Initiation of the accumulation of microcracks in granite under combined static and impact loading. Trigger effect

The hypocenter accumulation and crack propagation processes were studied in a laboratory model experiment. The impact wave in the uniaxially compressed granite was excited by a pendulum directed transversely to the compression. The acoustic emission method was used for recording the release of energy during the accumulation of microcracks and the formation of local damage. It was found that release of energy of microcracks induced by the pendulum under static pressure far from the pre-measured threshold of the sample global failure has several stages with a different slope of the accumulation curve. The impact damage threshold decreased by 5–20% in case of an increase of the compressive pressure on the sample depending on the installed pendulum energy, i.e. a trigger effect was manifested.