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One of the typical failure modes of composite repairs in oil and gas pipelines is the formation of a blister underneath the repair. Exceeding the critical pressure, and therefore the critical energy release rate ( GC) for crack propagation, will result in failure of the repair. Knowing the magnitude of the energy release rate ( G) is therefore key to understanding the interfacial debonding between a steel pipe and a composite repair. This paper describes two approaches for calculating the value of G for crack advance: (a) a purely analytical evaluation and (b) finite element analysis computation using the pressure–volume method (PVM). The design guidance by ASME and ISO standards for correct dimensioning is also presented, incorporating the most recent changes, which are derived from the work presented here.

The post-fire integrity of pultruded phenolic/glass and polyester/glass floor gratings, of the type used offshore and elsewhere, was investigated. The aim was to determine whether glass/phenolic gratings may be safely walked on, post-fire, by offshore workers and fire-fighting teams. The load to be resisted was identified as equivalent to a running person carrying a load, the combined mass being 150 kg. The maximum resulting dynamic strains were determined by strain gauges on the undersides of the individual beam elements of gratings. This enabled target values of post-fire bending resistance to be identified. Individual beam elements from the gratings were exposed to heat fluxes of 12.5, 37.5 and 100 kW/m2 using a propane burner, for periods up to 16 min, after which the residual strength was measured. Phenolic gratings showed longer ignition times, with lower flame and smoke emission, as well as greater post-fire strength compared to polyester ones. At the lowest flux, 12.5 kW/m2, all gratings remained serviceable beyond 16 min. At higher heat fluxes, the phenolic gratings retained some post-fire strength, assisted by the formation of a carbonaceous char binding the fibres. However, this was somewhat below the target level. A study of the effect of testing speed indicated that fire-exposed gratings are not especially strain-rate sensitive.

This article presents relationships for the through-thickness elastic constants of composite laminates, needed for modelling three-dimensional stress problems. Using the individual ply elastic constants together with the in-plane laminate elastic constants (from laminate theory) results in explicit relationships for, ν13, ν23, E3, G13 and G23. The Poisson’s ratio and Young’s modulus expressions apply to all laminate types. The shear modulus relationships apply to balanced laminates, but are extendable to other laminate types. Relationships are developed here for angle-ply, cross-ply and quasi-isotropic laminates. Results are presented for glass/epoxy and carbon/epoxy laminates, above and below the resin Tg. The importance of coupling stresses and their influence on ν13, ν23 and E3 is underlined. It is shown, for instance, that equating the laminate through-thickness modulus to the individual ply value can result in a significant under-estimate.

This article presents an experimental study of fusion-bonded polypropylene (PP) glass composite joints in the context of small craft manufacture. The objective is to investigate the manufacturing of lap joints and T-joints as a structural part of a small boat and study their properties, because a joining technique is a fundamental requirement of any boat construction technology. Results show that PP interlayers improve bond quality for both lap joints and T-joints and that woven precursor materials are preferred for T-joint manufacturing. It was also found that this technique produces higher lap shear strength values than adhesives and resistance welding, and that pull-out strength values were comparable with those of thermosetting designs, demonstrating that fusion bonding is a suitable joining technique for thermoplastic composite craft.

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 coupled thermo-mechanical model is presented for calculating the compressive strength and failure of polymer laminated composites with thermal barrier when exposed to fire. Thermal barriers are used to protect composite structures from fire, and this article presents a model for calculating the improved structural survivability under compression loading. The thermal component of the model predicts the through-thickness temperature profile of the composite when protected from fire using a passive thermal barrier insulation material. The thermal analysis is coupled to a mechanical model that calculates the loss in compressive strength with increasing temperature and heating time. The model predicts the strength loss and failure time of an insulated composite supporting a static compressive load when exposed to fire. The accuracy of the model is evaluated using failure times measured in fire-under-compression load tests on a woven E-glass/vinyl ester composite protected with a passive thermal barrier. The model predicts reductions to the failure time with increasing heat flux (temperature), applied compressive stress, and reduced insulation thickness, and this is confirmed by experimental testing. It is envisaged that the thermo-mechanical model is a useful analytical method to design thermal barrier material systems to protect composite structures exposed to high temperature or fire.

The delivery of the power from remote power generation sites to urban centres is increasingly becoming a challenge for the power industry since the use of traditional power transmission technology, such as overhead cables, is a politically sensitive issue and alternative approaches are sought. This paper reports the findings of a feasibility study into long distance power transmission via small diameter underground tunnels, constructed using trenchless technologies such as horizontal directional drilling or microtunnelling. The study examines the suitability of AC or DC transmission for such underground tunnels, various cooling methods to remove the heat generated in the conductors from electrical losses, type of conductor to be used, and issues concerning conductor installation. As a result of this study a number of technical challenges have been identified that need to be overcome in order to make the method an economically and technically viable option.

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.

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.

An experimental and modeling study is conducted on the vacuum consolidation of commingled glass/thermoplastic composites as part of a larger project on manufacturing large monolithic structures from these precursors. Two polyethylene terephthalate (PET) matrices are employed: semicrystalline PET and an amorphous PET copolymer. Samples of commingled fabric are processed into consolidated composites by means of both a convective oven, as will be used in practice, and a small-scale experimental characterization rig, designed to measure consolidation accurately. The samples are then cooled to room temperature. In this article, the thermal and consolidation characterization of these fabrics is reported. Thermally induced consolidation is observed to occur in two stages: a low temperature solid state de-bulking near to Tg, followed by full melt impregnation at a higher temperature. Both stages are modeled separately using an empirical model based on the Kamal equation. The measured consolidation versus time profiles suggest a rapid impregnation and wetting of the fibers, occurring near to the melting point of the semicrystalline polymer. The PET melting endotherm and crystallization exotherm have little effect on the observed thermal profiles, suggesting that these effects can possibly be neglected when modeling the process.

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.

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.

A model for the stress-strain behavior of fiber composites with matrix cracks is presented and applied to the deformation of fiber reinforced composite pipes under different combinations of hoop and axial stress. The model provides a good description of the nonlinear stress-strain relationship that develops in composites when the matrix is damaged by the progressive nucleation of microcracks during loading. Ply properties are expressed as a function of crack density, calculated as a function of increasing stress using the shear-lag approximation. The predictions are in very good agreement with data from multiaxial tests on ±55° filament wound glass-reinforced epoxy pipes.

A programme of tests has been conducted to characterise the strength reduction of three GRP composite materials as a function of temperature and testing environment. The GRP materials and testing environments were selected to be of maximum relevance to the offshore industry. The materials were E-glass fibre reinforcement in matrices of three resins: two epoxies and phenolic. The testing environments were sea water and crude oil condensate. The temperature range tested was ambient to 150°C. The tests were performed on coupon specimens laminated using fibres in the form of woven mat. The fibres were aligned so as to allow testing in both fibre dominated and resin dominated failure modes. A novel environmental tensile testing facility was developed for this work, and this is described briefly. Results are presented for the strength of the three materials as a function of temperature in each failure mode when tested dry. These are then compared with equivalent results for the specimens tested in the liquid environments. This is the first reported measurement of GRP strength in liquid environments at these very high temperatures. It is demonstrated from the results that strength reduction is exhibited predominantly in the matrix dominated failure mode, though a small but significant strength reduction is also seen in the fibre dominated mode. Causes for these strength reductions are identified as lowered glass transition temperature and degradation of the glass and glass/resin interfaces respectively. The strength reductions for each combination of material and environment are quantified. Finally, appropriate limits to operating temperature are suggested for each material when used in an offshore environment.

This paper reviews the techniques available for the impregnation of reinforcing fibres with thermoplastic resins. The key material factors influencing the achievement of impregnation are outlined and the current impregnation technologies are discussed. The techniques are divided into two categories: those that aim to reduce the viscosity in order to achieve rapid impregnation, the pre-impregnation processes, and the processes that attempt to reduce the distance that the resin is required to flow, which involve intimate mixing of constituents prior to melting, the mingling processes. The techniques used to mingle fine polymer powder and continuous fibres are examined and the various elements of a powder impregnation line are reviewed. Applications of powder impregnated composites are also discussed.