Phenolic and Polyester Pultrusions Under Load in Fire

The failure of polyester and phenolic pultrusions under tensile and compressive load and a one sided heat flux of 50 kW m-2 has been studied. A thermal/mechanical model, based on the Henderson equation and laminate theory, has been used to model their behaviour. In tension, significant load bearing capacity was retained over a period of 800 s, due to the residual strength of the glass fibres. However, pultruded composites are susceptible to compressive failure in fire, due to the loss of properties when the resin Tg is reached. The fire reaction properties reported here showed the phenolic pultrusions to perform better than polyesters in all fire reaction properties (time to ignition, heat release, smoke and toxic product generation). The measurements under load in fire showed that the phenolic system decayed at a slower rate than the polyester, due mainly to the very shallow glass transition of the phenolic, but also the char forming characteristic of the phenolic. The behaviour described here for phenolic pultrusions is superior to that reported for some phenolic laminates, the main reason probably being their lower water content.

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Long term creep and stress rupture of aramid fibre

The present paper describes a creep rupture investigation on aramid fibre yarns (Twaron 1000 and Kevlar 29) supplied by Teijin and Du Pont respectively. The ISO 9080 extrapolation procedure, which was developed for thermoplastic pipe materials, was used to model and interpret the results. The 4 parameter version of this procedure fitted the results well and gave useful predictions of the long term stress rupture behaviour, lending confidence to existing qualification procedures for the use of aramid fibre in reinforced thermoplastic pipe (RTP), and other applications involving continuous high tensile loads. Creep strain measurements on yarns showed a near constant degree of creep deformation per decade. Although they may involve some of the same mechanisms the creep and stress rupture processes appear to operate independently and on different time scales. It was found that creep deformation in aramid yarns is unlikely to be a significant problem at stress levels corresponding to a 20 year lifetime.

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Simulation of Z-cored sandwich panels

The failure behaviour of fibreglass sandwich panels with structured internal cores (z-cored panels) was studied in bending. A finite element model was developed for the simulation of three point bending tests and this has been validated against experimental results. The model was able to predict both the elastic response and, more importantly, the failure behaviour of the structure. It is therefore suitable for use in the optimising the design of z-core sandwich panels for transport applications. The same modelling approach was also applied to the structural behaviour of a larger sandwich panel with a metallic insert which was employed in the design of a semitrailer as part of a demonstration of the viability of the technology.

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Moisture absorption of GRP using Magnetic Resonance Imaging

The diffusion of water in an isophthalic polyester glass reinforced composite laminate, typical of marine applications, has been studied through accelerated water uptake tests involving total immersion in seawater at 40 and 60uC. Gravimetric analysis has shown that at 40uC, a Fickean type diffusion is operative with a saturation level of 0?8% whereas at 60uC, a two stage Langmuir type diffusion is operative. The nuclear magnetic resonance technique has also been used in an attempt to image the diffused water and determine its position within the composite. This has revealed that the water concentrates on the fibre/matrix interface and the concentration there is twice that of bulk of the matrix. The diffusion process appears to be aided by ‘wicking’ along the fibre/matrix interface.

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Quantitative DMTA of composites

The Cole–Cole (C2C) and Havriliak–Negami models respectively, have been identified as suitable for the parametric modelling of time and temperature behaviour of composites with
symmetrical and asymmetrical retardation time spectra. For C–C a shortcut method is proposed to enable the prediction of creep behaviour from dynamic mechanical and thermal analysis (DMTA) data. The C–C plot provides a method of characterising the retardation time distribution, independently of any time–temperature equivalence relationship. The isophthalic polyester resin and composites examined in the present study all showed symmetrical retardation time spectra (in log time) and their DMTA behaviour was well modelled using C–C, with the added assumption of Arrhenius relaxation kinetics. The proposed creep model also worked well for these materials.

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The Integrity of Polymer Composites During and After Fire

This paper reports on changes to the mechanical properties of woven glass laminates with polyester, vinyl ester and phenolic resins during fire exposure. Two sets of experiments were carried out. First, unstressed laminates were exposed to a constant one-sided heat flux (50kWm-2) for various times, and the residual post-fire strength at room temperature was reported. In a second series of experiments, laminates were tested under load. The times corresponding to a given loss of properties were 2–3 times shorter than in the previous case. It was found in both cases that modes of loading involving compressive stress were more adversely affected by fire exposure than those involving tension.

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Vacuum bag moulding of large thermoplastic parts in commingled glass/PET copolymer

This paper describes the development of a thermoplastic composite system for structural application in the chassis of an electrically propelled bus. The work involved the characterisation and modelling of a vacuum bag moulding process using a woven commingled thermoplastic composite precursor. The matrix materials were PET and a PET copolymer. The process employs an ambient pressure oven, with tooling that can be made from composite, metal or ceramic. The process results in good quality laminates, with a void content generally lower than 1%. The temperature profile through the part and the consolidation behaviour were characterised and modelled. It was found that the thermal profile could be modelled with adequate accuracy using ‘single point’ values of thermal properties. Experimental measurements showed, for the first time, that consolidation occurs in two stages: a low temperature solid state debulking near to Tg, followed by full melt impregnation at a higher temperature (above Tm in the case of the homopolymer). Both stages in the consolidation process were modelled separately using a simplified version of the Kamal equation.

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Modelling residual mechanical properties of polymer composites after fire

A thermomechanical modelling approach is proposed for estimating the residual properties of fibre reinforced polymer composites damaged by fire. The modelling was carried out in two parts: (i) prediction of the extent of thermal decomposition (or charring) using a thermal model; and (ii) prediction of the post-fire behaviour using a two layer model that combines the properties of the undamaged laminate and the residual char. Fire experiments were performed on glass–polyester, vinyl ester, and phenolic laminates using a cone calorimeter operated at heat fluxes in the range 25–100 kW m−2, for times up to 30 min. After cooling to room temperature the thickness of the thermal damage layer was determined, along with values of the residual tensile, compressive and flexural properties. For the ‘two layer’ model it was found that the effective boundary between char material and undamaged laminate corresponded to the point where the residual resin content (RRC) of the laminate was 80%. Surprisingly, this value was found to hold for all three resin types tested. Using this RRC value, excellent agreement was found between the measured and predicted post-fire char thickness and the residual mechanical properties. The approach presented is the first reliable method for accurately predicting the residual properties of composites after fire.

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The Cost Effective Use of Fibre Reinforced Composites Offshore

This report reviews areas where composites are finding application in the oil and gas industry, onshore and offshore. The most significant advances have been made in the areas of pipework and fluid handling, driven by light weight and corrosion resistance compared to metals. Modest, but significant progress has also been made in structural applications. Lessons are being learned from successful applications with the result that operators, design houses and contractors are now beginning to take a serious interest in their wider use. Expansion is therefore expected to continue into all sectors of the oil and gas industry.

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Composite angle ply laminates and netting analysis

This paper relates to the ‘netting-analysis’ approach, often used in calculations of the behaviour of helically wound reinforced pressure vessels and tubes. Numerical calculation of the stress–strain relations for angle ply composite laminates often gives the impression of unexpected instability in the underlying equations. For instance, when the in-plane principal stresses are in the ratio 2:1 and the fibres are inclined at an angle close to θ ≈ ±arctan√2, the stress–strain relations are very sensitive to the value of θ and to the relative stiffness of the fibres and matrix. There is a simple explanation for this, which is most clearly understood by developing analytical approximations for the stress–strain relations.

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