Time correlation between different forms of energy emitted from rocks under compression

It is possible to demonstrate experimentally that the failure phenomena, in particular when they occur in a brittle way, i.e. with a mechanical energy release, emit additional forms of energy related to the fundamental natural forces. The authors have found experimental evidence and confirmation that energy emission of different forms occurs from solid-state fractures. The tests were carried out at the Laboratory of Fracture Mechanics of the Politecnico di Torino, Italy. By subjecting quasi-brittle materials such as granitic rocks to compression tests, it was observed for the first time bursts of neutron emission (NE) during the failure process, necessarily involving nuclear reactions, besides the well-known acoustic emission (AE), and the phenomenon of electromagnetic radiation (EME), which is highly suggestive of charge redistribution during material failure and at present under investigation. In this work acoustic, electromagnetic and neutron emission were measured during new laboratory compression tests on rock specimens loaded up to failure. All the signals were acquired by a National Instruments Digitizer with eight channels simultaneously sampling. The aim was to find a time correlation between these three different forms of energy emissions from rocks under compression. Tests were performed on magnetite and basalt specimens at constant displacement rate. AE signals were detected by applying to the specimen surface a piezoelectric (PZT) transducer with resonance frequency of about 150 kHz. EME signals were revealed by the induced current in a closed circuit due to change of the magnetic flux during specimen compression. The specimens were also monitored by means of He3 proportional neutron detector. During the tests were first detected AE signals, then EM emission. All the recorded signals were correlated with the load vs time diagrams. The EM signals were obtained, in particular, during the typical snap-back instabilities, which characterize the stress-strain curves of brittle materials such as rocks in compression. Neutron emission signals were generally identified at the end of the tests. As a matter of fact, neutron bursts usually occur when the behavior of the specimens in compression is particularly brittle or catastrophic. Applications of these monitoring techniques to earthquake forecasting seem to be possible.