7 ACOUSTIC EMISSION DETECTION IN CONCRETE SPECIMENS: EXPERIMENTAL ANALYSIS AND SIMULATIONS BY A LATTICE MODEL I. Iturrioz1, G. Lacidogna2, A. Carpinteri2 (1) Federal University of Rio Grande do Sul Department of Mechanical Engineering Porto Alegre, RS, Brazil (2) Politecnico di Torino Department of Structural Engineering and Geotechnics Torino, Italy ABSTRACT In civil engineering, materials subjected to stress or strain states a quantitative evaluation of damage is of great importance due to the critical character of this phenomena, which at certain point suddenly turns into catastrophic failure. An effective damage assessment criterion is represented by the statistical analysis of the Acoustic Emission (AE) amplitude distribution signals that emerges from the growing microcracks. The amplitudes of such signals are distributed according to the Gutenberg- Ritcher (GR) law and characterized through the b-value which systematically decreases with damage growth. The b-value analysis was conducted on two experimental tests carried out on concrete specimens loaded up to failure. The first one is a prismatic specimen subjected to uniaxial compression load, the second one is a pre-cracked beam subjected to three point bending test. The truss-like Discrete Element Method (DEM) was used to made numerical simulation on the experimental tests. The comparison between experimental and numerical analyses, in terms of load vs. time diagram and AE data, elaborated throughout the b-value and signals frequencies variations, provided results in good agreement. Key words: Concrete; Lattice model; AE technique; b-value; Damage parameter. 1. INTRODUCTION The most advanced method for a non-destructive quantitative evaluation of damage progression is the acoustic emission (AE) technique. Physically, AE is a phenomenon caused by a structural alteration in a solid material, in which transient elastic-waves due to a rapid release of strain energy are generated. AEs are also known as stress-wave emissions. AE waves, whose frequencies typically range from kHz to MHz, propagate through the material towards the surface of the structural element, where they can be detected by sensors which turn the released strain energy packages into electrical signals Traditionally, in AE testing. a number of parameters are recorded from the signals, such as arrival time, velocity, amplitude, duration and frequency. From these parameters damage conditions and localization of AE sources in the specimens are determined, Carpinteri et. al. (2009). Using the AE technique, an effective damage assessment criterion is provided by the statistical analysis of the amplitude distribution of the Acoustic Emission (AE) signals generated by growing microcracks. The amplitudes of such signals are distributed according to the Gutenberg-Richter (GR) law, N(≥A)∝ A−b, where N is the number of AE signals with amplitude ≥ A. The exponent b of the GR law, the so-called b-value, changes with the different stages of damage growth: while the initially dominant microcracking generates a large number of low-amplitude AE signals, the subsequent macrocracking generates fewer signals of higher amplitude. On the other hand, the damage process is also characterized by a progressive localization identified through the fractal dimension D of the damaged domain. It may be proved that 2b=D (Aki(1967), Carpinteri 1994;
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