3D wing crack growth in a natural transparent brittle material under uniaxial compression

Detailed investigations of three-dimensional (3D) crack growth under uniaxial compression is necessary for understanding brittle failure in rocks and concrete. Importantly, a transparent modelling material is needed to observe the geometric features of crack growth. To date, the PMMA (polymethyl methacrylate) and casting resin have been used, which are only brittle upon freezing, such that the crack growth can be affected by the outside temperature and the time delay between freezing and testing. In this study, we use rosin, a naturally transparent and brittle material, which does not require pre-freezing and is considerably less sensitive to ambient temperature. The use of rosin involves some challenges. In particular, due to its delicate nature, traditional machining (cutting and polishing) methods often result in specimen damage. We use high-­precision PMMA inserts with the elastic modulus similar to that of rosin, integrating them to the specimens during casting to create flat and parallel ends (loading surfaces). Results from uniaxial compression tests (static) consistently showed that wing crack wrapping around the initial crack is the dominant crack growth geometry. Subsequently, only limited axial extension of the order of the initial crack size is observed, regardless of the loading rate, initial crack inclination and shape. Furthermore, specimens with initial cracks close to a specimen surface (lateral) were tested. The free boundary effect was observed not to lead to extensive crack growth either. Remarkably, even in this extremely brittle material that can be fractured under minimal manual force, extensive 3D crack growth does not occur under uniaxial compression. This study confirms that the arrest of 3D wing crack growth observed in artificial (man-made) materials (e.g. resin, PMMA and glass) is also observed in a natural material. These findings suggest that wing crack wrapping is an intrinsic feature of 3D crack growth under uniaxial compression.