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Optimisation of optical methods for strain field measurements dedicated to the characterisation of the fracture behaviour of refractories : Application to magnesia based materials
Abstract and Figures
Magnesia-spinel and magnesia-hercynite bricks destined for thermal shock applications in cement rotary kilns often show an enhanced crack propagation resistance due to an engineered microstructure design. In these materials, microcrack networks resulting from the thermal expansion mismatch between magnesia matrix and spinel/hercynite aggregates promote the activation of energy dissipating mechanisms within the so-called Fracture Process Zone (FPZ) during loading. In this research, the fracture behaviour of magnesia-based model materials was investigated by coupling a refined Digital Image Correlation method (2P-DIC) with the Wedge Splitting Test (WST). The coupling of these advanced characterisation methods has proven to be very effective in measuring important fracture parameters accurately and in highlighting characteristic fracture mechanisms, such as crack-branching. The investigation of microstructure-property relationships underlined the impact of thermally induced microcracks on the thermomechanical behaviour of magnesia-spinel and magnesia-hercynite materials. Despite the rather similar elastic and dilatometric properties of spinel and hercynite single constituents, peculiar microcracking patterns were observed, especially in magnesia-hercynite. In fact, extensive diffusion between magnesia and hercynite during sintering led to the formation of spinel solid solutions around hercynite aggregates. As a result of thermal expansion mismatch with magnesia, these solid solutions contributed to creating numerous fine microcracks confined within the diffusion zone. Initially present within the microstructure, microcrack networks promote an increase of the specific fracture energy during WST experiments. Moreover, the analysis of strain fields measured by 2P-DIC revealed extensive crack branching for magnesia-hercynite materials. In essence, 2P-DIC and WST measurements showed that microcrack networks promoted the development of the FPZ, which in turn induced higher fracture energies. In a refined R-curve approach, effective fracture energies were calculated using crack lengths measured by 2P-DIC, which helped establish strong links between FPZ development and an enhanced crack propagation resistance. The tendencies observed at room temperature during WST experiments were confirmed during thermal cycling experiments using a novel thermal shock device.
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