Effect of non-uniform loading on brittle fractures and 3D crack growth under uniaxial compression

The uniaxial compression test is a fundamental method for simulating the stress conditions that experienced by rock masses in the walls or pillars of underground openings. While such tests are typically assumed to be conducted under uniform loading, where the loading surfaces (top and bottom specimen ends) are the principal planes, non­uniform loading is often unavoidable in real testing. The loading non-uniformity may arise from specimen preparation or experimental set­up. In this study, a transparent and inherently brittle modelling material, rosin, was used to investigate the effects of non­uniform loading on brittle fractures and 3D crack growth under uniaxial compression. A key criterion proposed for sufficiently uniform loading is that wing cracks initiate from a centrally located 3D initial crack without the development of premature fractures prior to final failure. Results from both physical experiments and finite element modelling show that even minor unevenness at specimen ends can induce inclined tensile fractures near the ends, while the use of soft lubricant produces tensile stresses distributed across the entire loading surfaces, leading to splitting fractures sub-parallel to the loading direction. Both conditions could suppress 3D crack growth. Eccentric loading results in edge spalling and asymmetric wing crack growth from the upper and lower parts of the initial crack contour. This study provides new insights into the influence of loading conditions on brittle fracture mechanisms and offers practical guidance for reliable 3D crack growth experiments under compressive loading.