Influence of snap-back instabilities on Acoustic Emission damage monitoring

Experimental tests are carried out to evidence the energy emitted during snap-back instabilities of rock specimens in compression. Several gypsum samples with different slenderness are considered, and all the tests are monitored by means of the Acoustic Emission (AE) technique. In particular, the cumulated number of AE signals is used to represent the emitted energy. The performed tests have been controlled in compression in two different ways: through vertical or circumferential displacement, by imposing in both cases a constant displacement rate. For the tests controlled by the vertical displacement, in the more brittle cases crack propagation suddenly occurs with a catastrophic drop in the load carrying capacity, and the resulting emitted energy appears with a burst in the AE cumulated curve. When the tests are controlled by the circumferential elongation, the damage increment is measured with a roller-link chain wrapped around the cylindrical samples at mid-height in order to obtain the complete load vs. displacement response. In the brittle cases, both load and vertical displacement decrease to obtain a controlled crack propagation: the energy is totally dissipated and consequently no emissions due to snap-back phenomena are observed, proving that crushing or fracture energy and AE energy are independent physical quantities, the latter resulting from local instabilities in the structural response.