Biaxial compression mechanical properties of NPR anchor solid under different crack dip angles

With the rapid development of deep resource extraction and underground space construction, the design of anchored support systems for jointed rock masses in complex stress environments faces significant challenges. This study investigates the influence of prefabricated crack dip angles on the mechanical properties of anchored rock masses in deep soft rock roadways. By constructing similarity models of NPR (Negative Poisson's Ratio) and PR (Positive Poisson's Ratio) anchored solids, biaxial compression experiments under varying crack dip angles were conducted. Strain gauges, 3D Digital Image Correlation (3D DIC), and acoustic emission monitoring were employed to systematically analyze the strength characteristics, deformation-damage evolution, and energy dissipation mechanisms of the two types of anchor systems. The results show that: (1) The stress-strain curves of anchored solids with prefabricated cracks exhibit a distinct bimodal characteristic. Compared to PR anchors, NPR anchors show 20% and 23% improvements in peak strength and elastic modulus, respectively, with residual strength enhanced by up to 34%. (2) Owing to high pre-tightening force and large deformation capacity, NPR anchors maintain superior integrity under increasing crack dip angles, demonstrating more uniform free-surface displacement and localized shear-tensile composite crack patterns. (3) Acoustic emission analysis reveals that NPR anchors exhibit higher cumulative energy absorption (300% improvement over PR anchors) and lack low-rate energy development phases, indicating enhanced ductility and impact resistance at high crack dip angles. (4) Crack dip angle critically governs failure mechanisms by modulating the connectivity between shear cracks and prefabricated fissures: bimodal effects dominate at low angles, while vertical tensile crack propagation replaces bimodal behavior at high angles. The study proposes prioritizing NPR anchor cables in deep engineering applications and optimizing support parameters based on crack dip angles to mitigate stress concentration and ensure the long-term stability of surrounding rock.