13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- All specimens were tested, after the fracture loading, using pencil lead break to simulate the burst signal propagation. The device (pencil lead break) is an aid to simulate an acoustic emission event using the fracture of a brittle graphite lead in a suitable fitting. This generates an intense acoustic signal, quit similar to a natural AE source, which the sensors detect as a strong burst. Figure 3 presents the principle of the pencil lead break method preformed on the fractured sample (Fig. 3.a) and the calibration curve of the acoustic emission location (Fig. 3.b). The amount of AE wave attenuation depends on the proprieties on the material. For the tested samples, attenuation curve (Fig. 3.d) is performed by using the AST procedure (Auto Sensor Test, Fig. 3.c). AST provides an automated means of pulsing and receiving of simulated AE burst that is coupled to the structure. Similar results of attenuation curve can be also obtained by using the pencil lead break method. The attenuation curve plotted in figure 3.b is the consequence of several phenomena taking place as AE waves propagate along the sample: dispersion, scattering and eventually dissipation. The results presented in these figures highlight the importance of the AE calibration procedure in the analysis of AE data. 3.2. Analysis of Energy Balance The energy approach states that crack extension occurs when the energy available for crack growth is sufficient to overcome the resistance of the material. The material resistance may include the surface energy, plastic work, or other type of energy dissipation associated with a crack propagation. Figure 4.a illustrates the load-displacement behavior of tested wood specimens with a growing crack. Consider point A on the presented curve. The crack has grown with Δa length from initial length a0. The crosshatched area represents energy that would be stored in the material where considering linear elastic behavior; the remainder is the energy dissipation associated with a propagation crack. Examination of the plot of figures 4.c&d illustrates a number of things. First, crack growth initiation detected by image analysis occurs closely after the end of the linear elastic behavior. The progression of the crack front propagation according to the displacement loading is quasi-linear showing stable crack growth in the tested specimens. A second point to observe is the over shape similarity when we compare the fracture energy curve and the crack front propagation curve. Thus we can assume validity of the initial hypothesis, linear release of stored elastic energy, used in the energy balance analysis.
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