Abstract
The response of samples of unidirectional and cross-ply Nicalon fibre-reinforced calcium aluminosilicate (CAS) to a variety of thermal regimes has been examined using microscopy techniques and retained mechanical property measurements. The degree of matrix damage has been investigated by observation and measurement of cracking features and the results used in simple models in order to relate the occurrence of matrix cracking to stress distributions in the laminates. Thermal shock induced matrix crack damage was first seen to appear on the end faces of the unidirectional [0]16 laminate at a temperature differential of 400 °C and in the transverse plies, parallel to the longitudinal fibre direction, in the [0/90]3s cross-ply composite at a temperature differential of 350 °C. At more severe thermal shocks the next damage in both laminates was cracking in the matrix perpendicular to the fibre direction. The density of matrix cracking was seen to increase, initially, with increasing severity of thermal shock, but then to be less extensive at the highest temperature differentials (800 °C) used in this study. Crack density data for the unidirectional material at increasingly severe thermal shocks were compared with literature data for cracking under quasi-static loading using a simple thermal shock analysis incorporating a stress reduction factor. The effect of matrix cracking on retained mechanical properties has been examined by means of three-point flexure testing and values for Young's modulus, onset of non-linear behaviour and retained strength of the composites have been determined. Multiple thermal shock tests indicated that thermal treatment of previously cracked samples accelerated the rate of deterioration in the retained properties of the composite. It was proposed that the response of the samples to changes in the duration and severity of thermal treatments was consistent with interfacial modifications that have been reported to occur in this composite system at elevated temperatures. The suitability of applying a modified ACK model to predict critical temperature differentials for matrix cracking in the unidirectional laminate and longitudinal plies in the cross-ply composite has been tested. This approach combined applied thermal stresses, calculated using the simple thermal shock formula, with residual stresses, obtained from the model proposed by Powell et at. (1993). This method was found to be valid for the unidirectional material providing that some of the key parameters were determined independently. The use of a tunnelling crack model to predict thermal shock induced matrix cracking in the transverse plies of the cross-ply composite was less successful. This was partially attributed to the observed cracking patterns generated in the cross-ply material by flexure tests not conforming to those expected from stress calculations or reported from tensile tests.