Deformation-softening behaviors of high-strength and high-toughness steels used for rock bolts

In deep ground engineering, the use of high-strength and high-toughness steels for rock bolt can significantly improve its energy absorption capacity. However, the mechanisms and effects of rock loading conditions on this kind of high energy-absorbing steel for rock bolt remain immature. In this study, taking Muzhailing highway tunnel as the background, physically based crystal plasticity simulations were performed to understand the effect of rock loading rate and pretension on the deformation behaviors of twinning induced plasticity (TWIP) steel used for rock bolt. The material physical connecting to the underlying microscopic mechanisms of dislocation glide and deformation twinning were incorporated in numerical modeling. The rock loading conditions were mimicked by the real-time field monitoring data of the NPR bolt/cable equipment installed on the tunnel surrounding rock surface. The results indicate that the bolt rod exhibits pronounced deformation-softening behavior with decrease of the loading rate. There is also a sound deformation-relaxation phenomenon induced by the dramatic decrease of loading rate after pre-tensioning. The high pretension (>600 MPa or 224 kN) can help bolt rod steel resist deformation-softening behavior, especially at low loading rate (<10−1 MPa/s or 10−2 kN/s). The loading rate was found to be a significant factor affecting deformation-softening behavior while the pretension was found to be the major parameter accounting for the deformation-relaxation scenario. The results provide the theoretical basis and technical support for practical applications.