Assessment of viscoelastic crack bridging toughening in refractory materials

Viscoelastic bridges can be formed in refractory ceramics while cooling from high temperatures. Such bridges can shield crack tips, thus reducingthe effective crack tip stress intensity factors leading to higher resistance to creep and thermal shock. The extent to which the crack tip stress intensityis reduced can be estimated from fracture mechanics models that include experimental measurement of crack bridging and microstructuralparameters. In this paper a novel approach is proposed for the assessment of the effective crack bridging toughening from combining destructiveand non-destructive test methods. Fracture toughness values were determined applying chevron notched specimen technique and surface damage ofthe specimen was monitored by image analysis. Different cordierite–mullite compositions characterized by different microstructure morphologiesand crack propagation behaviour were investigated. A brief discussion about the correlation between thermo-mechanical properties, microstructure,crack propagation behaviour and thermal shock resistance is presented. Moreover, an empirical model able to determine the presence andeffectiveness of the viscoelastic crack bridging ligaments acting in the microstructure under thermal shock conditions and their degradation withincreasing thermal shock cycles from parameters measured at room temperature is presented.