress waves propagating from edge to edge of the panel. Tests of panels containing a single, centered glueline show that transit times increase and amplitudes decrease when tests are conducted directly across a skip in the glueline. A study of the relationship between wave velocity and grain angle indicates that stress waves deviate around skips in the glueline. A geometric limitation of the technique exists with short, wide panels due to interference created by waves that reflect from panel ends because of greater longitudinal versus transverse velocities. Tests of panels with varying glueline thicknesses indicate that the technique is not sensitive to differences in glueline thickness. No apparent differences in transit time or signal amplitude and no apparent differences in wave histories are observed. Extremely complex wave signals were observed, making it difficult to measure energy loss of the stress wave.

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