Analysis Method And Application Of Multi-Scale Damage Evolution Of Weld Specimen With Meso-Defects

Welding connection for steel structure is common and with many advantages. However the internal meso-defects within weld components are inevitable. It can cause stress concentration and result in deterioration of components and whole structure, even failure. Such damage evolution in the weld components steps across meso-scale and macro-scale. In order to track the influence of the meso-defects on multi-scale evolution of damage, it is necessary to investigate the relationship between damage scalars of different scales in weld components, and establish an approach to describe damage and a method to analyze the damage evolution of weld components with meso-defects.This thesis conducts experimental research on weld specimen with tensile tests to capture the mechanical response and damage evolution. The mechanism and features of the evolution of meso-defects during plastic deformation is analyzed to reveal the evolution law of defects with crack type and void type and the relationship between damage evolution and property deterioration of the specimen. A method to describe the multi-scale damage with fractal variables is established for both defect types based on the observation of distribution of defects. Moreover this work can be applied to the analysis of steel truss in engineering. The main achievements accomplished in this thesis are summarized as:1. The AE (Acoustic Emission) events of metal specimens in tensile tests are tracked. The features of AE signal in different deformation stages are analyzed to investigate the evolution mechanism of nucleation, growth and aggregation of micro voids, as well as the feature of damage evolution in elastic stage, plastic stage, hardening stage until failure.2. X-CT (X-ray Computed Tomography) and electrical measurement are adopted to capture evolution of meso-defects and macro mechanical response, the evolution law of the macro mechanical response with plastic deformation is studied. It is observed that crack type and void type are two major types of defects, and are randomly distributed. The evolution of these defects with plastic deformation is studied, the relationship between the macro mechanical property and meso-defects is revealed.3. According to the geometry and mechanical feature of defects in the weld specimen, the geometry of defects within the RVE (Representative Volume Element) has effect on the damage evolution. GTN model is applied to analyze such effect with finite element method.4. A method to describe the damage in case that crack type defect is in dominate is proposed. According to the observation of crack propagation in weld zone, the propagation of meso-cracks show feature of fractal. Therefore the evolution of fractal dimension of meso-cracks and total crack length are taken into account to describe the meso-cracks propagation in a multi-scale approach with fractal damage variable, which is verified by multiple specimen tests. The coefficient to determine the relationship between crack length and fractal dimension with plastic deformation is obtained. The effect of meso-cracks geometry on damage evolution is studied as well as the damage evolution with plastic deformation.5. A method to describe the damage in case that void type defect is in dominate is proposed. The quantity and volume of meso-voids also show feature of fractal. The evolution of fractal dimension of meso-voids and quantity and volume of meso-voids are studied to establish the description of damage, which are also verified with multiple tests. The coefficient to determine the relationship between damage and fractal dimension as well as plastic deformation is obtained.6. The multi-scale analysis method on weld component with meso-defects are applied to steel structure in engineering. According to the geometry of defects and deformation of components, the damage degree and its evolution can be quantified. Results show that the method to describe and analyze damage in this thesis can achieve an accurate measurement of damage evolution on both micro- and macro- scale. Meanwhile the effect of geometry of meso-defects on macro mechanical property can be considered.