Effects of stress path on brittle failure of sandstone: Difference in crack growth between tri-axial compression and extension conditions

The tri-axial testing of rocks under compression is a widely popular measure for the assessment of strength of geological materials in the laboratory. In this regard, mechanical complexities of true tri-axial testing machines impel researchers to opt for simpler conventional tri-axial testing machines. To account for influential factors that affect quality of experimental data, observations derived from laboratory experiments that enable overcoming some drawbacks are presented herein. In the present study, a series of tri-axial tests on Kimachi sandstone under Conventional Tri-axial Compression (CTC), Conventional Tri-axial Extension (CTE), Reduced Tri-axial Compression (RTC), and Reduced Tri-axial Extension (RTE) conditions were carried out. The results suggested higher compressive strengths for the rock under tri-axial extension tests than the compression tests. This observation proves that the effect of intermediate confining pressure on material behavior is important and if feasible, should not be sacrificed for more economical alternatives. Generally, the loading and unloading experiments for a specific test type rendered the same results and the conventional and reduced paths in both the compression and extension tests did not affect the failure strength of the specimens. The effect of pre-existing cracks orientation with respect to the principal loading directions on crack growth was focused upon and related conclusions were subsequently drawn.

Automated summary

This study investigated the strength performance of rocks under different conditions through a series of tri-axial tests, finding that the compressive strength of the rock was higher in tri-axial extension tests. Additionally, the study focused on the influence of crack orientation on crack growth.