The Integrity of Polymer Composites during and after Fire

Abstract

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.