Abstract

Introduction

Magnesia and magnesia spinel materials show a completely different fracture behaviour. For the latter one, due to pre-existing microcracks, cracks initiate easily in the magnesia spinel material, but the development of the process zone enhances the resistance to crack propagation. This is related to the considerable brittleness reduction by spinel addition.

Materials and Methods

In this research, a magnesia spinel material and a pure magnesia material are studied by digital image correlation to investigate the fracture process. A wedge splitting test was applied for both materials. During the test strains as well as the crack propagation were detected by digital image correlation. Fracture mechanical data have afterwards been determined from the load/displacement curves.

Results and Conclusions

The microcrack network already present in a virgin magnesia spinel material supports the formation of a pronounced fracture process zone. The mechanical properties measured by the wedge splitting test indicate a reduction in strength and an increase in fracture energy for the magnesia spinel material. The special microstructure characterized by pre-existing microcracks increases the strain bearing capacity, which is very important for improving the thermal shock resistance. For pure magnesia, no pronounced fracture process zone could be detected. Crack propagation in magnesia refractories commences immediately after reaching the maximum load. For magnesia spinel, the fracture process zone development starts in the pre-peak region. The fracture process of magnesia spinel is characterized by the development of the fracture process zone and the subsequent development of the macro-crack. The transition is indicated by a change of the fracture process zone dimensions. Due to the closure of the microcracks and propagation of the macro-crack, the fracture process zone width and height both decrease. The onset of the macro-crack is in the post-peak region when the load has already decreased to 66% of its maximum value. The fracture process zone of magnesia spinel contributes to the large post-peak region, stable crack propagation and high strain tolerance before failure. All of these properties are associated with lower material brittleness.