Elastic emission or very low frequency acoustic emission in solids under compression

In this paper are summarized the recent findings on detection of acoustic emissions at very low frequencies (i.e. from few Hertz to 10 kHz) in solid materials, such as rock and concrete, under compression until fracture in laboratory tests. These emissions, defined Elastic Emissions (or ELE), are characterized by wavelength greater than the maximum specimen size and by a very important amount of released energy. The ELEs are the effect of the propagating micro-shock waves due to the macro-cracks forming in the bulk of the materials detected at the specimen surface. A spectral analysis of the ELEs, realized with metrological reliability, provides quantitative informations about the macro-crack effects in terms of released energy, measuring the local acceleration in the accelerometer point of application.