Composites made of ceramic fibres and ceramic matrices (CMC) show a high thermal and chemical stability and high mechanical properties, including a damage tolerant failure behaviour caused by microscopic failure processes such as controlled crack evolution and fibre pull-out. This allows their use in high temperature applications under aggressive atmosphere and high load.
Until today no specific tests for determination of crack propagation and fracture mechanical behaviour of ceramic matrix composites exist, complicating the characterization of the failure behaviour of different materials or processing methods. To develop a reliable testing routine, the applicability of the Norm ASTM-E 1820 was investigated. Single-Edge Notched Bend-Tests (SENB) were performed on samples of all-oxide CMC prepared according to the geometric requirements of the norm. Focus was set to the determination of crack length, which is an important parameter in calculation of J-Integral, used to quantify the strain energy release rate. Other than metals or monolithic ceramics, CMCs show complex crack formation, e.g. crack branching. To estimate the real crack length considering the expected crack branching, the SENB-tests were in-situ monitored with an optical system to calculate the strain-distribution on the sample surface based on the displacement of a speckle-pattern. It was found that the regions of maximum strain coincide with the main crack paths formed during crack opening with increasing load. The obtained crack paths are compared to the crack tip opening displacement calculated by the compliance-method according to ASTM-E 1820. In addition, ultrasonic microscopy and x-ray computed tomography are used for high resolution investigation of the crack geometry, allowing determination e.g., of the crack length and the number of large cracks. Combining all experimental results, calculation of the J-Integral can be adapted for all-oxide CMCs to proceed in the development of a specific fracture mechanics testing procedure for these materials.