| Damage evolution of welding structure often initiates from the mesoscale congenital defects of welded details, and these defects gradually develop into maroscale in service after evolution and finally lead to component or structure failure. In evolution process, the macroscopic mechanical properties of component or structure have a continuous deterioration, and the intrinsic physical mechanism of which is the evolution of initial defects of materials. So far, on one hand, the characterization methods of damage evolution mostly stay on the single scale, meso-scale or macro-scale, and lack specific characterization method of damage evolution by virtue of a poor understanding of the intrinsic physical mechanism of component or structure. On the other hand, the influence of defects on overall configuration is neglected in many damage numerical analyses. Therefore, firstly, from the physical mechanism of metal component or structure damage, the method of combining macro and meso is used to established characterization of damage, the damage evolution equation and the constitutive relation are analyzed based on the comparative analyses of numerical results and test results. Then, the finite element model included meso-defects after the refactoring of meso-defects, and after analyzing the numerical results of welded steel components, the rules and meso physical characteristics of the damage evolution of welded steel components with defects are discussed, as well as the influence of initial defects on the evolution of the structural partial details. Specifically, following are the main work and the results of the study accomplished in this paper.First of all, in order to describe the damage behavior of meso-crack stage, fractal damage variable is established based on the fractal feature of meso-crack propagation. Through the compression of uniaxial tensile numerical simulation and experiments, it is found that damage distribution and fracture morphology of simulation are consistent with the experiment results, and the characterization results of the fractal damage variable have a good agreement with the results of the traditional macroscopic representation, which indicate that it is reasonable and correct of using fractal dimension to describe the damage evolution caused by meso-crack propagation. Second, due to the meso-crack initiation begins in the meso-crack band formed by meso-pores aggregation and in order to describe the entire damage evolution process dominated by meso-pores and meso-cracks, the way of porosity defined in GTN model is used to describe damage behavior of meso-pores stage. It is marked as the dominant form of the meso-crack when the damage value reaches the meso-pore critical aggregate volume fraction. The fractal dimension in meso-crack propagation process will be further used to describe the damage behavior in the meso-cracks stage.In order to investigate the influence of the initial defects of meso-pores and meso-cracks on the damage evolution of the welded joint during the loading process, fractal damage variable and volume-fractal damage variable are embedded in constitutive equation of the material in the form of strain equivalent, and the finite element model is established according to the existing test scheme. The results of the damage evolution simulation show that when the initial defects in the weld are meso-pores, the damage evolution has experienced the stages of meso-pores dominant and meso-cracks dominant; When the cumulative plastic strain reaches 0.018, the damage in the weld is dominated by the meso-cracks, and the damage evolution rate of meso-pores stage is much smaller than that of the meso-cracks stage. While in the situation of the initial defects are different, the results shows that meso-pores will weaken the subsequent evolution rate of mesoscopic crack. Hence, the physical form of defects has an effect on the rate of damage evolution in the late stage.Moreover, a typical truss structure finite element model of large span bridge steel box girder structure by considering weld joints is established. The numerical simulation of steel truss under cyclic loading of displacement and force control is carried out to analyze damage distribution and damage evolution of overall structure and partial material after considering weld joints and material defects. The results indicate that dangerous area of the structure is the place where bottom chord joints and inclined web are welded together, and the larger damage value appears on the connection surface between the welds and joints, and then it can be determined that the failure path of the structure is along the welds and joints, which is extended from one end to the other end. So, it can realize the content of the initial defects of the setting by controlling the initial damage value. When the initial defects are all meso-pores, the content of the defects has no significant effect on the evolution rate of the different damage dominated stages. The larger initial defect content can enhance the distribution of meso-cracks in the weld at the late stage of damage evolution, and the partial failure path of the structure is diversified. Compared with the weld defects, the initial defects in the base metal are not obvious to the overall mechanical properties in the later, so that it can be ignored in the calculation of the actual engineering application.In summary, the damage characterization method established in this paper is based on the physical mechanism, the damage evolution mechanism and laws are described according to the material congenital defects, and a distinct physical meaning is given to the damage characterization of the structure. The results of numerical simulations show that the evolution of initial defects of partial material undergo cross scales and cross physical form under the complicated stress level and ultimately leads to the failure of the whole structure. The works in this paper provide a new method and idea for the numerical analysis of damage evolution from material to structure and meso-scale to maro-scale. |
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