Experimental Study on the Size Effect of Fracture Process Zone of Mixed Mode I-II Crack in Sandstone

Rock cracks tend to grow in the form of tensile and shear failure, i.e., mixed mode I-II crack. Size effect of fracture process zone (FPZ) should be highlighted due to it may alter the fracture growth. To study the size effects of FPZ on the development characteristics of mixed mode cracks, three-point bending experiments with off-center initial crack were carried out on sandstone with ratio of length and height of 3. Combined with digital image correlation (DIC) method, the variation of deflection angle and displacement field around the crack tip were analyzed. The results show that the FPZ length increases with the specimen size while the normalized length of the FPZ decreases. The FPZ begins to develop at 60-70% pre-peak and is fully developed at 90% post-peak. The cracks of different sizes begin to deflect when the FPZ is fully developed, and the deflection is completed at 60% post-peak. The deflection angle of FPZ decreases with the sample size. To ensure that the local tensile stress is the maximum, the cracks will deflect and cause the local shear stress to decrease and disappear, which is more obvious in small-size specimens. This study further revealed the fracture mechanism of mixed mode I-II crack under different sizes. 1. INTRODUCTION Cracks are common internal structure in rock materials, due to different external forces, there will be different forms of expansion, generally can be divided into three modes: mode I (open) cracks, mode II (slip) cracks and mode III (tear) cracks (Surberg and Tschegg, 2001). In view of different shapes of prefabricated cracks, Lin and Labuz, (2013) et al. studied mode I fractures of sandstone. The results show that the critical crack opening displacement does not change with the prefabricated crack shape. The FPZ length is about 10 times the maximum particle size of the sample. The critical crack opening displacement and FPZ length in mode II cracks are larger than those in mode I cracks (Lin et al., 2014; Moazzami et al., 2020). Because cracks in actual rock structure often do not exist in a single fracture mode, such cracks are called mixed mode cracks. In practical engineering, mixed mode I-II cracks (hereinafter referred to as mixed mode cracks) are the most common (Aliha and Ayatollahi, 2013). In the mode I fracture, the fracture receives only tensile stress (Meirong. et al., 2024). Compared with mode I cracks, mixed mode cracks are not only subjected to tensile stress, but also to shear stress (de Moura et al., 2016). Since the shear resistance of rock materials is stronger than the tensile resistance, cracks generally start and expand in the direction perpendicular to the maximum tensile stress, that is, tensile failure is still the main type of mixed mode cracks (Song et al., 2018). Due to the existence of shear stress, there is a deflection Angle in the initiation of mixed mode cracks compared with mode I cracks.