Experimental Investigation of Fracture Toughness Reduction and Fracture Development in Basalt Specimens under Microwave Illumination

This is a part of a research program at McGill University, employing microwave assisted rock breaking system as an alternative to the explosive fragmentation. Given the fact that microwave radiation can reduce the strength of rock, a combination of microwave and mechanical rock breaking methods can potentially contribute in better and efficient mechanical breaking of hard rock. Microwave-assisted method can be used in mechanical breaking machinery such as full face tunnel boring machines and space excavators for rock sampling in space exploration missions. An experimental investigation of fracture toughness of basalt specimens under microwave illumination (BSUI) was carried out. Approximately 100 semi-circular basalt specimens with 50 mm diameter and 15 mm thickness were exposed to microwave radiations for 5 to 30 seconds at power level of 1000-5000 watts. Fracture toughness BSUIs were measured employing Semi-circular Bending Method (SCB) and compared with those of untreated samples. Note that terrestrial basalts have close chemical composition and physical property similarity to lunar and Martian rocks. Petrographic standard test has been employed to investigate the effect of micro-structural characteristics of BSUI on microwave susceptibility, thermo-mechanical cracks development in differential mineral phases. Such micro-structural characteristics include mineralogy proportion, grain size, pre-existing crack shapes, and pre-existing crack sizes. This study shows that although both power level and exposure duration lead to fracture toughness reduction and increase of fracture density; however, applying higher powers in shorter durations causes more fracture density and fracture toughness reduction. Given that heating rate of absorbent mineral dependent upon applied power and sudden expansion of neighbouring mineral due to the higher heating rate increases more grain bonds breaking and develops more micro crack in illuminated rock.