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This paper shows how to improve delamination resistance in composite laminates by the use of thermoplastic matrix resins and by stitching. Discussed are fatigue crack growth characteristics in thermoplastic matrix composites as well as the differences between the crack growth processes in thermoplastic and thermosetting matrix composites under cyclic fatigue and mono-tonically increasing loads. Further improvement of delamination resistance achieved by stitching shows a 20 to 30% increase in the critical crack-opening load, a 20 times higher fatigue threshold, and a two-orders-of-magnitude increase in fatigue life. Fractography evidence is presented to illustrate the role of the stitching yarns in resisting delamination. A fracture model was constructed to simulate delamination crack propagation in the presence of stitching yarns. The effects of geometric stitching parameters, as well as intrinsic and extrinsic yarn properties, were simulated with this fracture model. Finally, practical issues on the effective and economical application of stitching to composite structures are discussed.
thermoplastic matrix composites, 3-D composites, stitching, toughness, delamination, interlaminar fracture, fracture model, crack closure, crack propagation, fatigue, stress intensity factor, critical strain-energy release rate, plastic deformation, deformation process
Senior research engineer, E. I. du Pont de Nemours and Co., Inc., Wilmington, DE