Performance enhancement of nylon/kevlar fiber composites through viscoelastically generated pre‐stress

Abstract

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-...

Prestressed polymeric matrix composites: Longevity aspects

“…Nevertheless, as reported in the “Introduction” section, VPPMCs have now been demonstrated with UHMWPE fibers, which are ∼4 times stronger and >20 times stiffer than nylon 6,6 fibers ; hence, their mechanical performance is competitive with glass and carbon fibers. VPPMC strength and stiffness properties can also be enhanced by commingling nylon 6,6 fibers (for prestress) with other mechanically superior fibers, such as Kevlar . Since however, VPPMCs require prestressing fibers that are heat‐sensitive (such as nylon 6,6 or UHMWPE), it may be expected that applications involving high thermal loadings would favor the use of EPPMCs.…”

“…The matrix material was a clear‐casting polyester resin, as used in recent work , i.e., Cray Valley Norsodyne E9252, mixed with 1% MEKP catalyst, supplied by CFS Fibreglass, UK. Gel‐time (room temperature) was ∼0.25 h. Unidirectional continuous fiber composites were open‐cast in two aluminum molds, the process being completed within 0.5 h of the fiber stretching procedure.…”

Towards optimisation of load-time conditions for producing viscoelastically prestressed polymeric matrix composites

“…In contrast with previous VPPMC studies using nylon 6,6 fibre [1e6, 8,9,11], the fibre used in this study was obtained from an industrial supplier, Ogden Fibres Ltd, UK. Both new and old (i.e.…”

“…VPPMC methodology requires the use of polymeric fibres with appropriate viscoelastic properties and most of the research to date has involved nylon 6,6 fibres [1e6, 9,11]. Clearly, these fibres are, in terms of strength and stiffness, mechanically inferior to the fibres that can be used for EPPMCs, although performance enhancement has been recently demonstrated with nylon 6,6 fibres (for prestress) commingled with Kevlar fibres [8]. Moreover, VPPMCs using viscoelastically generated prestress from other reinforcements have been successfully demonstrated, i.e.…”

Application of time–stress superposition to viscoelastic behavior of polyamide 6,6 fiber and its “true” elastic modulus

“…Of particular interest, is polyamide 6,6 fiber, as this has become the most established reinforcement in the development of viscoelastically prestressed polymeric matrix composites (VPPMCs). The first VPPMCs to be successfully demonstrated utilized polyamide 6,6 fiber and their development has progressed, leading to, for example, commingled polyamide 6,6‐aramid fiber VPPMCs for enhanced mechanical performance . Most recently, polyamide 6,6 fiber has been used to produce the first bistable morphing structure based on VPPMC principles, and this may provide opportunities to develop morphing aerofoils for aerospace and other applications .…”