Modelling composite fire behaviour using apparent thermal diffusivity

Abstract

In this study, a new model has been developed for the prediction of thermal profiles for fibre reinforced plastic composites exposed to high heat ux. The model involves expressing the thermal diffusivity of the composite as a function of temperature. Apparent thermal diffusivity (ATD) can take into account the decomposition of the resin, which is endothermic, as well as the consequent changes in specific heat capacity and thermal conductivity of the composite. This offers the possibility of significantly simplifying computational procedures needed for modelling thermal behaviour with decomposition. The possibility of extending thermal analyses to two and three dimensional cases was explored. Techniques for the direct measurement of the apparent thermal diffusivity are presented for different composite systems over a wide range of temperatures: from ambient to ~600°C . Two different techniques were needed for different ranges of temperatures: from ambient to 80°C and from 80-100°C to 600°C. To measure the ATD in the low range, a step temperature change was applied to the surface of a slab-shaped piece of material. Theta, the difference between the middle plane temperature and the outer surface temperature was recorded. The value of the thermal diffusivity at each temperature was calculated from the values of theta. The high range measurement involved the application of a linear temperature rise to the surfaces of a slab of material. The ATD was calculated by means of the Laplace heat transfer equation. The thermal diffusivity function obtained through these measurements was used to model the Fire behaviour of these materials under different heat transfer conditions. Quasi isotropic glass/polyester slab shaped composite specimens were tested under one dimen- sional heat transfer conditions. A one-sided heat flux was applied to the samples and the remaining surfaces were isolated to obtain repeatable boundary conditions. The temperatures were recorded at different depths within the samples during the exposure. The ATD of this material was mea- sured through the techniques mentioned above and implemented in a one-dimensional heat transfer FORTRAN model. I-beam shaped pultruded sections were subjected to two-dimensional heat transfer conditions. The temperatures were recorded at different locations on the cold side. Thermal properties were determined by means of the apparent thermal diffusivity of the material and implemented in a two-dimensional FE thermal model. Carbon fibres reinforced wing box materials were used to perform three dimensional fire tests. To describe analytically the tests, the ATD was measured along the three principal directions by means of the techniques mentioned before. These data were implemented into finite element models. The suitability of the ATD to model complex cases was verified. The failure of polyester and phenolic pultrusions under tensile and compressive load and a one- sided heat flux of 50 kW/m² was studied. A thermal/mechanical model, based on the Henderson equation and laminate theory, was used to model their behaviour. In tension, significant load- bearing capacity was retained over a period of 800 seconds, 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. In all cases the experimental data and the predicted temperatures were compared. The ATD modelling proved capable of capturing the main features of the temperature curves that relate with the effects of re exposure of composites. This study allowed to determine the characteristics of the ATD curve at different temperatures and relate it to the phenomena occurring to composites exposed to fire.