The paper describes the results of laboratory experiments in which:
granite samples were preliminary loaded with a testing machine, and then
were instantly unloaded.
Acoustic emission signals due to the rock fracture process were detected during both of the above stages. As was established, the fracture process after the unloading of samples had an avalanche-type nature. The phenomenon was analytically described as a quick stochastic release of high density latent energy accumulated in a rock sample during the loading period.
Most studies consider rock fracture process exclusively as a result of action of external forces, which cause critical deformations and stresses in rock. However real rock masses even in a load-free condition may contain substantial internal local elastic deformations. They present a source of latent energy accumulated in rock. Such latent energy may appear in rock as a result of mechanical action to which rock was subjected earlier. Non-mechanical factors, for example, chemical processes, which took place or are taking place in rock, may also cause accumulation of latent energy.
Granite specimens were preliminary loaded uniaxially by a regular testing machine until single acoustic signals appeared. The specimens remained under the load for 5 minutes. Then the specimens were instantly unloaded and acoustic emission (AE) signals were again detected. Granite specimens were shaped as rectangular prisms of 50x50x 100 cm. Specimen sides were polished to provide better contact with the sensor and testing machine. Loads were applied to the specimens longer side edge surfaces. In order to conduct AE measurements, piezoelectric sensors were attached to the sides of a specimen, including grease, to provide a reliable acoustic contact. The AE frequency band ranged from 20 to 500 kHz. AE signals coming from the sensors were recorded with the AF-15 pulse detector connected to an additional amplifier. Total signal amplification was 113 dB. A non-linearity of the amplitude-frequency response consisted of not more than 10 dB. The experiments were carried out at loads of 5, 10 and in some cases 15 tons.
As was established during a series of experiments, a burst-like AE activity was observed immediately after unloading of pre-loaded granite specimens. The AE activity then decayed during relatively long time of about several minutes. In the following chapters we will try to give an analytical description of the observed phenomenon using a kinetic model of the rock fracture process recently developed by Anikolenko (1992,1998), Anikolenko & Mansurov (1996), and Kuksenko (1984).
It is established that the rock failure process is to a great extent controlled by micro stresses, which exist in rocks due to their heterogeneity and anisotropy. In a stressed state, micro cracks or clusters are generated in rocks, which then grow in size as deformation develops. During this process pairs of cracks join to create defects of a higher level by overcoming potential barriers caused by interatomic interactions.