A.G. Gibson

Laminate Theory Analysis of Composites under Load in Fire

Laminate analysis is used to model a loaded composite plate under one-sided heat flux. The input to the laminate analysis comes from a thermal/ablative model, which predicts the temperature evolution through the thickness. It also gives the profile of residual resin content, which reflects the extent of thermal damage. Relationships are proposed to enable the computation of the elastic constants and other mechanical properties as functions of temperature and resin content. The model was applied to a 12 mm thick woven glass/polyester laminate exposed to a heat flux of 75 kW m 2. The laminate A, B, and D matrices were modeled, along with the variation of failure loads in compression and tension. The predictions agreed well with experimental values for compression of a constrained plate. Both the local buckling load, which is proportional to √D1 D2, and the compressive failure load fall rapidly on exposure to heat flux. The bending/tensile coupling matrix, B, which is zero initially, becomes finite due to the asymmetric thermal profile, then declines as the thermal front progresses. For tensile loading, the residual properties after fire were accurately modeled, but the fall in tensile failure load was somewhat over-predicted.

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 (50 kW m 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. A simple ‘two-layer’ model is proposed, in which the laminate is assumed to comprise (i) an unaffected layer with virgin properties and (ii) a heat-affected layer with zero properties. For residual properties after fire, the ‘effective’ thickness of undamaged laminate was calculated using this model and compared with measured values. A thermal model was employed to predict the temperature and the residual resin profile through the laminate versus time. Comparing the model predictions with the measured values of effective laminate thickness enabled simple criteria to be developed for determining the position of the ‘boundary’ between heat-affected and undamaged material. For post-fire integrity of unloaded laminates, this boundary corresponds to a Residual Resin Content (RRC) of 80%, a criterion that applies to all the resin types tested. For polyester laminate under load in fire, the boundary in compressive loading (buckling failure) appears to correspond to the point where the resin reaches 170 C. In tensile loading, significant strength is retained, because of the residual strength of the glass reinforcement. The model was used to produce predictions for ‘generic’ composite laminates in fire.

Fire behaviour of composite laminates

The thermal response of laminated glass fibre reinforced panels to severe fire conditions has been investigated by furnace fire testing and thermal modelling. Excellent fire resistance has been demonstrated for several matrix materials and the materials response has been modelled to a high degree of accuracy. The thermal resistance properties are due to a combination of low thermal conductivity, good structural integrity and significantly, the endothermic decomposition of the matrix, which slows down the heat transmission through the laminate.

Tensile Strength Modeling of Glass Fiber—Polymer Composites in Fire

A thermal-mechanical model is presented to calculate the tensile strength and time-to-failure of glass fiber reinforced polymer composites in fire. The model considers the main thermal processes and softening (mechanical) processes of fiberglass composites in fire that ensure an accurate calculation of tensile strength and failure time. The thermal component of the model considers the effects of heat conduction, matrix decomposition and volatile out-gassing on the temperature—time response of composites. The mechanical component of the model considers the tensile softening of the polymer matrix and glass fibers in fire, with softening of the fibers analyzed as a function of temperature and heating time. The model can calculate the tensile strength of a hot, decomposing composite exposed to fire up to the onset of flaming combustion. The thermal-mechanical model is confined to hot, smoldering fiberglass composites prior to ignition. Experimental fire tests are performed on dry fiberglass fabric and fiberglass/vinyl ester composite specimens to validate the model. It is shown that the model gives an approximate estimate of the tensile strength and time-to-failure of the materials when exposed to one-sided heating at a constant heat flux. It is envisaged the model can be used to calculate the tensile softening and time-to-failure of glass—polymer composite structures exposed to fire.

High speed pultrusion of thermoplastic matrix composites

The main problem with using thermoplastic matrices for composites is the difficulty in impregnating the fibrous reinforcement with the high viscosity resin. This has led to the development of a number of different manufacturing techniques, which are used to fabricate thermoplastic matrix composites. One method is to provide the matrix in fibre form and intermingle, or co-weave, the polymer fibres with the reinforcing fibres. These commingled fibres should ideally be combined in the same strand, allowing a high degree of intimacy to be achieved and minimising the flow distance for impregnation. An alternative technique is to impregnate the reinforcing tow with polymer powder particles and then melt fuse the particles in place. This method, the dry powder impregnation technique, allows for the formation of resin bridges between adjacent fibres, and with the application of applied pressure, longitudinal resin flow takes place. This differs from the transverse impregnation which occurs with the commingled fibres.These two consolidation mechanisms have been characterised and modelled using compression moulding techniques on commingled and powder towpregs, and the results of these experiments have been applied to the on-line consolidation which occurs during pultrusion processing. Successful correlation was achieved between the experimental results and the models with commingled polypropylene/glass fibres and dry powder-impregnated PA12/glass fibre-reinforced towpregs. The models then enable users to produce well-impregnated continuously reinforced composites of minimal void content at high line speeds, those reported in this work are speeds up to 10 m/min. With more powerful processing equipment, even higher line speeds could be achieved, demonstrating the potential cost effectiveness of pultruded thermoplastic composites.

Failure of composite cylinders under combined external pressure and axial loading

Abstract An experimental and theoretical investigation was carried out into the collapse behaviour of filament wound glass fibre/epoxy cylinders under combinations of external pressure and axial loading in the third quadrant of the stress plane. Samples were tested with length-to-diameter ratios from 2·5 to 20 and diameter-to-thickness ratios in the approximate range of 20 to 40. Four ratios of hoop to axial stress were employed: ∞, 2, 1 and 0·5. The theoretical study employed a special purpose finite element program to calculate first ply failure (FPF) and buckling loads for shells of revolution made from multi-layered orthotropic materials. In all cases the experimental collapse pressure was strongly influenced by the predicted buckling failure mode. For those samples predicted to fail by buckling, agreement between the model and the experimental results was excellent. With the samples predicted to undergo FPF prior to buckling it was found that the residual strength was often sufficient to permit the buckling load to be approached.

Environmentally enhanced fatigue damage in glass fibre reinforced composites characterised by acoustic emission

A series of tests has been conducted to investigate the effect of sea water absorption on fatigue damage accumulation in a glass fibre reinforced polyester laminate of the type widely used in the marine and offshore industries, using four-point bend flexural loading to ensure peak strain in the outer layers of the material most subject to seawater absorption. Pre-exposure was found to reduce the flexural strength and enhance damage accumulation in fatigue by stimulating matrix cracking, fibre debonding and delamination. Acoustic emission (AE) was used to characterise damage accumulation. These results were found to correlate well with independent measurements of changing bending stiffness and with microstructural observations of the damaged sections.

Modeling composite high temperature behavior and fire response under load

This paper discusses the characterization and modeling of thermoplastic and thermosetting matrix composites under load in fire. Small-scale tests were found to provide a cost-effective means of characterizing load-bearing behavior of composites in fire and a useful framework for materials development. This paper demonstrates the modeling of thermal and decomposition behavior during the test and the extension of this modeling to include mechanical response and failure behavior. The work necessitated measurement of strength and stiffness over a wide temperature range, with interesting results up to the point of resin decomposition. The approach was applied to three 12 mm thick glass reinforced systems: vinyl ester, polyester, and polypropylene. The laminates were subjected to a one-sided 50 kW·m−2 heat flux, using a propane burner. Thermal behavior was modeled using a simplified version of the Henderson equation to predict the evolution of temperature and residual resin content through the thickness. These parameters were then used, along with a material model, to predict the mechanical response in fire.

A model for the consolidation of aligned thermoplastic powder impregnated composites

This paper describes the role of surface energy effects, externally applied pressure, resin flow and fiber bed elasticity on the consolidation of thermoplastic-matrix composites manufactured by the powder impregnation route. Surface energy effects in the spreading of polymer droplets on fiber surfaces are discussed; then a model for the consolidation process is developed, relating the variables mentioned above. Consolidation experiments on powder-impregnated composites of the FIT type (Fibres Impregnees de Thermoplastique) were carried out using a mold attached to a servo-hydraulic testing machine. The model accurately predicts variations in void content during consolidation of carbon fiber/PEEK (CF/PEEK) and carbon fiber/PEI (CF/PEI) laminates. It was found that, at the pressures needed to achieve rapid consolidation, surface energy has a negligible influence on impregnation rate, but its effects on the void topology can be considerable. It was also shown that, when laminates of low void content are required, a minimum pressure is needed to overcome the effect of fiber bed elasticity.

Modeling Compressive Skin Failure of Sandwich Composites in Fire

A thermal-mechanical model is presented for calculating the residual compressive strength of flammable sandwich composite materials in fire. The model can also estimate the time-to-failure of the laminate face skin to sandwich composites exposed to fire. The model involves a two-stage analysis: thermal modeling and mechanical modeling. The thermal component of the model predicts the temperature profile and amount of decomposition through sandwich composites exposed to one-sided heating by fire. The mechanical component of the model estimates the residual compressive strength of the sandwich composite and the onset of skin failure. The model is tested for sandwich composite materials with combustible glass/ vinyl ester skins and balsa core. Experimental fire tests are performed on the sandwich composites under combined compressive loading and one-sided heating at constant heat flux levels between 10 kW/m 2 (Tmax · 250°C) and 50 kW/m2 (·600°C). The model predicts that the time-to-failure increases with the skin thickness and decreases with an increase to the applied compressive stress or heat flux. The predictions are supported by experimental data from fire-under-load tests. It is envisaged that the model can be used to design sandwich composite materials with improved compressive load capacity in fire.