Experimental Study on Fatigue Behavior of Quasi-NPR Steel Bolted Rock Joints under Prepeak Cyclic Shear Loading

The primary objective of this work is to investigate the shear fatigue of rock joints reinforced by negative Poisson’s ratio (NPR) bolts. First, the prepeak cyclic shear tests are conducted on both quasi-NPR steel and conventional steel bolted joints. The results show the quasi-NPR bolted joints exhibited a 0.9 MPa increase in shear strength and a 4.2% reduction in bolt plastic deformation, demonstrating the fatigue resistance of the quasi-NPR bolt. Furthermore, the effects of joint dip angle, normal stress, cyclic number, shear rate, and shear amplitude on fatigue resistance of the quasi-NPR bolted joints are explored. Increasing the joint dip angle, normal stress, and cycle number enhances fatigue resistance, while higher shear rates and shear amplitudes weaken it. Finally, the microscopic analysis reveals that prepeak cyclic loading activates more coherent twinning in quasi-NPR steel, thereby enhancing the negative Poisson's ratio effect and reducing fatigue damage in rock joints. The excellent fatigue resistance of quasi-NPR steel highlights its promising application prospects in the seismic design of engineering rock masses.