Thermal cycling effects on the dynamic behavior of granite and microstructural observations

This study investigated the thermal cycling effects on the dynamic behavior of granite and described its microstructure. The specimens were subjected to various numbers of thermal cycles (0, 1, 3, 5, and 7 cycles) at temperatures ranging from 25 to 500 °C. Then, ultrasonic wave tests and split Hopkinson pressure bar (SHPB) tests (with different impact gas pressures of 0.25, 0.28, and 0.31 MPa) were performed to study the thermal cycling effects on the P-wave velocity, P-wave modulus, dynamic compressive strength, and impact failure pattern of the granite specimens. Finally, scanning electron microscopy (SEM) was performed to analyze the micromechanism of the dynamic property degeneration of the granite specimens. The results show that the dynamic properties of the P-wave velocity, P-wave modulus, and dynamic compressive strength exponentially decrease as the number of thermal cycles increases. The decreases in the dynamic properties mainly occur during the first thermal cycle, and the P-wave modulus and dynamic strength decrease by 67.5% and 8.4–16.3%, respectively. Moreover, a higher dynamic compressive strength, smaller fragments, and more fine powder are generated by impact failure with a larger strain rate. Smaller fragments and more fine powder are observed after impact failure as the number of thermal cycles increases. The tests further reveal that the dynamic properties of thermally damaged granite are closely related to the microcracks induced by thermal cycling.