Study on the Length-to-Diameter Ratio Effect of Rock Indirect Tensile Deformation Evolution Based on AE and DIC Technologies

This study investigates the effects of the length-to-diameter (L/D) ratio on the indirect tensile strength of red sandstone using Brazilian splitting tests combined with acoustic emission (AE) and digital image correlation (DIC) technology. Brazilian splitting tests were conducted on red sandstone disk specimens with a constant diameter of 50 mm and L/D ratios of 1.0, 0.8, 0.6, and 0.5. The results show that the apparent splitting strength decreased from 5.05 MPa at L/D = 0.5 to 1.35 MPa at L/D = 1.0, a reduction of 73.3%. As the L/D ratio decreased, the AE ring counts and energy release became increasingly concentrated near the peak load and exhibited more pronounced burst-like characteristics. Shear-type AE events increased from 62.4% to 82.9%, and tensile-type events decreased from 37.6% to 17.1%. The decrease in the AE b-value prior to failure indicates a transition of damage evolution from distributed small-scale microcrack activity to localized large-scale crack initiation, propagation, and coalescence. DIC showed that primary cracks initiated near the specimen center, whereas lower L/D ratios produced stronger localization, more secondary inclined cracks, and more abrupt failure. The higher calculated strengths obtained under low L/D conditions should be interpreted as apparent Brazilian splitting strengths resulting from the combined effects of specimen geometry and boundary constraints, rather than being directly regarded as the intrinsic tensile strength of red sandstone.