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Dynamic crack coalescence attracts great attention in rock mechanics. However, specimen preparation in experimental study is a time-consuming and difficult procedure. In this work, a gypsum-like material by powder bed and inkjet 3D printing technique was applied to produce specimens with preset cracks for split Hopkinson pressure bar (SHPB) test. From micro X-ray CT test, it was found that the 3D printing technique could successfully prepare specimens that contain preset cracks with width of 0.2 mm. Basic mechanical properties of the 3D printing material, i.e., the elastic modulus, the Poisson's ratio, the density, the compressive strength, the indirect tensile strength, and the fracture toughness, were obtained and reported. Unlike 3D printed specimens using polylactic acid, these gypsum-like specimens can produce failure patterns much closer to those observed in classical rock mechanical tests. Finally, the dynamic crack coalescence of the 3D printed specimens with preset cracks were captured using a high-speed camera during SHPB tests. Failure patterns of these 3D printed specimens are similar to the specimens made by Portland cement concrete. Our results indicate that sample preparation by 3D printing is highly competitive due to its quickness in prototyping, precision and flexibility on the geometry, and high material homogeneity. [ABSTRACT FROM AUTHOR]

In order to observe meso-destruction process of coal in real-time, CT scan test on coal bearing uni-axial compression was implemented in State Key Laboratory of China University of Mining and Technology (Beijing). CT images were analyzed based on the fractal theory. Fractal dimension of CT images which include indications of crack expansion and rupture process of coal samples were calculated. Calculation result shows that the changing trend of CT images' fractal dimension with the stress matches the one of stress-strain curve obtained from the coal uni-axial compression test. It also indicates that the half-Gauss function could well describe the variation of fractal dimension with the change of coal loading under uni-axial compression CT test. The results of this research have important significance for providing intuitive observation data on coal meso-damage model and evaluating the performance of coal materials. [ABSTRACT FROM AUTHOR]