Thermomechanics of multiphase refractories
Refractory materials must, in their everyday environment, withstand high stress levels which are a result of mechanical and thermal loadings. Any failure which results from these applied stresses can have serious financial and human consequences and therefore should be avoided. One key aspect to understanding the thermal shock behaviour of refractories is the mechanical behaviour at low temperatures. In this thesis the mechanical behaviour of a small range of multiphase refractories is explored. In particular the stress-strain response and its influence on the fracture behaviour is investigated. Experiments, performed on magnesia and magnesia spinel composites, indicate that non-linear stress-strain behaviour accompanied by permanent deformation upon unloading is a result of the release of microscale residual stresses by microcracking. A micromechanical constitutive model combining these features was developed using linear elastic composite theory and isotropic continuum damage mechanics. This non-linear stress-strain behaviour also gives rise to increasing toughness as crack propagation occurs. This increase in toughness results from an expansion which occurs when microscale residual stresses in front of the crack tip are relaxed by microcracking. A micromechanical model has been developed based upon the specifically developed constitutive model and previous work on transformation toughening. These models are capable not only of simulating experimental results, but can also indicate the microstructures which are most likely to exhibit extensive non-linear stress-strain behaviour and strongly rising toughness curves.