Multi-scale Analysis On Concrete Material And Structure Failure Due To Meso-cracks’ Growing

Concrete is a composite material composed of mortar, aggregate and interface, in macro-scale it can be regarded as continuous homogeneous solid, but in meso-scale it contains a lot of meso-cracks which can be regarded as initial damage. The root cause of concrete failure is damage initiation and evolution of some material part, the failure of concrete material or structure is a trans-scale damage evolution process leaded by meso-cracks. So it is necessary to select appropriate meso numerical analysis method based on concrete’s meso-scale structure and failure mechanism, analyze the concrete material failure caused by trans-scale damage evolution in meso-scale, discuss its effect on concrete member’s mechanical property, then explore the process of concrete structure failure with multi-scale analysis.Based on the damage and failure mechanism of concrete material, this thesis idealized concrete’s meso-scale structure and selected meso-crack model as numerical research tool, simulated and analyzed the failure process caused by trans-scale damage evolution in concrete material, member and structure level, then discussed the impact on the macro and meso-scale mechanical properties in different levels. The main research activities and achievements are listed as follow:Combining the theory of damage mechanics and fracture mechanics, the characteristics of concrete damage and failure under different scales were analyzed. By comparing of existing concrete meso-scale numerical analysis methods, the meso-crack model based on concrete failure mechanism was selected as this thesis’ numerical research tool. With meso-crack model, concrete material failure caused by trans-scale damage evolution leaded by meso-cracks was firstly analyzed. Based on the idealization of concrete material’s physical model and the selection of model parameters, established the finite element model and simulated the process of concrete material under tension. Then discussed the concrete material’s macro and meso mechanical properties and the characteristic of trans-scale damage evolution with macro and meso damage analysis method. The results show that the damage evolution process and the stress-strain curve of the sample based on the meso-crack model agree with the experiment test. The damage evolution curves based on macro or meso damage analysis method both can reflect the cross scales characteristic, and with the effective bearing area can establish relationship between macro and meso damage variable. Trans-scale damage evolution will affect mechanical properties of concrete material, with the increase of initial damage, concrete material ductility increases, intensity decreases, the failure mode is more complicated.The concrete specimen’s failure caused by trans-scale damage evolution then was calculated and analyzed. With meso-crack model, the failure of specimens with or without precast crack under static load were simulated and optimized the meso numerical analysis method. By numerical experiment of concrete specimens, size effect on concrete was discussed from the angle of trans-scale damage evolution. Based on the fatigue failure mechanism of concrete, improved the meso-crack model and simulated the failure process of specimen with few macro-cracks or lot meso-cracks under fatigue load. By numerical experiment of concrete specimens, fatigue damage and fatigue life were discussed from the angle of trans-scale damage evolution. The results show that the failure of concrete specimen is a trans-scale damage evolution process, meso-cracks at low stress area can be ignored and use multi-scale modeling method. The trans-scale nonlinear damage evolution process may be a mechanism of concrete size effect, with the increase of concrete specimen’size,the fracture energy increase, but the nominal strength and data discrete decreases. With simplified Paris’law, successfully simulated the fatigue failure caused by trans-scale damage evolution. Damage evolution of concrete specimen under different fatigue stress level exist distinction, fatigue damage and fatigue life can be discussed based on the description of stiffness degradation.Bases on the analysis of concrete material and member, the concrete structure’s trans-scale damage evolution process was discussed by multi-scale method. After comparing the current material and structure multi-scale problems, established the method of structure multi-scale analysis based on the homogenization theory and meso-crack model. Chosen concrete gravity dam as the research object and simulated its failure under different working condition, then analyzed its trans-scale damage evolution process. The results show that appropriate cell can introduce macro and meso scale at the same time, initial macro-cracks can be released based on the initial damage analysis in dam, then numeral simulation of damage evolution can be done. Under different working conditions, the dam’s trans-scale damage evolution process and final failure mode exist distinction, material failure is the root cause of member or structure’s fracture.In conclusion, this thesis explored the concrete’s failure caused by meso-cracks’extension based on meso numeral analyze method, discussed the damage and fracture in different structure levels with multi-scale analysis. This method is based on concrete’s meso structure characteristic and failure mechanism, can reveal the material failure process caused by damage evolution leaded by meso-cracks, can apply to concrete member’s analysis and explore its size effect and fatigue fracture from the angle of damage evolution, can apply to concrete structure’s multi-scale analysis based on homogenization theory and guide the structural design.