Size effect on fracture behavior of quasi-brittle materials during uniaxial compression tests

Understanding the size dependence of quasi-brittle materials' fracture behavior is useful in assessing the safety of flawed rock engineering structures. In this study, a set of uniaxial compression tests were conducted to scrutinize the mechanical properties and crack morphology of cubic specimens with sizes of 75 mm, 100 mm, 125 mm and 150 mm. The results showed that the specimens had significant size dependence in terms of strength and crack initiation angle. The crack type and fracture pattern were not affected by the specimen size and were mainly controlled by the pre-existing crack inclination angle. Next, the test results were compared with the maximum tangential stress criterion considering the T-stress. The results also showed that the crack initiation angles of specimens with different sizes were in good agreement with the theoretical calculations. The non-singular compressive stress perpendicular to the crack surface inhibited the generation of the fracture process zone (FPZ). In addition, the stress required for tensile failure was smaller for large-size specimens, leading to a decrease in the length of the FPZ with increasing specimen size. The FPZ variation law of the uniaxial compression tests was greatly different from that of the mode I loading.