Quantitative Characterization of Rock Fracture Surface Damage using Visualizable Triaxial Shear Experiment

The shear strength of rock fractures governs the mechanical behavior and engineering stability of rock masses, with the damage evolution of asperities on fracture surfaces playing a crucial controlling role in the shear behavior. To overcome the limitations of traditional shear testing methods in real-time observation of the fracture surface failure process, this study employed a self-developed CT-visualized triaxial shear testing system to conduct triaxial shear tests on red sandstone and granite specimens containing prefabricated fractures under identical confining pressure conditions, acquiring real-time CT images during the shearing process. By introducing a high-absorbance lead particle marking technique, precise tracking of the three-dimensional displacement field of fracture blocks throughout the entire shearing process was achieved. Combined with a high-precision contact model, a quantitative analysis method for the shear failure area ratio of asperities was proposed. The results show that the root mean square error (RMSE) between the displacements obtained from CT images and those measured by the axial displacement sensors of the instrument was smaller than 0.097 mm, confirming the accuracy and reliability of the CT measurements and their capability to globally capture real-time deformations in the axial, normal, and tangential directions. Under the same confining pressure and roughness conditions, approximately 19% of asperities on the red sandstone fracture surface have damage area ratios concentrated in the 10% to 15% range, with a maximum value of 64%. In contrast, about 11% of asperities on the granite fracture surface fall within the 25% to 30% range, with a maximum value of 99%. This indicates that granite fractures contain a higher proportion of severely damaged, or sheared asperities, whereas red sandstone fractures are dominated by slight wear. The CT-visualized testing method and quantitative analysis techniques developed in this study provide an effective new approach for the mesoscopic analysis and damage mechanism investigation of rock fracture shearing.