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®) fibre-reinforced borosilicate glass matrix composites after thermal cycling and thermal shock are presented. The thermal shock tests involved quenching the samples from high temperatures (600–650°C) in a water bath at room temperature. For the thermal cycling tests, the samples were alternated between a furnace at high temperature (700°C) 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 in the material after thermal loading. To detect the onset of unstable microcracking during the chevron-notch experiments, an acoustic emission technique was used. The fracture toughness values measured were in the range 18–26 MPam1/2, in agreement with literature reports, and they were little affected under the thermal shock and thermal cycling conditions investigated (for low number of cycles). The present results were shown to be in agreement with data of previous studies in which other techniques (Youngʹs modulus and internal friction determination, fibre push-out test) were used to assess damage development under similar thermal loading conditions. The overall finding is that, for these conditions, no major degradation of the fibre–matrix interfaces occurred, and therefore the material retains its apparent fracture toughness and flaw tolerant behaviour. However, for a high number of thermal cycles in air (>800), severe microstructural damage occurred in the form of porosity development and interfacial oxidation. Under these conditions, the material showed also macroscopic delamination, which made impossible the use of the chevron-notch test.