Z. Mathys

Post-fire mechanical properties of marine polymer composites

Changes to the tensile and flexure properties of marine-grade glass-reinforced polyester, vinyl ester and resole phenolic composites after exposure to radiant heat are investigated. The properties were determined at room temperature after the composites had been exposed to heat fluxes of 25–100 kW/m2 for 325 s or to a heat flux of 50 kW/m2 for increasing times up to 1800 s. The stiffness and failure load of all three composites decreased rapidly with increasing heat flux or time due mainly to the thermal degradation of the resin matrix. The post-fire tension and flexure properties of the resole phenolic composite were similar to the properties of the other composites, despite its superior fire resistance. Models are presented for determining the post-fire mechanical properties of fire-damaged composites, and are used to estimate the reductions in failure load of composite ship materials caused by fire.

Post-fire mechanical properties of glass-reinforced polyester composites

An investigation into changes in the mechanical properties of glass-reinforced polyester composites after exposure to intense radiant heat is presented. The tension, compression, flexure and interlaminar shear properties fell rapidly with increasing heat flux and heat-exposure time owing, mainly, to charring and delamination cracking caused by burning of the composite. Substantially higher post-fire mechanical properties were attained when the composite was protected from the radiant heat with a thermal barrier coating that delayed the onset of combustion. Analytical models for determining the post-fire tension, compression and flexure properties are presented. The potential use of the models for making preliminary predictions of reductions to the failure loads of glass-reinforced polyester composite structures on marine craft and naval ships, such as decks and bulkheads, caused by fire is discussed.

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.

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.

Plasma surface modification of advanced organic fibres: Part V Effects on the mechanical properties of aramid/phenolic composites

Aramid fibres have been treated in ammonia and oxygen plasma to enhance adhesion to resole phenolic resins. The plasma treatments resulted in significant improvements in interlaminar shear strength (ILSS) and flexural strength of composites made from these materials. Composites containing aramid fibres with epoxide groups reacted on to the ammonia plasma-treated fibre surface also showed further improvements in ILSS and flexural strength. Scanning electron and optical microscopic observations were used to examine the microscopic basis for these results, which have been compared with those obtained previously for aramid/epoxy and aramid/vinyl ester composites. For composites containing oxygen and ammonia plasma-treated fibres, the enhanced ILSS and flexural strength are attributed to improved wetting of the surface-treated aramid fibres by the phenolic resin. However, for those containing fibres with reacted epoxide groups on the ammonia plasma-treated fibre surfaces, the enhanced composite properties may be due to covalent chemical interfacial bonding between the epoxide groups and the phenolic resin. Effects of catalyst levels and cure cycle on the ILSS of composites laminated with untreated fabric has also been examined and optimum values have been determined. The catalyst concentration has an influence on the phase-separated water domain density in the matrix which in turn, affects the available fibre/matrix bonding area and hence the composite ILSS and flexural strength.

Mechanical properties of fire-damaged glass-reinforced phenolic composites

Changes to the mechanical and physical properties of a glass-reinforced resole phenolic composite due to intense radiant heat and fire are investigated. Fire testing was performed using a cone calorimeter, with the composite exposed to incident heat fluxes of 25, 50, 75 or 100 kW/m(2) for 325 s and to a constant flux of 50 kW/m(2) for different times up to 1800 s, The post-fire tensile and flexural properties were determined at room temperature, and these decreased rapidly with increasing heat flux and heat exposure time due mainly to the chemical degradation of the phenolic resin matrix. The intense radiant heat did not cause any physical damage to the composite until burning began on exposure to a high heat flux. The damage consisted of cracking and combustion of the phenolic matrix at the heat-exposed surface, hut this only caused a small reduction to the mechanical properties. The implication of the findings for the use of glass-reinforced resole phenolic composites in load-bearing structures for marine craft and naval ships, where fire is a potential hazard, is discussed.

Reinforcement and matrix effects on the combustion properties of glass reinforced polymer composites

Cone calorimetry has been used to determine the combustion properties of glass reinforced polymer (GRP) composite materials containing various matrix resins and different types of glass reinforcement. Powder bound chopped strand mat (CSM) and woven roving (WR) GRP composites containing polyester, vinyl ester and phenolic matrix resins were examined at various irradiance levels. Values for time to ignition, rate of heat release, effective heat of combustion, smoke obscuration and evolved carbon monoxide and carbon dioxide are reported for the composites and the resin components, with the phenolic materials displaying clearly superior combustion properties. The different reinforcements also have a significant effect on composite combustion, with the CSM composites consistently showing lower ignition times, and lower and broader rate of heat release vs time profiles. Reasons for these differences are discussed.

Plasma surface modification of advanced organic fibres

Aramid and extended-chain polyethylene fibres have been treated in ammonia and oxygen plasmas in order to enhance adhesion to vinylester resins and thereby improve fibre/resin interfacial properties in composites made from these materials. For both aramid/vinylester and extended-chain polyethylene/vinylester composites, the plasma treatments result in significant improvements in interlaminar shear strength and flexural strength. Extended-chain polyethylene/vinylester composites also exhibit increased flexural modulus. Scanning electron and optical microscopic observations have been used to examine the microscopic basis for these results, which are compared with results previously obtained for aramid/epoxy and extended-chain polyethylene/epoxy composites. It is concluded that the increased interlaminar shear and flexural properties of vinylester matrix composites are due to improved wetting of the surface-treated fibres by the vinylester resin, rather than covalent chemical bonding.

Plasma surface modification of advanced organic fibres: Part II Effects on the mechanical, fracture and ballistic properties of extended-chain polyethylene/epoxy composites

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.

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.