CHARACTERISTICS OF ACOUSTIC EMISSION GENERATED BY IMPACT DAMAGE OF UNIAXIALLY COMPRESSED GRANITE

Uniaxially compressed granite samples were subjected to pointed impact damage directed orthogonally to the compression. Generation of the impact-induced acoustic emission (AE) was recorded by a wide-band piezoelectric transducer made of high-sensitivity PZT ceramics. The AE time series were analyzed in the frequency ranges of 80–200 kHz (conventionally low-frequency range) and 300–500 kHz (high-frequency range). Energy distributions in AE time scans recorded in the lower frequency range follow the power law, which is characteristic of the cumulative process of microcrack formation, whereas distributions recorded in the high-frequency range are described by an exponential (Poisson-like) function characteristic of random, non-interacting AE-events. Concentration of microcracks in the low-frequency range was characterized by the Gutenberg-Richter b-value. At continuous impact energy, the b-value increased progressively with increasing uniaxial compression applied to the samples from zero to the highest subcritical value, which suggests lowered contribution of larger cracks. As the compression load approached (but without reaching) the ultimate strength, the impact-induced subthreshold macrofracture of the granite sample (trigger effect) occurred.