Source characterization of fracture in quasi-brittle materials

A common feature of fracture propagation in quasi-brittle materials is the development of microcracking, which releases energy in the form of elastic waves called acoustic emission (AE). TheAE technique can be used to monitor the evolution, including location and mechanism, of damage at various stages of loading. For an idealized point source of displacement discontinuity, the localized stress release fromAE can be represented by dipoles of forces called the moment tensor.To determine the characteristics of theAE, a simplified transducer calibration is suggested, where constant values of the amplitude sensitivities were taken to capture the response up to the first peak associated with the P-wave arrival. Calibration breaks on at least three different points were performed to obtain an amplitude sensitivity for each transducer, owing to the dependency on the coupling between the transducer and specimen. Components of the displacement discontinuitywere retrieved by minimizing the error between measured displacements obtained from the AE signal and the displacements calculated from the moment tensor at the source, subjected to restrictions on the form of displacement discontinuity and the moment tensor. A separable time dependency for all components of the moment tensor was assumed.A three-point-bend fracture test conducted on a typical quasi-brittle material and theAE data were analyzed with the source model. Even though mostAE events were characterized as shear predominant, the displacement behavior associated with macroscopic crack opening was observed. This anomalous behavior can be explained by examining a torturosity angle, which is a measure of microcrack orientation on the horizontal plane.