Mechanical and electromagnetic emissions related to stress-induced cracks

The present research focuses on Acoustic Emission (AE) and Electromagnetic Emission (EME) detected during laboratory compression tests on concrete and rocks specimens. We investigated their mechanical behaviour up to failure by the AE and EME due to micro- and macro-crack growth. Among the tested specimens, a concrete sample was analyzed by applying to its surface both piezoelectric (PZT) transducers for detection of high-frequency AE waves, and PZT accelerometric transducers for detection of low-frequency AE (elastic emission, or ELE)1. Besides the high-frequency acoustic emissions (AE), the emergence of low-frequency elastic emissions (ELE) just before the failure describes the transition from diffused micro-cracking to localized macro-cracks which characterizes the failure in brittle materials. For all the specimens, a simultaneous analysis of magnetic activity was performed by a measuring device calibrated according to metrological requirements. In all the considered specimens, the presence of AE events has been always observed during the damage process, whereas it is very interesting to note that the EME, detected by a dedicated Narda ELT-400 device, were generally observed only in correspondence of sharp stress drops or the final collapse 2. The experimental evidence confirms AE and EME signals as collapse precursors in materials like concrete and rocks. As AE and EME coincide when certain types of rocks fail, the influence of EM fields on the AE transducers has been previously evaluated in order to be minimized. Given a fracture process, the AE activity behaves as fracture precursor, since it precedes EME events, which accompany stress drops and related discontinuous fracture advancements. While the mechanism of AE is fully understood, being provided by transient elastic waves due to stress redistribution following fracture propagation 3-8, the origin of EME from fracture is not completely clear and different attempts have been made to explain it.