Micromechanisms of fracture from acoustic emission

Acoustic emission (AE) events were located and characterized from a mode I fracture test on Charcoal granite. An AE source was modeled as a displacement discontinuity, a microcrack, with a displacement vector b, normal n, and area ΔA. For convenience, the source parameters were recovered through a moment tensor representation, the components of which were estimated by minimizing the error between theoretical and measured displacements at sensor positions, with an appropriate constraint on the moment tensor for the microcrack model. Amplitude sensitivity was obtained through a calibration method with a step-unloading point source, providing the linkage between the AE signal and the measured displacement. The source mechanisms of some 400 events were identified as mixed-mode but shear dominant, with both opening and closing normal displacements. Decomposition of the microcrack volumes biΔA into the global coordinate system of the specimen revealed a positive accumulation of opening volume compatible with the mode I fracture mechanism, while the change in the vertical component agreed with the trend of beam deflection, even during a snap-back response. An out-of-plane, mode III microcrack volume component was also detected, which reflects the nature of the local deviation of the fracture path due to heterogeneity on the grain scale. Microcrack orientations, defined by horizontal and vertical angles called tortuosity and inclination angles, respectively, provided the explanation for the shear-dominant mechanisms on the local scale but opening on the structural level. Thus, shear-type AE events can be consistent with tensile fracture, as microcracks are locally based at various orientations.

► Acoustic emission is modeled as a point source of displacement discontinuity, a microcrack, described by displacement vector b, normal n, and area ΔA. ► From a moment tensor representation of a microcrack, the source mechanisms from a mode I fracture test are identified as shear dominant. ► Decomposition of biΔA reveals a positive accumulation of opening volume compatible with mode I fracture. ► Due to fracture tortuosity, shear-dominant mechanisms on the local scale can be consistent with opening on the global level.