Study On Fracture Properties Of 8.8S Grade High Strength Bolts

In the steel structure building,welded connection and bolted connection are two important connection forms.Bolted connections are used in large numbers in engineering practice because of their ease of installation and the ability to be disassembled at will,so the use of high-strength bolts has also begun to spread.Building steel structures can suffer local damage due to incidental loads,and this damage can propagate in all directions along new transfer paths due to changes in the structure’s own transfer paths,leading to local damage or overall collapse of the building over larger areas.In such large continuous collapses,the bolts as the main connection often pulled or sheared off,fires may occur in the continuous collapse of steel structures,and bolts may also be pulled off at elevated temperatures.Therefore,the study of the collapse performance of steel structures requires the study of the strength and fracture properties of bolts at different temperatures.Currently,The fracture properties of structural steel in construction have been studied by many scholars,and fracture prediction models suitable for the corresponding steels have been proposed,but research on the fracture properties of bolts is lacking,and there is a lack of corresponding fracture models to use.Among many high-strength bolts,8.8S high-strength bolts are widely used in actual steel buildings,so it is necessary to study the fracture performance of 8.8S high-strength bolts at different temperatures.In this paper,the corresponding strain-strengthening model is proposed,and the VGM model,KM model and Lou M model are calibrated and validated by studying the fracture performance of 8.8S high-strength bolts at room temperature;The Johnson-Cook constitutive model and the Johnson-Cook fracture model are modified by studying the fracture performance of 8.8S high-strength bolts at elevated temperatures,respectively.The corresponding modified models were proposed,and the finite element results were used to calibrate and verify the Johnson-Cook fracture model.The main research process and conclusions of this paper included the following points.(1)In this paper,five specimens,including round bar tensile specimen,flat plate shear specimen,flat plate notch tensile specimen,cylindrical shear specimen and cylindrical pure compression specimen,were designed to study the fracture properties of8.8S high-strength bolts at room temperature,and the five specimens corresponded to the stress state under different stress triaxiality and Lode parameters;By loading all specimens,the corresponding load displacement curves and smooth round bar specimens’ engineering stress-strain curves are obtained.Analysis of the loading results of the round bar specimens shows that the notch radius of the round bar specimens has significant influence on the mechanical properties of the specimens,and the smaller the notch radius is,the fracture displacement and ductility of the round bar specimens will decrease sharply.(2)Based on the Swift strain hardening model and Voce strain hardening model,a hybrid Swift-Voce strain hardening model was proposed in this paper,and all specimens were subjected to finite element analysis using the strain hardening model;the corresponding fracture parameters were extracted from the critical cells of the completed finite element model of the simulation,and three fracture models,VGM fracture,KM model and Lou M model,were calibrated,and the finite element model was finally used for verification.(3)Through the finite element analysis and verification of the normal temperature specimens,it can be found that the Lode parameter and the stress triaxiality are important parameters that influence the fracture performance of the material,and the introduction of the Lode parameter term in the fracture model can greatly improve the prediction accuracy and applicability of the fracture model;For the round bar specimens,when the notch radius is large,there is also good accuracy in the fracture prediction of the specimens using the VGM model.(4)In order to study the elevated temperature fracture performance of 8.8S grade high-strength bolts,three round bar specimens were loaded at five temperatures of 20°C,200°C,400°C,600°C and 800°C to obtain load displacement curves and engineering stress-strain curves in this paper.It can be found that the ultimate strength and 0.2%plastic strain strength of 8.8S high-strength bolts first slowly increase with the rise of temperature and then sharply decrease,while the modulus of elasticity slowly decreases with the rise of temperature and then sharply decrease.(5)The simulation results validate the correctness of the modified Johnson-Cook constitutive model by performing finite element simulations of the round bar specimens using the modified Johnson-Cook constitutive model;the fracture parameters at the critical unit were extracted using the simulated finite element model,and then the modified Johnson-Cook fracture model was calibrated,and finally fracture prediction was performed for all specimens using the modified Johnson-Cook fracture model.By analyzing the fracture prediction results,it can be found that the fracture failure prediction accuracy of the modified Johnson-Cook fracture failure model is higher for smooth round bar specimens than for notched round bar specimens;the Johnson-Cook constitutive model and the fracture model cannot be applied to all materials,and their applicability can be expanded by modifying the two models.