Viscoelastically prestressed polymeric matrix composites: An overview

Elastically prestressed polymeric matrix composites exploit the principles of prestressed concrete, i.e. fibres are stretched elastically during matrix curing. On matrix solidification, compressive stresses are created within the matrix, counterbalanced by residual fibre tension. Unidirectional glass fibre elastically prestressed polymeric matrix composites have demonstrated 25–50% improvements in impact toughness, strength and stiffness compared with control (unstressed) counterparts. Although these benefits require no increase in section dimensions or weight, the need to apply fibre tension during curing can impose restrictions on processing and product geometry. Also, fibre–matrix interfacial creep may eventually cause the prestress to deteriorate. This paper gives an overview of an alternative approach: viscoelastically prestressed polymeric matrix composites. Here, polymeric fibres are subjected to tensile creep, the applied load being removed before the fibres are moulded into the matrix. Following matrix curing, viscoelastic recovery mechanisms cause the previously strained fibres to impart compressive stresses to the matrix. Since fibre stretching and moulding operations are decoupled, viscoelastically prestressed polymeric matrix composite production offers considerable flexibility. Also, the potential for deterioration through fibre–matrix creep is offset by longer term viscoelastic recovery mechanisms. To date, viscoelastically prestressed viscoelastically prestressed polymeric matrix composites have been produced from fibre reinforcements such as nylon 6,6, ultra-high molecular weight polyethylene and bamboo. Compared with control counterparts, mechanical property improvements are similar to those of elastically prestressed polymeric matrix composites. Of major importance, however, is longevity: through accelerated ageing, nylon fibre-based viscoelastically prestressed viscoelastically prestressed polymeric matrix composites show no deterioration in mechanical performance over a duration equivalent to ∼25 years at 50℃ ambient. Potential applications include crashworthy and impact-absorbing structures, dental materials, prestressed precast concrete and shape-changing (morphing) structures

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

A viscoelastically prestressed polymeric matrix composite (VPPMC) is produced by applying a tensile creep load to polymeric fibres, the load being released before the fibres are moulded into a polymeric matrix. The viscoelastically recovering fibres induce compressive stresses within the matrix, which can improve mechanical properties by up to 50%. This study investigates the feasibility of reducing the creep loading period for VPPMC production. By using nylon 6,6 fibres, we have demonstrated that the previously adopted viscoelastic creep strain, requiring 330 MPa for 24 h, can be achieved over a shorter duration, tn, using increased creep stress. Thus tn was 92 min at 460 MPa and 37 min at 590 MPa. Subject to avoiding fibre damage however, it may be possible to reduce tn further. From the three creep settings, elapsed recovery strain values were similar, as were the Charpy impact test data from corresponding VPPMC samples; i.e. there were no significant differences in impact energy absorption, these being ∼56% greater than their control (unstressed) counterparts

A bistable morphing composite using viscoelastically generated prestress

Elastically generated prestress within polymeric composites can be used to create bistable morphing structures; i.e. they can ‘snap through’ between one of two states. In this paper, a morphing bistable structure has been produced, utilising viscoelastically generated prestress. Here, polymeric fibres are subjected to a tensile (viscoelastic) creep load which is released before the fibres are moulded into a matrix. Following curing, the previously strained fibres continue to attempt viscoelastic recovery, creating compressive stresses within the matrix that are counterbalanced by residual tension in the fibres. The bistable structure consists of prestressing strips bonded to the sides of a thin, flexible resin-impregnated fibre-glass sheet. Bistability is achieved through pairs of strips orientated to give opposing cylindrical configurations within the sheet. It is envisaged that viscoelastically prestressed morphing structures may overcome the potential limitations of elastic prestressing; i.e. production flexibility and product longevity

Snap-through behaviour of a bistable structure based on viscoelastically generated prestress

A novel form of shape-changing bistable structure has been successfully developed through the use of viscoelastically generated prestress. Bistability is achieved through pairs of deflecting viscoelastically prestressed polymeric matrix composite (VPPMC) strips, which are orientated to give opposing cylindrical configurations within a thin, flexible resin-impregnated fibreglass sheet. This arrangement enables the structure to ‘snap through’ between one of two states by external stimulation. Deflection from the VPPMC strips occurs through compressive stresses generated from the non-uniform spatial distribution of nylon 6,6 fibres undergoing viscoelastic recovery. In this study, snap-through behaviour of the bistable structure is investigated both experimentally and through finite element (FE) analysis. By using experimental results to calibrate FE parameter values, the modelling has facilitated investigation into the development of bistability and the influence of modulus ratio (fibreglass sheet: VPPMC strip) on the snap-through characteristics. Experimental results and FE simulation show good agreement with regard to snap-through behaviour of the bistable structure and from this, the bistability mechanisms are discussed

An investigation into phenomena which influence the optimisation of ion plating systems

Ion plating involves the transportation of vapour through a low pressure glow discharge to form solid coatings on negatively biased substrates. Although the method has gained commercial acceptance, particularly for engineering applications, much of the underlying science of the process is poorly understood. This work is concerned with investigating certain mechanisms occurring within the ion plating environment, and their role in process optimisation.The work has concentrated primarily on electron beam evaporation in thermionically enhanced discharges. Investigations have centred on various aspects of discharges used in ion plating and the influence of gas scattering mechanisms on the vapour species during deposition. This has been achieved by analysing the published literature and performing experiments involving cathode sheath thickness measurements, sputter weight loss determinations, optical emission spectroscopy and coating thickness evaluation.The main findings are:(i) Thermionically enhanced discharges can considerably reduce cathode sheath thickness, providing benefits in terms of bombardment uniformity and energy transportation. However, the influence of plasma bombardment is anisotropic and also falls off exponentially with distance from the thermionic emitter. This can be offset by a comparable reduction in coating flux arrival rates if the thermionic emitter is positioned close to the vapour source.(ii) The incorporation of nitrogen in (reactive) ion plating discharges may reduce the rate of ion generation, particularly in the presence of thermionic emitters. Nitrogen dissociative charge transfer collisions within the cathode sheath may be signif icant but their practical importance is questionable.(iii) There is evidence to suggest that the metal vapour in an argon ion plating discharge transports most of the ion current to the substrate and at least some of the material arrives as atomic clusters.(iv) A model which unifies coating thickness uniformity with source to substrate distance has been developed. This predicts the existence of a virtual source phenomenon

Experimental investigation on performance of fabrics for indirect evaporative cooling applications

© 2016 Indirect evaporative cooling, by using water evaporation to absorb heat to lower the air temperature without adding moisture, is an extremely low energy and environmentally friendly cooling principle. The properties of the wet channel surface in an indirect evaporating cooler, i.e. its moisture wicking ability, diffusivity and evaporation ability, can greatly affect cooling efficiency and performance. Irregular fibres help to divert moisture and enlarge the wetted area, thus promoting evaporation. A range of fabrics (textiles) weaved from various fibres were experimentally tested and compared to Kraft paper, which has been conventionally used as a wet surface medium in evaporative coolers. It was found that most of the textile fabrics have superior properties in moisture wicking ability, diffusivity and evaporation ability. Compared with Kraft paper, the wicking ability of some fabrics was found to be 171%–182% higher, the diffusion ability 298%–396% higher and evaporation ability 77%–93% higher. A general assessment concerning both the moisture transfer and mechanical properties found that two of the fabrics were most suitable for indirective evaporative cooling applications

Viscoelastically prestressed polymeric matrix composites: An investigation into fibre deformation and prestress mechanisms

© 2018 Elsevier Ltd A viscoelastically prestressed polymeric matrix composite (VPPMC) is produced by subjecting polymeric fibres to a creep load, which is removed before moulding the fibres into a polymeric matrix. The resulting fibre viscoelastic recovery creates compressive stresses within the cured matrix. Although mechanical properties can be improved by up to 50%, the effect of fibre creep stress magnitude on VPPMC performance is unknown. In this paper, viscoelastic effects were investigated for 24 h creep stress values of 330–590 MPa. This involved recovery force measurement and wide-angle X-ray diffraction (WAXD) on nylon 6,6 fibres and Charpy impact testing of nylon fibre-polyester resin VPPMCs. Greatest performance was achieved with an intermediate value (460 MPa), suggesting an optimum creep stress condition. Moreover, with increasing creep stress, WAXD demonstrated a progressive reduction in regions with viscoelastic energy storage capability. By considering polymeric three-phase microstructural and latch-based m echanical models, a viscoelastic fibre-generated prestress mechanism is proposed

Viscoelastically active sutures – A stitch in time?

We present results to show that a commercially available polypropylene suture filament (Ethicon Prolene), following annealing and tensile creep can, after creep load removal, release viscoelastically stored energy over a period of several weeks. Specifically, over 0.1–1000 h, the suture undergoes a time-dependent contraction of ~4% and, following a short recovery time (~3 min) to a fixed strain, produces a progressively increasing recovery force of ~0.1–1 N. We suggest that this time-dependent energy release may facilitate wound healing by the action of viscoelastically induced mechanotransduction (VIM). Moreover, our recent (published) findings have led to evidence of reduced hydrophobicity from viscoelastically recovering polymeric filaments and speculation that this may emanate from the long-term release of electric charges. Thus, we propose that the latter may enhance the VIM mechanism. In this paper, we report on the direct detection of these charges and the first findings from an investigation involving the presence of cell cultures on Prolene samples that are (i) viscoelastically recovering, (ii) annealed only and (iii) in as-received condition. From (i), the results demonstrate a significant increase in cell motility, with migration towards the suture, compared to (ii) and (iii). This suggests greater stimulation of the wound healing process, an effect which is expected to continue for the duration of the viscoelastic recovery period

Drop weight impact behaviour of viscoelastically prestressed composites

Viscoelastically prestressed polymeric matrix composites (VPPMCs) are produced by subjecting fibres to tensile creep, the creep load being released prior to fibre moulding. Following matrix curing, the viscoelastically recovering fibres generate compressive stresses within the matrix which, from previous studies, can improve mechanical properties by up to 50%. This paper reports on the first study of thin flat-plate VPPMCs, using nylon 6,6 fibre-polyester resin to form cross-fibre composite plates (CCPs) with 0°/90° fibre layers and randomly distributed discontinuous fibre plates (RCPs). Drop-weight impact testing was performed on CCPs with impact velocities of 1.9 – 5.8 m/s and, compared with (unstressed) control samples, VPPMC damage depth was reduced by up to 29%; however, this difference decreased with impact velocity, indicating little improvement above 7.7 m/s. RCPs, tested at 3.0 m/s, showed a ~30% reduction in VPPMC damage depth, compared with ~20% for CCPs, but with no changes in debonded area

Viscoelastically prestressed polymeric matrix composites – effects of delayed moulding on Charpy impact properties

Viscoelastically prestressed polymeric matrix composites (VPPMCs) are produced by subjecting fibres to creep, then releasing the creep load before moulding. Previous work has demonstrated mechanical property improvements up to ~50% from nylon 6,6 fibre-polyester resin VPPMCs, compared with control (unstressed) counterparts. Since fibre stretching and moulding processes are decoupled, the time interval between releasing the fibre stretching load and moulding (delayed moulding) offers considerable production flexibility. This paper investigates delayed moulding over 0–1272 h, using fibres stored at 20 °C and-25.4 °C. Charpy impact tests demonstrated increased energy absorption from all VPPMC samples compared with control counterparts, this increase reducing with delayed moulding time. A 1272 h delay gave an increase of ~23% for fibre storage at 20 °C, and ~40% at-25.4 °C, the latter demonstrating " decelerated " ageing. For all samples, the magnitude of fibre-matrix debonding (the principal energy absorption mechanism) increased linearly with impact energy data