Real-time computerized tomography (CT) experiments on limestone damage evolution during unloading

The computerized tomography (CT) images and CT values for the process of unloading, micro-cracking, and dilation up to the failure of limestone specimens under different unloading levels have been obtained by using the real-time CT technique. Clear CT images and CT value of the stages from compression of micro-cavities → emergence of micro-cracks → bifurcation → development → coalescence → suddenly collapse → rapid stress drop → unloading can be observed. The CT value, equivalent to rock density in the CT scan layer, is the most important parameter describing the damage evolution process of rock. The paper reveals that the mechanical behaviors of rock under unloading are different from those under loading due to its load-path dependency. It is clear from the experiment that it is easy for failure phenomenon of rock to happen suddenly under unloading. The mechanism of damage evolution of limestone is analyzed. Moreover, a damage model based on CT value and a new failure criterion are established.