Investigation of the nonlinear fracture behaviour of ordinary ceramic refractory materials

In many cases of mechanical and thermomechanical stress, refractories showing reduced brittleness corresponding with a pronounced nonlinear material behaviour are desired. A fracture mechanical characterization is based on a wedge splitting test which enables stable crack propagation for sufficiently large specimens. Figures-of-merit derived from the test results for the estimation of brittleness and resistance against thermal shock are the characteristic length lch and the parameters R′′′′ and Rst according to Hasselman. Moreover, the R-curve behaviour is calculated. In order to identify the microprocesses responsible for nonlinear material behaviour, a technique for the preparation of specimens was developed that allows the microscopical investigation before and after crack propagation. A decrease in the ceramic bond between grain and matrix as well as pre-existing cracks (microcracks), especially at the grain boundaries, reduce the energy consumed for the fracture of bonds and increase nonlinear effects such as friction in the process wake and grain bridging.