Electric Signals Accompanying Fracture of Granite

Pre-heated specimens of granite were uniaxially compressed. Electric charge of the order of 10-11 C accompanying fracture of the specimen was measured within a deformation accelerating precursory period until complete failure occurred. Specimens that had been pre-heated to 200°C exhibited the highest electric charge, and signal intensity decreased with increasing pre-heating temperature. Meanwhile, uniaxial strength of the specimen was the highest, 220 MPa, for the specimens that had been pre-heated to 200°C and it decreased with pre-heating temperature. Our results suggest that the intensity of the electric precursor signal detected prior to failure positively correlates with uniaxial strength. ESR signal intensity of the E′ center of quartz contained in the fractured granite specimen decreased upon failure. Stress-induced annealing of the E′ center is the suggested cause of the signal decrease.