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.
Experimental studies were made on the surface characteristics of glass fibers with two different finishes. The effects of the chemical composition and thermal stability of the finishes on the compatibility between glass fibers and high melting temperature thermoplastics were investigated. Thermogravimetric Analysis (TGA) was used to characterize the composition of surface finishes and their thermal stability. The chemical composition of surface finishes was investigated using a combination of X-ray photoelectron spectroscopy [XPS) and Time-of-Flight secondary ion mass spectrometry (ToF SIMS). Static contact angle measurements were performed at room temperature to characterize the interfacial tension between the glass fibers and thermoplastics. The study provides a fundamental understanding on some important compatibility issues between thermoplastic resins and glass fiber related to composite fabrication and applications.
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