a b s t r a c tA viscoelastically prestressed polymeric matrix composite (VPPMC) is produced by subjecting polymeric fibres to tensile creep, the applied load being removed before moulding the fibres into a resin matrix. After matrix curing, the viscoelastically strained fibres impart compressive stresses to the surrounding matrix, thereby improving mechanical properties. This study investigated the mechanisms considered responsible for VPPMCs improving impact toughness by performing Charpy impact tests on unidirectional nylon 6,6 fibre-polyester resin samples over a range of span settings (24-60 mm) and fibre volume fractions (3.3-16.6%). Comparing VPPMC samples with control (unstressed) counterparts, the main findings were: (i) improved impact energy absorption (up to 40%) depends principally on shear stressinduced fibre-matrix debonding (delamination) and (ii) energy absorption improves slightly with increasing fibre volume fraction, but the relationship is statistically weak. The findings are discussed in relation to improving the impact performance of practical structures.

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

The viscoelastic characteristics of ultra-high molecular weight polyethylene (UHMWPE) fibres are investigated, in terms of creep-induced recovery strain and force output, to evaluate their potential for producing a novel form of prestressed composite. Composite production involves subjecting fibres to tensile creep, the applied load being removed before moulding the fibres into a resin matrix. After matrix curing, the viscoelastically strained fibres impart compressive stresses to the surrounding matrix, to produce a viscoelastically prestressed polymeric matrix composite (VPPMC). Previous research has demonstrated that nylon fibre-based VPPMCs can improve mechanical properties without needing to increase mass or section dimensions. The viability of UHMWPE fibre-based VPPMCs is demonstrated through flexural stiffness tests. Compared with control (unstressed) counterparts, these VPPMCs typically show increases of 20-40 % in flexural modulus. Studies on the viscoelastic characteristics indicate that these fibres can release mechanical energy over a long-timescale and fibre core-skin interactions may have an important role.

Kevlar-29 fibers have high strength and stiffness but nylon 6,6 fibers have greater ductility. Thus by commingling these fibers prior to molding in a resin, the resulting hybrid composite may be mechanically superior to the corresponding single fiber-type composites. The contribution made by viscoelastically generated pre-stress, via the commingled nylon fibers, should add further performance enhancement. This paper reports on an initial study into the Charpy impact toughness and flexural stiffness of hybrid (commingled) nylon/Kevlar fiber viscoelastically prestressed composites at low fiber volume fractions. The main findings show that (i) hybrid composites (with no pre-stress) absorb more impact energy than Kevlar fiber-only composites; (ii) pre-stress further increases impact energy absorption in the hybrid case by up to 33%; (iii) pre-stress increases flexural modulus by 40% in the hybrid composites. These findings are discussed in relation to practical composite applications. POLYM. COMPOS., 35:931-938, 2014. V C 2013 Society of Plastics Engineers INTRODUCTIONAlthough pre-stressed concrete is an established structural material, the exploitation of pre-stress in polymeric composite structures seems to be comparatively rare. Residual stresses within composite moldings are normally seen as an unfortunate consequence of differential shrinkage from the processing route [1] or as a means (when purposely applied) to align fibers in filament-wound structures [2,3]. Research papers focused on enhancing the mechanical properties of polymeric matrix composites (PMCs) through pre-stress are uncommon.An elastically pre-stressed PMC (EPPMC) is directly comparable to pre-stressed concrete, in that fibers within the composite are stretched to maintain an elastic strain as the matrix cures. On solidification, this produces compressive stresses within the matrix, counterbalanced by residual fiber tension. Studies comparing unidirectional glass fiber EPPMCs, with unstressed counterparts, have indicated increases in tensile strength and elastic modulus of 25% and 50% respectively [4]. Impact resistance and flexural properties (stiffness and strength) have also been found to increase by up to 33% [5,6]. Explanations for these improvements emanate from matrix compression and fiber tension effects: these may (i) impede or deflect propagating cracks and (ii) reduce composite strains from external tensile or bending loads [4][5][6]. Although elastic pre-stressing should offer improved mechanical properties without the need to increase mass or section dimensions within a composite structure, there are potential drawbacks. Fiber orientation, length, and spatial distribution would be restricted by the application of fiber tension during matrix curing, thereby compromising mold geometry. Moreover, the matrix (being polymeric) may undergo localized creep at the fiber-matrix interface regions, in response to the compressive stresses imposed by the fibers: hence the pre-stress effect could deteriorate with time [7].A viscoelastically pre-...

a b s t r a c tThe impact properties of continuous unidirectional UHMWPE fibre-reinforced polyester resin composites have been investigated, to elucidate the effects of prestress on energy absorption characteristics. Prestress within composite samples was produced by subjecting the UHMWPE fibres to a creep load, which was then released prior to moulding. From Charpy impact tests, these viscoelastically prestressed samples absorbed $20% more energy than their control (unstressed) counterparts, with some batches reaching 30-40%. Generally, whether prestress is created through elastic or viscoelastic means, fibre-matrix debonding is regarded as being a major energy absorption mechanism in this type of composite, but this was not evident in the current study. Instead, evidence of debonding at the skin-core interface within the UHMWPE fibres was found, the skin regions possessing lower stiffness and longer term viscoelastic activity. Skin-core debonding appears to have a significant energy absorbing role within the prestressed samples and we believe it is a previously unrecognised mechanism.