Design and application of a direct shear testing apparatus for cylindrical rock specimens

The ongoing advancements of large-scale water conservancy projects, deep ultralong tunnels, and deep geological storage facilities in geotechnical engineering present severe challenges to the safety of rock mass engineering. The shear strength of rock masses is crucial for engineering construction, as numerous instability incidents stem from shear-failure mechanisms. Therefore, the experimental studies of the shear mechanical properties of rock masses are highly necessary. However, traditional rock shear testing machines are only suitable for square specimens, suffering from uneven stress distribution caused by a rigid contact between the loading indenter and specimen pads. To address this problem, this study optimizes the performance of an existing laboratory rock shear testing machine. The modification simultaneously enables direct shear testing on cylindrical specimens and incorporates acoustic emission monitoring. It also improves the uneven distribution of normal stress on the rock specimens. This optimization is crucial for expanding specimen specifications, enhancing rock shear testing accuracy and scalability, and maintaining the long-term stability of rock engineering structures.

This study focuses on optimizing the TEST60 rock shear testing machine. The main body of the device comprises an operating box equipped with horizontal load bars on both sides to apply shear loads and a vertical load bar at the top to apply vertical loads. The front end of the horizontal load bar connects to upper and lower pads featuring arc-shaped grooves. These pads are secured via mounting holes and incorporate dedicated holes for acoustic emission sensors. The indenter of the vertical load bar is modified into a spherical shape, and springs are uniformly distributed along its lower edge, connecting to the outer wall of the pressure base to ensure uniform pressure distribution. Using this optimized shear testing machine, shear tests are conducted on cylindrical rock specimens with real-time acoustic emission monitoring. During shear testing, the vertical load rod and spherical indenter are manually lowered. Fine adjustments to the spherical indenter are possible throughout this process to ensure uniform vertical loading.

The shear stress displacement curve exhibits typical prepeak and postpeak mechanical characteristics, effectively illustrating the shear failure mechanism of the specimen. Acoustic emission monitoring reveals minimal activity during the initial loading, followed by a highly active phase after the peak. Combined with the spatial localization of acoustic emissions based on phase segmentation, this clearly reveals the damage evolution mechanism in the specimen, from microcrack initiation to macroscopic fracture. The peak values of the shear stress displacement curve and acoustic emission precisely correlate with the test results, consistent with actual rock mechanical properties. This validates the effectiveness and applicability of the optimized rock shear testing machine. The spherical indenter ensures the uniform distribution of normal stress across the rock specimen, and the concealed acoustic emission sensors enable multimethod collaborative monitoring.

The optimized rock shear testing machine overcomes the limitations of traditional machines, notably their applicability to only square specimens, by incorporating cylindrical pads with holes for acoustic emission sensors, thereby expanding the range of suitable specimen dimensions. The vertical loading head is upgraded to a spherical design, resolving the issues of uneven normal stress distribution and enhancing the accuracy of rock mechanical parameter testing. The optimized direct shear testing apparatus, when combined with techniques such as digital image correlation, enables further investigation into shear failure mechanisms, enabling the execution of a broader range of rock mechanical tests.