Acoustic emission and electrical properties of quasi-brittle materials under compression

Acoustic emission (AE), electromagnetic emission (EME) and electrical properties measurements have been performed during laboratory compression tests on Green Luserna Granite dry rock loaded with increasing pressure up to their failure. Using a servo-controlled testing machine (MTS) each compression test was carried out at constant displacement rate and under ordinary room conditions. AE signals were detected by applying to the sample surface a piezoelectric (PZT) transducer, sensitive in the frequency range from 50 to 500 kHz. EME signals were detected using a loop antenna (80.0 dB gain at 500 kHz). The time-dependent outputs of the PZT transducer and the electromagnetic tester were connected to a DL708 Yokogawa oscilloscope (10 MSa s–1) in order to acquire simultaneously AE and EME signals associated with the same fracture event. Electrical properties, in terms of relative electrical resistance variation, were monitored by using copper electrodes coupled to specimens by conductive silver paint, and connected to a multimeter (Agilent model 34411A). The recorded AE, EME and electrical properties variation were related to the load vs. time diagram of the tested specimens. The experiments conducted in this research confirm that AE and EME are failure precursors in quasi-brittle materials, moreover EME signals and electrical property variations are related to sharp drops in stress vs. time diagram.