An improved fixture for better measuring the displacements and mode-I fracture toughness of chevron bend specimens

The load versus load point displacement (LPD) curve is significant for the nonlinearity corrections for the mode-I fracture toughness of chevron bend (CB) specimens. The testing fixture plays a crucial role in measuring load versus LPD curves. To better measure the LPDs of CB specimens, we developed an improved fixture system consisting of an understructure and a yoke based on the traditional saddle arrangement fixture suggested by ISRM. With the aid of the improved fixture, continuous synchronized LPDs and CMODs are measured, and nonlinearity corrections are conducted. After nonlinearity corrections, the data of mode-I fracture toughness become more comparable and present a better regularity among different specimen diameters and variant rock types. Repeated tests verify that the improved fixture has the advantages of higher stability and more convenient installation, removal, and adjustment, which can enhance the testing robustness and efficiency.