Microfractures occur in composites even if the load is far below the ultimate strength of the structures. The need to understand and monitor the initiation and/or propagation of microfractures, which are directly related to damage evolution in composites, motivated this study. Previous work has been done in modeling the response of a composite plate due to surface loading ([1]), but theoretical analysis of signals produced by sources in composites is very few. Analysis of signals produced by the initiation and/or propagation of microfractures can in principle be used to monitor damage evolution in defect-critical structural components. In this paper we describe our effort to model the microfracture phenomena in composite laminates and to find a casual relationship between the type of microstructures and the major characteristics of the signals. Previous experimental studies ([2]) have shown that signals from microfractures in graphic/epoxy composite plates can be grouped into several categories. This suggests that different fracture mechanisms in the plate are related to different signals. Localized displacement discontinuities are used here to model microfractures in composites. As a first step, Lamb waves from microfractures in a unidirectional composite plate are studies. The problem of wave propagation is then decomposed into a symmetric part and an antisymmetric part. The integral transform technique is used to solve the equations of motion in the frequency domain, and time domain solutions are obtained through Fourier inversion. The properties of the dispersion curves for both symmetric and antisymmetric motion are determined. Cutoff frequencies are used as starting points for tracing the dispersion curves for a given propagation angle. The slowness curve are calculated for each frequency. It is shown that some Lamb wave modes exist for certain propagation angles, but do not appear for other angles due to the existence of local minima in the dispersion curves. Contour integration is used to reduce the order of the integral resulting from the inverse double Fourier transform. For the far field the stationary phase method is applied to evaluate the integrals resulting from contour integration. The major contribution to the far field response is shown to come from the points on the slowness curve where the normals are parallel to the position vector of the field point, i. e., in the direction of the energy
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.full paper not received Publisher:Ashgate Publishing Company
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