New insights into the fracture mechanism of flattened Brazilian disc specimen using digital image correlation

Abstract As a simple and convenient testing method, the flattened Brazilian disc (FBD) specimen has been widely used to determine the tensile strength and mode-I fracture toughness of rocks. However, the precision of the mechanical parameters calibrated by FBD testing method needs to be further evaluated. In this paper, digital image correlation (DIC) technique is used to capture the progressive failure process of FBD specimen, and some new insights are provided for further understanding the fracture mechanism of FBD specimens. Firstly, the horizontal tensile strain history in the center region of FBD specimen is obtained via DIC technique, and combined with loading force history, the tensile stress–strain curve of rocks can be determined. Then, by analyzing the horizontal strain and displacement field near the central crack of FBD specimen, the central crack tip positions are identified. The progressive failure process of FBD specimen shows that the micro-crack and fracture process zone (FPZ) has grown at the center of FBD specimen at the moment of first peak force Pmax. After the Pmax, the central crack propagates unsteadily towards the flattened ends of FBD specimen, while is hindered after the loading force declined to the local minimal force Pmin. Therefore, it is not proper to determine the mode-I fracture toughness KIC using the local minimal force Pmin in FBD tests. In this study, a modified calculation method to determine KIC is proposed using the first peak force Pmax and its corresponding central effective crack length observed via DIC technique. In addition, the cracking nature of the secondary crack is identified, and the second rise of the loading force is induced by the compression-shear failure near the flattened ends of FBD specimen. These results are beneficial to better understanding the progressive fracture mechanism of FBD specimen and improving the accuracy of KIC calibrated by FBD testing method.