Prestressed polymeric matrix composites : longevity aspects

Elastically prestressed polymeric matrix composites (EPPMCs) are produced by stretching fibers (e.g., glass) within the composite during matrix curing. The resulting prestress can enhance mechanical performance, without increasing section dimensions or weight. Viscoelastically prestressed polymeric matrix composites (VPPMCs) can provide similar benefits, these being produced by subjecting polymeric fibers (e.g., nylon 6,6) to a creep load, which is released prior to molding. Although VPPMCs offer simplified processing and flexibility in product geometry, long-term viscoelastic activity within the prestressing fibers is sensitive to time-temperature limitations. In this study, nylon 6,6 fiber-polyester resin samples were subjected to accelerated ageing. Using time-temperature superposition, the samples were maintained at 70°C for 2,298 h, representing a 20-fold ageing increase over previous work. Subsequent Charpy impact testing (at 20°C) demonstrated that the VPPMC samples absorbed ?40% more energy than corresponding control (unstressed) counterparts; i.e., no deterioration in impact performance was observed, over a duration equivalent to ?25 years at 50°C. In contrast, the longevity of EPPMCs remains unknown, but it is suggested that progressive localized matrix creep at the fiber-matrix interface regions may cause a deterioration in elastically generated prestress with time and/or elevated ambient temperatures