Trigger Effect in Impact Fracture of Granite Sample under Uniaxial Compression

This study focuses on an impact wave generated in a uniaxially compressed granite sample by a pendulum hammer in the direction transversal to compression. The pressure was varied from zero to a pre-destructive value. Microcracking during impacts was recorded using the method of acoustic emission (AE). The energy distribution in the time series of AE pulses followed an exponential law both in the unloaded and in the statically loaded samples. The first impacts induced local damage with a splash in AE from small cracks which, under subsequent impacts, coalesced and initiated redistribution of larger microdamages. In generation of an impact wave in a sample under pre-limiting compression, the surface of the induced main crack exceeds the area of the local damages by a few orders of magnitude, which is typical of the trigger effects which lead to large-scale fractures under external impacts at a safe-density energy.