Chapter28 High-Frequency Resonance Phenomena in Materials Subjected to Mechanical Stress G. Lacidogna, B. Montrucchio, O. Borla, and A. Carpinteri Abstract The elastic energy released by micro-cracking yields to macroscopic fracture whose mechanical vibrations are converted into electromagnetic (EM) oscillations over a wide range of frequencies, from few Hz to MHz, and even up to microwaves. As regards Acoustic Emission (AE), the classical monitoring techniques allow an observation over a range of frequencies up to hundreds of kHz. In this paper the authors investigate if, during compression tests on brittle materials, which involve catastrophic fractures, it is possible to identify in the stressed materials mechanical oscillations in a frequency range higher than that characteristic of the AE and comprised between MHz and THz. This excited state of matter could be a precursor of subsequent resonance phenomena of nuclei able to produce neutron bursts, especially in the presence of sudden catastrophic fractures. This phenomenon has been also very recently argued from a theoretical physical point of view by Widom et al. In this investigation experimental evidences emerge by means of a confocal sensor able to measure the resonance frequency of the specimen. The basic idea is to use a laser light focused onto a spot of the specimens surface subjected to mechanical compression. A photo-detector measures the intensity of the reflected light and then gives the frequency variation that is proportional to the vibration frequency of the spot particles. Keywords Brittle Failure • Acoustic Emission • Electromagnetic Emission • Neutron Emission • Laser measurements 28.1 Introduction This paper discusses the phenomenon of energy emissions from brittle material specimens under mechanical loading. The authors have recently found experimental evidence that energy emission of different forms occurs from solid-state fractures. By subjecting quasi-brittle materials such as granitic rocks to compression tests, for the first time, bursts of neutron emission (NE) during the failure process were observed for the first time [1–5], necessarily involving nuclear reactions, besides the well-known acoustic emission (AE) [6–15], and the phenomenon of electromagnetic radiation (EM) [16–21], which is highly suggestive of charge redistribution during material failure. As regards neutrons, these emissions are due to piezonuclear reactions, which depend on the different modalities of energy release during the tests. For specimens with sufficiently large size and/or slenderness, a relatively high energy release is expected, and hence a higher probability of neutron emission at the time of failure [1–5]. The phenomenon of EM emissions is regarded as an important precursor of critical phenomena in Geophysics, such as rock fractures, volcanic eruptions, and earthquakes [22, 23]. For example, anomalous radiation of geo-electromagnetic waves are observed before major earthquakes. At the laboratory scale, rocks and concrete under compression generate AE and EM emission nearly simultaneously. This evidence suggests that also NE emissions are generated during crack growth, reinforcing the idea that the NE phenomenon could be associated with seismic activity [24]. The experimental analysis carried out by the authors may open a new possible scenario, in which the stress state of the elements firstly involves the generation of microcracks, accompanied by mechanical energy release in the field of ultrasonic G. Lacidogna ( ) • O. Borla • A. Carpinteri Department of Structural, Geotechnical and Building Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy e-mail: giuseppe.lacidogna@polito.it B. Montrucchio Department of Control and Computer Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy J. Carroll and S. Daly (eds.), Fracture, Fatigue, Failure, and Damage Evolution, Volume 5: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06977-7__28, © The Society for Experimental Mechanics, Inc. 2015 211
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