## 5 Results
### Failure Behaviour  
Acoustic emission technology works based on monitoring wave signals produced by the internal structure change, such as cracking, fracture propagation and dislocated displacement. In addition, stable defects such as elastic strain cannot generate acoustic signals, so cracking and failure activities can be revealed via AE signals. For an intact rock material, AE activities normally remain peaceful during a loading process and start to be active around and before the failure moment. Because of that, AE data can be used to predict material failure, which is also useful in practical engineering work. As for jointed rock mass, AE behavior differs from that of intact rock. As depicted in Figure 14a, AE behavior remains active first at the pre-compression stage. The entire rock mass is compressed during this period with a gradually enhanced stiffness. Specifically, 0\({}^{\circ}\) jointed rock tends to have a longer pre-compression duration than other joint conditions due to its higher pre-compressibility. As the loading continues, 0\({}^{\circ}\) jointed mass will present a short period of elastic behavior during which a linear elastic deformation occurs, with comparatively milder AE signals. Obvious elastic behavior happens in the cases of 45\({}^{\circ}\) and 90\({}^{\circ}\) jointed angles while with a longer lasting period due to (with 90\({}^{\circ}\) jointed rock mass, for instance) a lower starting point of elastic behavior and a higher yield stress. There are no evident AE activities during this period, which is a common phenomenon in three joint conditions. Moreover, it can be found that when the rock mass turns into a yield surface, an isolated AE behavior will emerge, and clustered AE columns will appear before the failure. Failure modes are recognized by the DIC measurement for three jointed cases, as depicted in Figure 14. As discussed earlier, four distinct regions are discovered as revealed in the stress evolution curves: region \(a\) of pre-compression, region \(b\) of elastic state, region \(c\) of sample yielding, and region \(d\) of post-yield process. Volumetric strain distribution measured by the DIC is attached for each region of respective cases. For three jointed cases, localized deformation emerges after the material comes into the plasticity, and significant localization occurs after the failure. With the 0\({}^{\circ}\) In a jointed case, for instance, cracks emerge near the center hole and tend to develop towards the exterior area as the compression continues. A similar situation can be found in the 90\({}^{\circ}\) jointed case with a higher degree of deformation (see Figure 14c). When it comes to the 45\({}^{\circ}\) jointed situation, localized deformation appears surrounding the pre-existed joints (see Figure 14b). According to the Mohr Column yield criterion, the joint angle of 45\({}^{\circ}\) is close to the failure plane, although for the sample with an internal hole. In such a case, localized deformation emerges from  
Figure 13: Stress-strain curves of uniaxial compression tests for the intact sample and jointed samples with angles of 0, 45 and 90 degrees. the joints and gets extended to the entire sample surface. A significant shear failure model is distinguished in the 45\({}^{\circ}\) jointed case. It should also be noted that the 45\({}^{\circ}\) jointed case experiences a ductile failure mode after the peak strength and the localization develops with a longer duration and an extensive influenced area. Figure 14: Evolution of strain diagrams and AE activities of (**a**) 0\({}^{\circ}\) joints, (**b**) 45\({}^{\circ}\) joints, and (**c**) 90\({}^{\circ}\) joints. The unit of coordinates is in pixels (1 pixel corresponds to 0.05 mm in reality). As described above, perpendicular-jointed condition (0\({}^{\circ}\) joints) is most dangerous because of its comparatively lower strength and brittle failure mode with a shift energy release.