Fracture Prediction and Analysis of Q690D High Strength Steel Based on Cyclic Void Growth Model

High strength steel has been gradually applied in practical steel structures. The ultra-low cycle fatigue fracture analysis of steel is the basis to evaluate the fracture failure of high strength steel structures under strong earthquake. The traditional fracture mechanics method assumes that the crack already exists, and there is a high strain constraint at the initial crack tip, so it is mainly suitable for the study of brittle fracture problems with extremely limited plastic deformation, but it is not suitable for the ductile fracture problems with no macroscopic initial defects and significant plastic deformation under ultra-low cyclic loading. The ultra-low cycle fatigue fracture model based on void growth mechanism has been gradually applied in the ductile fracture analysis of steel.
In order to study whether the cyclic void growth model (CVGM) is suitable for predicting the fracture failure of domestic Q690D high strength structural steel, 17 rod specimens and 3 dog-bone weakened plate specimens (DB) were designed and processed. The MTS axial servo fatigue test system was used to carry out the fracture test of round bar specimens under monotonic loading and ultra-low cycle loading. The stress-strain relationship, basic mechanical parameters and load-displacement curve of Q690D steel were obtained. The influence of loading system on the bearing capacity and deformation capacity of specimens was analyzed.
The experimental results show that:1) Q690D steel has no obvious yield platform, and there are obvious notch strengthening effect and cyclic softening phenomenon. Due to the damage of material during cyclic loading, the fracture displacement of cyclic loading is less than that of monotonic loading. 2) By analyzing the fracture morphology of the specimen, it is found that the cracks all start at the center of the minimum section of the specimen. The micro morphology of the specimen fracture is observed by scanning electron microscope. The fracture presents the ductile fracture characteristics of dimple, which conforms to the mechanism of void growth fracture mechanics. At the same time, the characteristic length of Q690D steel is about 0. 3 mm by scanning electron microscope.
The finite element model of Q690D high strength steel was established by using ABAQUS software. Based on the test results of Q690D rod specimen and combined with finite element analysis, the parameters of CVGM and DSPS of Q690D high strength steel were calibrated. Under monotonic loading, CVGM and DSPS degenerate into the void growth model (VGM) and the stress modified critical strain model (SMCS). Finally, with the help of user subroutine USDFLD in ABAQUS software, the CVGM model is programmed with Fortran language. In the process of numerical calculation, when the material element meets the requirements of failure criteria, the element failure is determined, the stress of the element is released, and then the finite element calculation is continued according to the new state until the specimen failure. The cyclic void growth model of steel is used to predict the fracture failure of dog-bone weakened plate specimen under different loading systems. The crack initiation position, load-displacement curve and fracture displacement of the specimen are in good agreement with the experimental results, and the prediction errors of ultimate load and fracture displacement are within 2% and 12% respectively.