Three-dimensional damage and rupture patterns of different thicknesses of rock mass with open joints under blast loading

There are an immense number of discontinuous structural surfaces represented by tensile joints in natural rock mass, in which the thickness of tensile joints determines the form how stress wave interacts with joint surface, which has a significant influence on the fracture behavior of jointed rock mass under blast loading. Open joint specimens were prepared using 3D printing technology, an indoor 3D blasting experiment was carried out with respect to different open joint thicknesses. Through CT scanning and three-dimensional reconstruction, the rupture and the distribution patterns of three-dimensional cracks of the post-blasting specimens were compared, and the rupture characteristics of the rock masses with different open joint thicknesses under blast loading were analyzed. The blasting process of the rock masses with open joints was inverted by AUTODYN numerical simulation software, the effects of open joint thickness on the propagation of stress wave and the three-dimensional damage characteristics of rock mass were revealed. Results showed that under the blast load, the wing cracks derived from the end of the open joints are greatly affected by the thickness of the joints and as the joint thickness increases, the annular derived cracks formed on the right outer surface of the specimen gradually decrease. In case of a change in the open joint thickness, the intensity at which the stress wave diffracts and transmits through the end of the joint will be directly affected. This makes it hard for the wing cracks at the end to initiate, the distribution density of the radial tension cracks traversing through the open joint surface attenuates obviously, resulting in the decrease of the fractal dimension and the damage extent of the specimen. As the thickness of open joints increases, the blast stress wave running through the joint surface will endure a time delay and reduced intensity, the closure of the joint surface gradually reduces, and the degree of stress concentration at the end gets weak. In the course of stress wave propagation, the rock mass on the back-blast side of the joint endures the combined actions of compressive stress field, tensile and tangential stress field and compressive and tangential stress field. As the thickness of open joint increases, the time point for the back-blast side to reach the peak stress is delayed, the intensity of the composite stress field subjected to compressive and tangential stress field decreases, and the normal displacements on the near-blast side and the back-blast side of the joint follow the opposite trend.