Study On The Mesoscopic Crack Evolution Law And Constitutive Model Of Concrete

At present,many railway tunnels in operation suffer from tunnel diseases due to external environment and other factors,and there are hidden dangers that threaten the safety of tunnel traffic.Among them,the crack of tunnel lining is the key to affecting the health of the tunnel,and the evolution of meso-cracks in the lining concrete is the cause of some diseases.Therefore,from a meso-level perspective in which concrete is divided into three components:coarse aggregate,cement mortar and interfacial transition zone to study the evolution of internal mesocracks,to understand the mechanical behavior of concrete materials,to evaluate the health of tunnels,and to formulate the treatment of diseased tunnels are of great significance.In this paper,combined with laboratory experiments,numerical simulations and theoretical analysis methods,the evolution of meso-crack of concrete materials under loading are systematically studied,and the relationship between the mesoscopic composition and the evolution of meso-cracks is revealed.Specifically,first,by carrying out experimental research,combined with the acoustic emission（AE）technology and the digital image correlation（DIC）technology,the evolution of meso-crack in different proportions of concrete under the uniaxial compression loading process was studied;secondly,three-dimensional AE positioning technology was used to obtain the spatial evolution law of meso-crack in the process of concrete beam under three-point bending.After that,in order to further reveal the evolution characteristics of the meso-crack of concrete under compression,based on the Laguerre-Voronoi tessellation method,a program for generating the 3D meso-structure of concrete was written by MATLAB program,and the generated concrete meso-structure was imported into the PFC^3d software to quantitatively analyze the characteristics such as the meso-crack cracking location and fracture type of the concrete sample.Finally,from the perspective of meso-crack evolution,a wing-crack model which was suitable for describing characteristics of meso-composition of concrete was established.Based on the above studies,the following conclusions are obtained:（1）According to the acoustic emission and digital image test results,combined with the macro load-time curve,the evolution of meso-crack in concrete materials under compression is divided into the following stages:（a）the compaction stage:a small amount of acoustic emission signals are generated,the surface strain field is continuous.（b）meso-crack stable growth stage:the amount of acoustic emission signal and energy increases steadily,the surface strain field has meso-cracks that can be identified by the machine,and the meso-crack have little influence on elastic modulus;（c）meso-crack unstable growth stage:the acoustic emission signal increases significantly,mesocracks that can be recognized by the naked eye appear on the surface and the nonlinear characteristic of the load-time curve appears.（d）strain softening stage:the unstable crack propagation stage after the post-peak stress stage.（2）According to the acoustic emission location data of the three-point bending concrete beam,the b-value,the fractal dimension and the minimum time-space distance method are used to analyze the time-space evolution characteristics of the meso-crack during fracture failure.It is found that the b-value,fractal dimension and minimum space-time distance will gradually decrease when the beam approaching failure,which illustrates that the active meso-cracks would become larger in size,closer in distance,and more clustered together.According to the b-value and the spatial distribution of acoustic emission event,the crack induced damage region contour map is established to reproduce the process that the damage region of the three-point bending beam would extend from the middle to the top of the beam along loading process.（3）A program according to Laguerre-Voronoi tessellation method is established to form a 3D concrete meso-structure that can satisfy the coarse aggregate particle size distribution.By analyzing the mesostructures generated by different volume contents and coarse aggregate particle sizes distribution,it is found that by adjusting the generation parameters,a polygonal particle sample that meets the requirements of coarse aggregate content and size distribution can be obtained.Particles produced by this program have the feature that larger size particles occupy more vertex number,more face number and larger sphericity.（4）The PFC^3d software which is based on the particle flow method are used to simulate cylindrical uniaxial compression specimens.The same calibration parameters are applied to study the influence of different coarse aggregate contents,particle sizes and water-to-cement ratios on the macro behavior,and similar observations to those test results are found.According to the analysis of the broken contact features,it is found that when the concrete material is damaged under pressure,the meso-crack in the material first occurs in the interfacial transition zone,mainly shear type failure;then the meso-crack gradually expand into the cement mortar as the load increases,and tensile meso-crack are mainly obtained to propagate approximately parallel to the loading direction;then the number of tensile meso-crack in the cement mortar increases significantly,and the material reaches the peak stress;after the peak stress,the tensile meso-crack in the cement mortar are still dominated,but the number decreases.（5）Combining the conclusions obtained from experiments and numerical simulations,the features of coarse aggregate,interfacial transition zone and cement mortar are reflected through equivalent straight crack method,and the influence of crack spatial evolution on the material is considered by introducing the meso-crack spatial evolution function,and the concrete wing-crack model is established based on above modification.By setting reasonable parameter calibration rules,the theoretical results match the test curve well.This model changes with the coarse aggregate content,water-cement ratio and particle size in the similar way to the test results.