Modelling of high temperature asymmetric creep behavior of ceramics

An extension of the Bingham-Norton's rheological model accounting for high asymmetric tensile-compressive creep behavior is proposed for ceramics subjected to high temperature loadings. Such behavior is observed in heterogeneous ceramics made of grains having a high creep resistance embedded in a softer glassy phase. Two mechanisms of deformation are introduced: namely, under compressive stresses, grain facets are in contact and transmit the load, whereas under tensile stresses the grains are separated and the glassy matrix transmits the load. It leads to a difference in elastic stiffness and strain rate under tensile and compressive stresses. A new three-dimensional constitutive model is developed considering an additive decomposition of the stress tensor into a positive and negative part. Each stress tensor is the driving force of one mechanism of deformation that is characterized in a classical manner. A simplified identification of the model is performed at high temperature on a bauxite-based refractory, which is used in steel ladle linings. It is shown that for this ceramic few material parameters are sufficient to account for major differences of behavior observed in compression tests and three-point bend tests.