Acoustic Emissions at High and Low Frequencies During Compression Tests in Brittle Materials A. Schiavi*, G. Niccolini*, P. Tarizzo*, A. Carpinteri † , G. Lacidogna † and A. Manuello † *National Research Institute of Metrology, INRIM, Strada Delle Cacce 91, 10135 Torino, Italy † Politecnico di Torino, Department of Structural Engineering and Geotechnics, Corso Duca Degli Abruzzi 24, 10129 Torino, Italy ABSTRACT: The damage process in a concrete specimen subjected to uniaxial compression test is investigated by detection of the propagating elastic waves because of micro- and macrocrack growth. Besides the high-frequency acoustic emissions (AEs), the presence of low-frequency elastic emissions (ELEs), from 1 to 10 kHz, is detected just before the specimen failure. A spectral analysis of the ELEs is performed by measuring with a calibrated transducer the local acceleration of the specimen surface. Quantitative information about the macrocrack effects in terms of released energy is thus obtained. Furthermore, the evolution of damage is followed through the analysis of the amplitude distribution of AE and ELE signals, distributed according to the Gutenberg–Richter (GR) statistics. KEY WORDS: acoustic emissions, b-value analysis, elastic emissions, fracture precursors, low- frequency Introduction The mechanical behaviour and the damage evolu- tion of heterogeneous materials under compressive loading conditions are of great interest. Acoustic emission (AE) because of crack growth in brittle materials is usually observed in the high frequency (HF) range, typically from 10 2 to 10 3 kHz, since the beginning of the damage process. The AE detection is used for damage localisation and damage level assessment in quasi-brittle materials such as concrete and rocks [1–5]. The increasing rate in the AE activity is observed as the specimen approaches impending failure, and it is correlated with the progressive degradation of mechanical proper- ties of material (decrease of the Young modulus) [6–13]. It is established by numerous investigations that the AE activity varies during damage process. The AEs released in various stages of this process are distin- guished by its own energy and furthermore are con- centrated in various sections of the frequency range; at the start of the damage process (microcrack initi- ation), AE is emitted at high frequencies, and later (microcrack coalescence into cracks of large length), the AE frequencies drop off to few kHz or less [13, 14]. In this perspective, the attention is focused also on the characterisation of low-frequency (LF) elastic emissions (ELEs), specifically in the range of 1– 10 kHz, detected in a concrete specimen under compression. The goal is analysing two well-separated frequency ranges of emitted signals to investigate the different stages of damage process (microcrack initiation and macrocrack growth), which precede the failure of quasi-brittle materials. Elastic Wave Propagation in High- and Low-Frequency Ranges Crack growth in a rigid stressed-strained specimen causes redistribution of the internal stresses in the form of transient elastic waves. Therefore, each crack event acts as an impulsive force that causes elastic vibrations of the specimen. During nucleation and growth of microcracks, high-frequency acoustic emissions (AE) are observable. As soon as microcracks coalesce to form cracks of large length, the frequen- cies drop off and ELE clearly appear in addition to AE [13, 14]. In the case of AE, the longitudinal and/or shear wave propagation is because of the oscillations of material particles around their equilibrium positions during microcrack growth (Figure 1A). While the growth of large cracks, forming new internal surfaces, is accom- Ó 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 105–110 105 doi: 10.1111/j.1475-1305.2010.00745.x An International Journal for Experimental Mechanics