Study on the pre-peak cyclic shear fatigue damage characteristics of 2G-NPR bolted rock joints

The shear support performance of 2G-NPR bolts has been verified; however，their damage behavior under fatigue loading remains to be further explored. Through laboratory pre-peak cyclic loading tests，the shear mechanical properties of PR steel(Q235 steel) and 2G-NPR steel bolted rock joints were compared，exploring the effects of different normal stresses，roughness，shear amplitudes，and rates on the shear fatigue damage characteristics of 2G-NPR steel bolted rock joints. The results show that：(1) As the normal stress increases，the shear strength of PR steel bolted rock joints exhibits a marked decrease after an initial increase in the number of cycles，whereas the shear strength of 2G-NPR steel bolted rock joints gradually increases. This trend underscores the significant role of the negative Poisson's ratio effect in enhancing the shear fatigue performance of the 2G-NPR steel bolts. (2) The peak and residual shear strengths of 2G-NPR steel bolted rock joints significantly increase with higher shear rates and normal stresses，but are less affected by changes in joint surface roughness and shear amplitude. Additionally，the pre-peak shear stiffness of these joints increases with roughness，shear rate，and normal stress，while shear amplitude has only a minor effect. The flexural angle of the anchor rod increases with the roughness，shear amplitude，and shear rate. (3) The ratio of shear force to axial force of 2G-NPR anchor bolts is consistently lower than that of PR anchor bolts under different cyclic loading conditions，indicating that the 2G-NPR bolts can effectively withstand axial force during shear deformation，thus maintaining the stability and support effectiveness of the rock mass. This research offers a valuable reference for the application of 2G-NPR anchor bolts in rock mass support engineering，particularly in regions prone to microseismic activity.