Research On Mechanical Response Characteristics And Failure Mechanism Of The Bolt Applied Dynamic And Static Load

The interaction between bolt and surrounding rock is the key problem of roadway surrounding rock control.During the service period of bolt,it is generally subjected to complex dynamic and static loads caused by installation pre-tightening,surrounding rock expansion,sliding and instantaneous failure.Studying the mechanical response characteristics and failure mechanism of underground bolt under static and dynamic loads can provide basic support for deepening the understanding of bolt support mechanism,improving the reliability of bolt support design,and solving the problems of bolt support in complex deformation and rockburst roadway.On the basis of theoretical analysis,this paper takes the self-developed comprehensive test system of bolt mechanical properties as the test platform,simulates the actual working conditions of underground bolt in coal mine,and carries out experimental research on bolt support unit under the conditions of tensile shear,tensile-single impact,tensile-shear-single impact,tensile-multiple impact,tensile-shear-multiple impact and multiple impact with different energy,and obtains the following main research results:(1)According to the statically determinate beam model,the analytical expressions of shear force,axial force and bending moment at the intersection of shear plane and bolt rod and at the plastic hinge point under tension-shear combined load are established,The method for determining the yield mode of the bolt under combined tensile-shear loads is obtained.A dynamic mechanical model of the support unit under impact is established,and it is analyzed that the stress wave can generate a tensile wave on the surface of the surrounding rock,causing tensile failure of the rock,and the analytical expression of the influence of the static load condition on the transmission of stress waves is obtained.(2)The interaction mechanism between the tensile-shear load of the bolt is studied: The results of tension-shear tests show that the initial tension load can improve the initial shear stiffness of bolt and enhance its ability to control the sliding deformation of surrounding rock,However,a high initial tensile load reduce the maximum shear load of the bolt.The results of the tension-shear-tension test show that the shear load borne by the a bolt cause an increase in the axial load but has almost no effect on the yield load,maximum load,and fracture load of the bolt.The influence on its axial deformation capacity is significant,and the axial deformation capacity decreases significantly with the increase of the shear load.(3)Mechanical response characteristics of bolt impact process: the tensile-single impact-tensile tests show that the impact load generated by bolt is much greater than the static breaking load.With the increase of pre-tensile load,the peak value of impact load on bolt increases,the maximum elongation decreases,and the energy absorbed by supporting unit increases at first and then decreases.When the impact energy exceeds a certain threshold,the plastic deformation of the bolt occurs,and the length of the yielding section and the strengthening section decrease at the same time.With the increase of impact energy or times,the length of the yielding section of the bolt gradually decreases until it disappears,and gradually presents a "hardening" characteristic,but the overall deformation after impact is similar,and the maximum load and breaking load have not changed significantly.The tensile-multiple impact-tensile-shear-impact-tensile-tensile test results show that the essence of shearing the bolt under impact is to increase the axial stiffness of the bolt,and the shear load is obviously reduced during the rebound of the bolt,and the reduction is positively related to the initial shear load,which leads to the reduction of its constraint on the shear slip of surrounding rock.(4)The law of energy evolution in the process of bolt impact: Under the action of impact load,the bolts oscillate and elongate before quickly rebounding,accompanied by energy absorption,consumption,and release.During the impact process,the energy absorbed by the bolt through deformation and the energy consumed by plastic deformation are negatively correlated with the initial tensile load.The results of multiple impact and fracture tests on bolts show that as the number of impacts increases,the energy absorbed by the bolts during a single impact response cycle remains relatively stable,while the energy consumed by plastic deformation shows a decreasing trend.Due to the continuous "hardening" of the rod body,the energy released by the rebound of the bolt shows an overall increasing trend.(5)Failure characteristics of bolt : The macro-and micro-fracture analysis results show that the failure form of the rod near the shear plane under the combined tension-shear load changes from oblique extension failure to tension-shear failure with the increase of pre-tension load,and the failure of bolt depends on the final loading conditions;Regardless of whether the bolt is subjected to quasi-static tensile or dynamic impact tensile loads,the fracture position occurs in the threaded section.With the increase of impact times and energy,the hardening characteristics of bolt becomes more obvious,and the probability of brittle fracture increases.(6)Based on the experimental results,it is suggested that in the design of support systems for complex and difficult tunnels,the pre-tension load of the bolt should be designed to be 40%-60% of its yield strength.which can improve the axial stiffness of the support system and the ability to resist the sliding of surrounding rock.On the other hand,it can reduce the fluctuation amplitude of dynamic load of bolt rod and improve the reflection ability of support system to shock wave in the event of impact.