## 2 Overview of Unloading Experiments
Micro-damage may be induced in the specimens during both the loading and unloading parts of the successive cycles. [PERSON] et al. [28] conducted specific experiments and compared the strength for conventional triaxial loading with the strength for a three-stage loading and unloading. [PERSON] et al. used the conventional triaxial cell and conducted five experiments per loading path for three different confinement stresses. Each of the five experiments corresponded with a different inclination angle of the bedding planes in bedded sandstone. Although the overall aim of the research was the storage of energy and the energy flow under rockburst conditions in a coal mine, the comparison of the measured strength values is extremely  
Figure 2: Micro-photograph of petrographic thin sections of cinoidal limestone specimens, tested in a true-triaxial stress cell applying an unloading scheme. (**a**) Test stopped prior to the formation of macro-fractures; (**b**) test until the failure of the specimen. Width of both pictures is about 1.5 mm. Orientation of major and minor principal stresses are indicated by arrows. interesting. Overall, the strength or failure envelope for the three-stage loading and unloading was smaller than the one associated with conventional triaxial loading. Their work is discussed in more detail below. [PERSON] et al. [29] applied different loading-unloading methods and rates for different confining pressure tests of marble specimens. They tried to approximate the in situ stress path as well as possible, resulting in relatively complex stress paths. They studied the influence of the initial confining pressure; loading and unloading rates; and the stress paths on rock dilatancy, failure, energy, and strength characteristics. Among other findings, they observed that the peak stress and the residual stress were smaller with larger unloading rates. The increase in unloading rates accelerates the deformation and failure of the rock. [PERSON] et al. [16] used a true triaxial rock testing machine to conduct loading and unloading tests of sandstone specimens. In comparison to the results of the loading tests, the peak strength of the unloading path was reduced. They presented their results with a Mogi-Coulomb criterion, but looking at the cohesion and friction angle, they obtained cohesion values of about 22.0 MPa for the true-triaxial loading and about 16.5 MPa for the true-triaxial lateral unloading with friction angles of 57.6\({}^{\circ}\) and 48.3\({}^{\circ}\), respectively. This means that the cohesion was reduced by about 25%. [PERSON] et al. [14] conducted minimum principal stress unloading tests of marble specimens under true triaxial compression conditions and with different initial stresses and unloading rates. They also investigated the effect of the middle principal stress. Although considering a large number of different values is very interesting, it also makes it difficult to interpret the failure values for the loading vs. the unloading paths and to judge whether the failure criteria of the unloading paths were indeed stronger than of the loading path as indicated on their graphs of the largest differential stresses (major principal stress minus minor principal stress) vs. the minor principal stress. When studying the effect of various stress paths, it is essential that one observes the occurrence of micro-fractures during the entire loading and/or unloading process. The recording of acoustic emission is hereby a useful tool (e.g., [8; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41]). The determination concerning whether the emissions are in shear mode, in tension mode, or in mixed mode (e.g., [31; 42; 43; 44; 45; 46]) allows for a proper interpretation of the fracturing mechanisms involved and how they evolve as a function of time or of the load level. An additional verification by the visualisation of micro- and macro-fractures provides a better understanding of the failure process.