| Due to its advantages of economical and convenient for construction, concrete is widely used in bridge and tunnel engineering. In the bridge made of concrete, cracks occurrence, expansion of the crack width over the limit, and excessive deflection of beam can be regarded as results caused by unstability of the deformation of concrete, which can be analyzed from meso-scopic view. Concrete is a kind of multiphase composite material which is composed of cement mortar, aggregate, pore and fissure, and its composition determines the heterogeneity in the structure and the discontinuity in physical and mechanical. The failure of concrete is a process in which the internal damage accumulation results in deformation localization, and the deformation localization evolves into macro cracks. The deformation localization zone will directly lead to the decrease of strength of concrete material as a whole, and the falling of bearing capacity. It is very important for macro and meso mechanics of concrete to research on damage evolution, onset, development and description of deformation localization zone. This study will provide a scientific basis for improving the mechanical properties of concrete and the development of high performance concrete materials.It can help to reveal the nonlinear characteristics of concrete mechanics and mechanics characteristics before and after failure Macro from macro and meso-scopic mechanics experiment, and provide a deeper understanding nonlinear nature, damage and instability mechanism of the concrete.In the lab, through a variety of test instrument to simulate different stress state, the the formation mechanism of the deformation localization in concrete is studied. With the help of measuring instrument, the damage and deformation localization are measured, and the geometry of them is described in quantitative. Photoelastic coating method, laser speckle method and digital image correlation method and so on were used to investigate damage and deformation localization in the concrete. These methods are limited to research on the surface of specimens, and can’t catch on their internal deformation characteristics. Heterogeneity of concrete material determines that these methods can not fully reflect the characteristics of the localized deformation. Acoustic emission(AE) is an indirect method of dynamic observation. Ulatrasonic will occur during the propagation of crack. AE is an effective tool to be applied on the cracks in the concrete. However, the biggest flaw of AE is its difficulty on quatification of mesoscopic damage. In X-ray Computed Tomography(CT) technique, the cross section images of a specimen are obtained by using X-ray CT scanning. The gray value in CT image is a function of physical density. Based on CT image, internal damage evolution, crack initiation, extension, and through the whole process can be observed. In these researches, crack or porosity is extracted by using image segmentation method, and statistics are analysed just on several slices, which cannot fully reflect the cracks distribution in space. Due to the limitation of the resolution, only cracks over CT resolution can be obtained, and the onset and development of the deformation localization before the formation of the cracks over CT resolution CT. By using CT scanning, three-dimensional structure of the concrete can be obtained undamaged. How to represent damage and fracture in the concrete, and reveal the process of damge evolution and deformation localization development, are the problems to be solved. It is helpful to the understanding of the mesoscopic damage mechanism of concrete.By using mesoscopic mechanics, damage mechanics, experimental mechanics and numerical simulation, this research focus on the identifications of damage and deformation localization. Combined high precision industrial computed tomography(CT), stereology principle, digital image processing and digital volumetric speckle photography, the quantitative characterization and visualization methods of damage accumulation and evolution, deformation localization in the concrete during uniaxial compression are researched. In this dissertation, the main research work and achievements are as follows:(1)Combined industrial computed tomography(CT) and special design loading device, concrete specimens under uniaxial compression were scanned in situ. The whole process of internal crack initiation, extension and coalescence were obtained. Fractures mainly develop along the interface between aggregate and cement paste, and when the angle between the long axis direction of the aggregate and loading direction is too large, the crack will be through the aggregate and development. The original large size fracture and pore does not collapse before the specimen is broken. These fractures and pores tend to become channels of crack extension;(2) Under different stress states, the difference of the mean gray value of different layers and variance of CT images reflects the inhomogeneity in the specimen. With the load increase, the specimen volume change caused the change of the gray value, the mean gray value of overall image has experienced a process of first increases and then decrease, while the gray variance increased after decreased first. The relationships between the changes and specimen deformation are nonlinear. From the crack percentage distribution, the change of micro crack in the interface between aggregate and cement are appearance; statistical analysis of image gray value can indicate the variant rule of crack under CT resolution;(3) Through image segmentation, internal fracture three-dimensional images in the different stages were extracted. The porosity and fracture three-dimensional fractal box-counting dimension were calculated and their changes with deformation are consistent. The box-counting dimension is a comprehensive reflection of the number and size of fractures. Three dimensional crack analysis method is more suitable for larger than CT scale analysis;(4) By using stereology method, fabric surface area tensor Sij, mean solid path tensor λij and porosity tensor Nij were constructed, and the second invariants of these tensors were used to describe the distribution uniformity of pores and cracks in the concrete specimen. The corresponding damage parameters the specific damaged surface area, the mean solid path and the mean porosity can reveal damage evolution in the concrete specimen. In the compaction stage, specific damaged surface area Sv and the mean porosity n0 are less, specimen become dense, the mean solid path increase. When the loading reach a certain stage, the occurrence of the new micro cracks result in the growing trend of Sv and n0, and falling of the mean solid path λ. The heterogeneity of the spatial distribution of cracks and solid paths is in a growing trend. Due to opening and closing of the microcracks, the second invariants of these tensors fluctuate;(5) Combined digital volumetric speckle photography(DVSP) with CT images, the interior displacement and strain distribution of the concrete specimen were calculated. 3D visualization of the strain fields was realized. With reference to the strain distribution strain localization area generation and development process caused by microcracks can be directly revealed. According to the volume strain curve, the onset points of cracks under CT resolution and over CT resolution were defined, which are 54.7% and 82.0% of peak stress, respectively.Based on the strain fields, damage strain threshold is defined, and the zone with its strain value greater than the threshold is defined as damage zone. A damage factor by using second moment of the damage zone position is proposed to describe damage evolution law.(6) Based on the high accuracy CT image of the concrete, aggregate, cement paste and pore and interface were extracted, respectively. By using MIMICS software, the three dimensional numerical model of concrete specimen reflecting the mesoscopic structure was built, and then was import ANSYS to simulate uniaxial compression. Compared with strain results from DVSP, the model was verified;(7) The resolution of CT system will influence on the CT image analysis. The scale of the different analysis method has its applicability. CT image grey value can be used to analyze the general rule of micro cracks in the voxel. The image segmentation method is more suitable for the crack greater than CT resolution. Those two methods is more suitable for the description of the whole deformation law of the specimen; With reference to the strain distribution from DVSP technique, strain localization area generation and development process caused by microcracks can be directly revealed.The innovation points are as follows:(1)Combined industrial computed tomography(CT), the volumetric images of concrete specimens under uniaxial compression were scanned and obtained in situ. By using stereology method, fabric tensors and the corresponding damage variants are proposed to describe the distribution uniformity of pores and cracks in the concrete specimen.(2)Combined digital volumetric speckle photography(DVSP) with CT images, the interior displacement and strain distribution of the concrete specimen were calculated. A damage factor based on strain fields is proposed to describe damage evolution law.(3)Based on the high accuracy CT image of the concrete, the three dimensional numerical model of concrete specimen reflecting the mesoscopic structure was built, and uniaxial compression of specimen was simulated. The results display the evolution of internal deformation and damage of specimen.The purpouse of this dissertation is to provide a noval and effective method for the quantitative analysis of the evolutions of meso damage and deformation localization in the concrete. |
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