# Acoustic Emission Activity for Characterizing Fracture of Marble under Bending
## 1 Introduction  
Acoustic emission (AE) is a technique used for many decades in structural health monitoring (SHM) applications. It uses sensors to record the elastic motion that follows crack nucleation and propagation events within the materials and transform them into electric waveforms. Their intensity (energy), number and other parameters reveal information on the crack density and propagation rate, the location and the mode of fracture [1; 2; 3; 4]. The use of the technique in a typical laboratory experiment is shown in Figure 1. In typical rock materials like marble and granite, acoustic wave techniques have been used for quite some time. The cumulative AE activity was studied in relation to the fracture process of pre-cracked marble under compression. It was shown that specimens with \"zigzag\" cracks failed in a more unpredictable way since most of the activity was registered at the moments of macroscopic failure [5]. The evolution of AE activity was very indicative of the strain at creep experiments of granite [6]. The AE activity and energy release during compression was used to indicate the damage level showing that in marble, main damage comes with the peak stress, whileearlier only micro-cracking is formed compared to granite and rock salt [7]. The frequency content of AE has shown to increase with the load, being essentially a precursor of failure in confining pressure experiments, while close to the peak load a strong drop was noticed [8]. The Kaiser effect (absence of detectable AE before the previous maximum load is overpassed) in Brazilian and bending tests was investigated to reveal the previous tensile stress imposed in marble [9]. Energy rate, hit counts, amplitude and location of AE events were used for characterization of crack propagation during the indentation test [10] and compression test in rock materials [11]. AE monitoring of rock material under compression showed mainly two time zones of high \"avalanche-like\" activity, interrupted by a long period of acoustic silence [12]. The effect of temperature on brittleness of marble was studied with AE corresponding very well to the \"rupture evolution law\" [13]. Complementary to AE, another elastic wave method, ultrasonic pulse velocity (UPV) has also been used for characterization of marble condition and mechanical performance. Specifically UPV has been used to derive elastic constants of marble [14] and has been positively correlated to the mean grain size of marble [15], while the anisotropy has also been highlighted due to the noticeable difference between the velocities at the three different axes [16]. The influence of water saturation in granite has also been examined by ultrasonic measurements. The effect of microstructure is revealed by the systematic increase of velocity for higher ultrasonic frequencies (150 kHz compared to 54 kHz) [17]. The present study is a continuation of preliminary tests on granite and marble. Previously, different fracture modes were targeted showing that AE is quite sensitive to the loading conditions [18; 19]. Specimens loaded in pure bending emitted higher frequencies and signals of shorter duration relatively to shear-loaded ones. After encouraging results were found, in this case, a large population of specimens of two marble types was tested in bending with concurrent AE recoding to increase the statistical adequacy in an effort to check possible correlations between passively monitoring parameters and mechanical properties. The AE behavior proved very consistent exhibiting three distinct stages with different AE rate. Specific parameters can be used from the early loading to characterize the capacity of the material, like average frequency and RA value, which have been adopted from concrete practice. The fractured specimens were consequently repaired and reloaded to failure.