13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- compression tests. In the following, the experimental results about the three-point bending configuration are also simulated numerically, exploiting a discrete element strategy. The main achievement of the present work consists in showing how the amount of cracking obtained from the numerical simulation and the experimentally detected AE events share the same temporal scaling laws 2. Experimental test 2.1. Acoustic Emission The estimation of active cracks is of significant importance for any structural inspection. For an early warning of crack nucleation, the classification of active cracks is a great deal of the AE technique [9]. AE signals due to microcracks are detected by AE sensors attached on the surface of the concrete specimen. The signal waveforms are recorded by the AE measurement system. In order to classify active cracks, AE parameters such as rise time and peak amplitude of each signal are considered to calculate the rise angle (RA) value, defined as the ratio of the rise time (expressed in ms) to the peak amplitude (expressed in V) [9–12]. Figure 1. Typical waveforms of tensile and shear events. A is the amplitude and RT the rise time (time between the onset and the point of maximum amplitude) of the waveforms [13]. The shape of the AE waveforms is typical of the fracture mode (Fig. 1). Shear events are characterized by long rise times and usually high amplitudes, whereas low rise time values are typical of tensile crack propagations. These conditions are synthesized by the RA value. Another parameter used to characterize the cracking mode is the Average Frequency (AF) expressed in kHz. The AF values are obtained from the AE ringdown count divided by the duration time of the signal. The AE ringdown count corresponds to the number of threshold crossings within the duration time. In general, the shift from higher to lower values of AF could indicate the shift of the cracking mode from tensile to shear [14]. Nevertheless, when a cracking process involves the opening of large cracks (Mode I), the frequency attenuation must be a function of this discontinuity. In other words, in this case the wavelength of the AE signals needs to be larger for the crack opening to be overcome, and the shift of the frequencies from higher to lower values could support
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