Fatigue deformation and damage characteristics of bolting system under stress-controlled cyclic pullout

Abstract

This study explored the fatigue mechanical properties and acoustic emission (AE) characteristics of bolting system under cyclic pullout tests controlled by different upper stress levels. A novel compression-pull load conversion device was designed to precisely apply cyclic pullout load on bolting system. Strain gauges and AE sensors were employed to capture the surface strain variations of rock bolts and microfracture development during static and cyclic testing. The fatigue strength, deformation, axial/shear stress distributions and AE characteristics of the bolting system were comprehensively investigated. Fatigue threshold, an important value determining the service life of a bolting system, was supposed to be lower than 85% of the static strength in this study. The bolt residual slip strain curve was S-shaped and comprised three stages: the initial stage, constant-velocity stage and acceleration stage. The constant-velocity stage accounted for most of the fatigue life and was the main stage of fatigue development. The fatigue failure of the anchorage system was presumably controlled by the entire static stress–strain process, and the residual strain at fatigue failure corresponded to the peak strain at the peak stress under static loading. The anchorage fatigue damage development completely coincided with the residual strain evolution, and the turning points were identical between the different deformation stages. Based on the residual slip strain development, nonlinear inverted S-shaped cumulative fatigue damage models were proposed for different upper stress levels. A comparison of the results from different samples revealed that the damage evolution was highly dependent on the upper stress level. The fatigue life of the bolting system decreased as the upper stress level increased due to the increased steepness of the second stage and the decreased proportion of the second stage to the entire fatigue process.