13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- applications. The application of AE monitoring in masonry structure is highly complicated, as attenuation and wave propagation are dependent on the heterogeneity of the material. Besides, the presence of voids, cracks and cavities does also influence the AE detection itself. The AE waves, generated at the other side of a large crack to which the sensors are placed, will generally not be detected by AE sensors. This, for example, would be the case between the disconnected layers of a multiple-leaf wall, which is often encountered in historical masonry structures. All these ingredients make the source localization in masonry structure rather difficult [9]. In order to address these issues regarding damage region assessment in masonry, a series of tests were designed and carried out on a masonry model bridge. The results for the tests are discussed in this paper. 2. Source location of AE events and classical localization method Localization of AE sources is important to assess the regions of active damage in the monitoring technique. Location problems are usually solved by various triangulation techniques based on mathematical analysis of acoustic wave trajectories [16]. This analysis cannot be simply performed if the structure of tested specimen is geometrically complicated. Generally, these techniques depend on the mode of propagation, the elasticity modulus, and signal attenuation due to the heterogeneity and anisotropy of material. In this Section, a general overview on the classical localization method and the corresponding knowledge is depicted. Figure 1. Localization of point AE source involving a generic array of n sensors. As shown in Fig. 1, in a theoretical model with wave propagation velocity Pi v , the onset time * it at sensor ix , unknown crack coordinates 0x and origin time 0t can be estimated by an integral along the actual ray path i :
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