| In recent years,the fracture of steel structure has attracted much attention with the rapid development of the construction industry.HRB400 hot rolled steel bar is the mainstream steel in China's construction industry at the present stage,and it is of a great engineering significance to study its fracture mechanics behavior.For a long time,the fracture study on ductile material mainly focuses on the high triaxial stress,while the study on the fracture problem under the lower triaxial stress is still controversial.Therefore,in this paper,a series of specimens with different stress states were processed by HRB400 steel,and the failure test of these specimens was carried out using the MTS testing machine.Combined with numerical simulation software,the relationship between failure strain and stress state of HRB400 steel under low triaxial stress was studied:(1)The elasto-plastic parameters of the material were obtained by the single-pull test of the smooth solid round bar and the pure torsion test of the hollow thin-walled round pipe.Then the hardening curve of HRB400 steel under uniaxial tension and pure torsion was calibrated by numerical simulation,and the differences of them were analyzed.(2)In order to study the damage rule of the material under the lower stress triaxiality,the center hole plate specimen,the different form of the shear plate specimen and the hollow notched pipe specimen were used for the deformation failure test.The crack initiation time and starting position of each specimen was measured.According to the pre-fractured deformation of each sample,their average fracture strains were estimated.It was found that the average equivalent fracture strain of HRB400 steel under tension-shear mixing is much lower than that of pure shear.(3)The local deformation and crack initiation position of each sample were observed under an three-dimensional microscope,and the stress state of the starting point of each sample was analyzed by numerical simulation.The results show that the sample of the center orifice plate and the shear plate have the characteristics of tensile stress,besides,the shear plates accompanied by shear characteristics.There is a obvious pure shear property of the crack position of the hollow notch tube specimen.(4)A11 experiments were simulated with numerical calculation software,with the rationality of the simulation is ensured,the stress and strain information at the crack point of each sample are extracted for analysis.The results show that the fracture strain of HRB400 material is large when the stress triaxial degree is 0,and between 0.4?0.6 in triaxial stress,the fracture strain along with the change of Stress triaxiality is the trend of decreasing first and then increasing.(5)The relationship between fracture strain and Lode parameter was established.The results show that the change of the fracture strain of HRB400 material with the Lode parameter in-1?0 is the law of decreasing first and then increasing,which means that the Lode parameter has important influence on the fracture of material.(6)In this paper,a fitting J-C fracture criterion is used to simulate the crack initiation process of each sample.The results show that the J-C model can be used to predict the crack points of most samples.However,there is a deviation between the simulation and the test of individual samples,which indicates that there are some limitations of the fracture is controlled only by the J-C model. |
PDF Full Text Request