Physical And Mechanical Approaches To Three-dimensional Visualization Of The Stress Field In Complex Porous Structures

Quantification and visualization of the three-dimensional(3D)whole-field stress distribution in complex porous geomaterials are significant for solving various engineering problems in which stress governs the deformation,fracture propagation,and fluid transport inside the materials.For instance,knowledge of the stress field in the 3D complex pore structures of natural rock masses has a great influence on the distribution of the mining-induced stress,pore pressure in the oil exploitation,permeability in shale gas development etc.;in geotechnical engineering,the long-term security of underground tunnels and caverns is intimately linked to the damage zones caused by the concentrated stress in the surrounding rocks.In essentially,the structures and whole-field stress inside the porous geomaterials governs the mechanical responses when the materials are under external loads.However,the complexity and invisibility of the complex porous structures and stress field make it extremely difficult to accurately describe and quantitatively characterize the hidden 3D porous structures and whole-field stress.Various techniques,including theoretical and numerical calculations and in-situ and laboratory tests,have been applied to quantitatively evaluate the stress distribution inside 3D porous structure geomaterials.Unfortunately,no approach based on current theory provided the ability to precisely calculate the whole-field stress distribution in a 3D complex structure characterized by abundant and geometrically irregular pores.In-situ monitoring methods that are based on a limited number of measurement points are not capable of determining the continuous evolution of the whole-field stress in rock strata induced by resource exploration or tunnel construction activities.Rock mechanical property tests can be used to evaluate the stress field inside geomaterials from the mechanical responses of the tested specimens instead of direct observations.However,the measurement results do not present the whole-field stress distribution inside the 3D test models but only indicate the deformation and failure information of the local zones.Because of the limitations of these methods in measuring and visualizing stress fields,numerical simulation methods have become a powerful tool to visualize and characterize 3D stress field distribution inside complex pore geomaterials.However,the challenges involved in choosing the physical and mechanical parameters of materials,the mesh generation method,the definition of calculation element,the selection of the constitutive relationship etc.still limit the stability and accuracy of calculation results,which also introduces complications to experimental verification.Thus,it is difficult for numerical simulation to fully replace experimental study in rock mechanics,and a physical stress measurement method should be developed to verify the accuracy of numerical simulation,especially the results of models with complex structures.But how to accurately extract and visualize the hidden 3D complex porous structures and stress field is the critical difficulty to realize the measurement of the 3D whole-field stress distribution.To accurately extract and measure the 3D porous structures and whole-field stress inside porous geomaterials,this thesis aims to propose a method to quantitatively visualize the complex porous structures and 3D stress field.In this study,the 3D porous structures in the geomaterials were extracted and the digital and physical porous models were established by combining the micro-CT scan,digital images reconstruction and 3D printing;the mechanical and optical properties of the transparent and photosensitive materials fabricated by 3D printer;a method to quantitatively identify and visualize the whole-field stress evolution in the 2D complex porous structures under continuous compressive loads was proposed;the 3D stress-frozen,phase shifting,and unwrapping techniques were developed and a method to quantitatively extract and visualize the whole-field stress in 3D complex porous structures was proposed.The comparison of the experimental and numerical results verified the efficiency and accuracy of the experimental and numerical simulation results,and the characterises of the stress distribution in porous structures,especially the potential shear fractures bands and high-concentration stress zones around the local pores were evaluated based on these results.The major innovation points in this thesis:(1)The 3D digital and physical models with complex porous structures were established by combining micro-CT scan,digital image reconstruction and 3D printing techniques that is capable of physically and transparently characterizing the complex 3D porous structures in geomaterials.(2)To satisfy the study of the quantitative visualization of 3D porous structures,the mechanical and optical properties of the transparent and photosensitive materials fabricated by 3D printer and the influence of photocuring techniques to the optical and mechanical properties of the 3D transparent models was analyzed.(3)A method was proposed to quantitatively identify and visualize the whole-field stress evolution in the 2D complex porous structures under continuous compressive loads that provided an emerging method to quantitatively evaluate the whole-field stress evolution in the complex porous structures that is hard for traditional photoelasticity.(4)The techniques including 3D stress-frozen,phase shifting and unwrapping methods were developed and a method to quantitatively extract and visualize the 3D whole-field stress in complex porous structures under compressive loads was proposed that created the conditions for realizing the physically characterizing of the 3D whole-field stress evolution in complex porous structures.