28. High-Frequency Resonance Phenomena in Materials Subjected to Mechanical Stress

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.