Investigating the failure mechanism of X-shaped non-persistent joints under uniaxial loading: Experimental and numerical analysis

In this paper, rock samples containing non-persistent cracks of "X"- shape were prepared and experimentally studied under uniaxial compression. Then, the numerical simulations of these tests were established using dynamic finite element software (LS-DYNA). Various joint configurations are assumed and their lengths are considered to change from the center of "X" in different rock specimens. The cracks propagation patterns, the failure processes, and energy absorptions during the tests are studied. The gypsum samples' dimensions were 10 x 10 x 5 cm and one "X" shaped non-persistent crack was provided inside each specimen. The tensile strength of gypsum samples was 1 MPa. The notch's lengths are similar in some three cases while the length of one wing of the crossest joint varied in two other cases. In two residual cases, the length of the two wings of the crossest joint was varied. The opening of the crack was 1 mm. Angel between the notch trajectories was 90 degrees. The model was subjected to an applied axial load rate of 0.05 mm/min. Results show that the failure mechanism of specimens was affected by notch length. The larger notch has a dominant effect on the breakage mode and failure strength. Newborn tensile cracks originated at the notch tip and propagated parallel to the loading axis to merge with the model boundary. Both the crack initiation stress and breakage stress were decreased by increasing the notch length. The absorbed energy has a minimum value when two large notches with a length of 6 cm exist in the model while it has a maximum value when notch lengths are equal to 2 cm. The increase in notch length resulted in a decrease in the difference between strains associated with crack initiation stress and final stress, indicating progressive breakage. Whereas the difference between strains associated with crack initiation stress and final stress was increased by decreasing the notch length, therefore delay failure occurred after crack initiation in the model. The results obtained from both laboratory experiments and numerical simulations show good agreement in the failure progression.