Analysis of Damage Induced in Multi-Stage Triaxial Tests Using Acoustic Emissions Data

The purpose of this research is to develop a technique to quantify damage and support the assertion that a Poisson’s ratio (PR) of 0.5 is an appropriate point of unloading for Multi-Stage Triaxial tests (MST). Acoustic Emissions (AE) were measured on five different samples which underwent Multi-stage Triaxial Tests. Two different behaviors were observed. For the quartz dominated samples (Miocene sandstone, Wilcox and Cambrian sandstone), the number of AE events did not show a strong dependence on confining stress. They did however have an exponential increase of AE events with increasing load at each stage. The total axial strain for the other set of samples (mudrock samples) was significantly larger than the quartz dominated samples indicating a more ductile failure mechanism. The choice of PR of 0.5 was validated using AE data as the point of unloading for quartz dominated samples in the MST. The number of AE events for quartz dominated samples showed a sharp increase as Poisson’s ratio approached 0.5. In contrast, the mudrock samples (Niobrara Marl and Niobrara Chalk) followed a different behavior. Plots of PR versus axial strain however did show a change in slope around a PR of 0.5 and then sharply increase further along the stress path for all samples. Even though the correction between maximum compressive strength and the point of positive dilation is about 1.2 for these samples, the simple connection between AE events is not observed. This would be investigated in future work.