Determining the fracture resistance of fibre-reinforced glass matrix composites by means of the chevron-notch flexural technique

Results on fracture toughness determination on SiC (Nicalon (R)) fibre-rein forced borosilicate glass matrix composites after thermal cycling and therm al shock are presented. The thermal shock tests involved quenching the samp les from high temperatures (600-650 degreesC) in a water bath at room tempe rature. For the thermal cycling tests, the samples were alternated between a furnace at high temperature (700 degreesC) and room temperature for up to 1000 cycles in air. Fracture toughness and work of fracture were measured using the chevron-notched specimen technique. Supported by fracture surface observations, the results were used to assess the microstructural damage i n the material after thermal loading. To detect the onset of unstable micro cracking during the chevron-notch experiments, an acoustic emission techniq ue was used. The fracture toughness values measured were in the range 18-26 MPam(1/2), in agreement with literature reports, and they were little affe cted under the thermal shock and thermal cycling conditions investigated (f or low number of cycles). The present results were shown to be in agreement with data of previous studies in which other techniques (Young's modulus a nd internal friction determination, fibre push-out test) were used to asses s damage development under similar thermal loading conditions. The overall finding is that, for these conditions, no major degradation of the fibre-ma trix interfaces occurred, and therefore the material retains its apparent f racture toughness and flaw tolerant behaviour. However, for a high number o f thermal cycles in air (> 800), severe microstructural damage occurred in the form of porosity development and interfacial oxidation. Under these con ditions, the material showed also macroscopic delamination, which made impo ssible the use of the chevron-notch test. (C) 2001 Elsevier Science B.V. Al l rights reserved.