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 k N)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.

Experimental behavior and fracture prediction of a novel high-strength and high-toughness steel subjected to tension and shear loading tests

In this study,laboratory testing and numerical simulation methods are used to investigate the mechanical behavior and perform fracture prediction of a novel high-strength and high-toughness steel with a negative Poisson's ratio(NPR)effect under combined tensile-shear loading conditions.A test device capable of meeting different tensile-shear combination test angles is designed and manufactured,wherein the mechanical experiments on the NPR(Negative Poisson's Ratio)steel specimens are carried out at various testing angles.Q235 steel and MG400 steel are used as experimental control groups.The results show that the mechanical deformation of NPR steel is significantly better than that of Q235 steel and MG400 steel.Its tensile-shear test curve has no yield plateau and it has quasi-ideal elastic-plastic mechanical properties.The loading direction gradually changes from tension-dominated to shear-dominated as the tension-shear angle increases,and the strength and deformation of the specimens show a decreasing trend.Based on the laboratory test results,a finite element numerical model of NPR steel is established.A series of numerical simulations are carried out under the conditions of different tension and shear angles and the average stress triaxiality and fracture strain data are obtained.The fracture data of NPR steel are fitted using the Johnson-Cook fracture criterion,and the Johnson-Cook fracture parameters under the tensile-shear test conditions of NPR steel are thus obtained.The numerical simulation verifies that the fracture model can accurately predict the tensile-shear fracture behavior of NPR steel.

Influence of notch volume on mechanical properties of a novel high-strength and high-toughness steel

The effect of notch volume on the mechanical behavior of a novel high-strength and high-toughness steel with negative Poisson’s ratio effect(called NPR steel)was studied.First,the quasi-static tensile test of NPR steel with different notch volumes was carried out,and its failure characteristics and mechanical properties parameters were studied.Then,the modified Johnson-Cook(J-C)constitutive model with coupled notch volume ratio was proposed.The model was validated based on three-dimensional finite element numerical simulations.The results show that the engineering stress–strain curve of NPR steel has no yield platform,and has the mechanical properties of high strength,high elongation,and high energy absorption.Notch volume significantly affects the mechanical properties of NPR steel.The elongation,yield strength,tensile strength,and energy absorption characteristics of steel bar gradually decrease with the increase in notch volume.The notch volume ratio V^{*},a characteristic parameter describing the notch volume of reinforcement,is defined,and the quantitative relationship between this parameter and mechanical parameters is established,which can accurately characterize the mechanical properties of specimens with different notch volume ratios.Based on the true stress–strain curves with different notch volume ratios,a constitutive model with modified Johnson-Cook model parameters is proposed.The finite element results show that the modified J-C model can accurately fit the quasi-static tensile mechanical behavior of NPR steel.