DEVELOPMENT AND HEALING OF MATRIX MICROCRACKS IN FIBER-REINFORCED GLASS MATRIX COMPOSITES - ASSESSMENT BY INTERNAL-FRICTION

A non-destructive forced resonance technique was used as an indirect m ethod to assess the development and healing of microcracking damage in SiC fibre reinforced glass matrix composite materials. Matrix microcr acks were induced by both mechanical stressing and thermal-shock cycli c loading. Mechanical stressing involved applying a series of successi ve incremental loadings in the range 43-90% of the flexural strength v ia three-point flexure. The thermal-shock loading involved cyclic quen ching of the sample from a high temperature (620 degrees C) to room te mperature in a water bath. A semi-empirical equation for the variation of the internal friction with microcrack evolution was derived using the experimental results and a model available in the literature for t he effective Young's modulus of a microcracked composite. The possibil ity of thermal healing of matrix microcracks was investigated by subje cting microcracked samples to a heat-treatment (annealing) for 12 h at 550 degrees C. Internal friction was shown to be an excellent indicat or of microcracking evolution by comparison of the measured data for a s-received, microcracked and heat-treated materials. (C) 1998 Elsevier Science S.A.