Three Dimensional Mesoscale Structure And Visualization Analysis Of The Stress Field Of Aggregated Heterogeneous Materials

Aggregated heterogeneous solids are widely encountered in the field of civil engineering,geological and mining engineering,etc.,such as glutenite,concrete,soil and rock mixtures.These heterogeneous engineering materials exsist in many practical engineering applications,such as building construction,underground structures,hydropower dams,tunnels and bridges.Aggregated materials characterized by randomly distributed and geometrically irregular aggregates always exhibit nonlinear behaviour and exert various damage and failure patterns when under applied forces.In most cases,static compression or tension,cyclic loadings,freeze-thaw cycles or even dynamic loads of blasting,impact,shock and seismic actions,etc.,can easily induce brittle damage and failure in the aggregated materials and structures.The macroscopic and overall behaviour of aggregated materials,as well as their nonlinear characteristics,are directly controlled by the distribution pattern and evolution rules of their interior stress field.However,the fact that the physical nature and complex meso-level structure of concrete cannot be easily visualized creates difficulties in accurately describing their mesoscale stress states.Physical visualization and quantitative characterization of the stress field are of vital importance to understanding mesoscopic damage,deformation,stress redistribution,and the macroscopic mechanical behaviour and failure mechanisms of the heterogeneous materials.Many researchers have performed a variety of laboratory tests and in situ tests to study the internal stress field of aggregated heterogeneous materials.Most experimental results pertaining to the influence of inclusions on the mechanical response and physical behaviour of a solid have been derived mainly from macroscopic analysis and statistics.As such,they face challenges in directly detecting the role played by individual components or their effects on physical processes that control meso-scale phenomena in aggregated solids.Indeed,there have been very few experimental results that have directly related meso-level stress fields to the macro-level physical and/or mechanical responses of aggregated solids,which is largely due to the complexity of their meso-level heterogeneity and the lack of any direct observation techniques.In overcoming some of the shortcomings of experimental techniques,numerical simulations have provided significant contributions in terms of visualizing and quantifying the internal stress field in heterogeneous solids under load.Although complex meso-scale features of concrete can be characterized via computed tomography scanning,wide acceptance of numerical analysis is hindered by a number of unresolved problems that include the deviation between simplified models and real heterogeneous structures;contact and separation of units;the choice of algorithms;the selection of suitable material parameters;the ascertainment of constitutive relations;and the scale and efficiency of calculation,especially in situations in which calculations are difficult to validate in experiments or field tests.Physically characterizing and visualizing real-world meso-structures and experimentally validating simulation results therefore remains a major challenge to reliably describing and quantifying the internal stress field and its effects on the macroscopic mechanical behaviour of aggregated solids.Based on the the above analysis,this paper therefore have carried out the several aspects of research as follows:(1)manufactured a kind of coarse aggregate concrete using limestone,which serves as the prototype for aggregated heterogeneous solids;three-value-segmented model was constructed by employing X-ray microfocus computed tomography(CT)imaging technique and threshold segmentation method;three dimensional reconstruction method was adopted to build a 3D digital models of the aggregated structure.Meanwhile,the geometrical characteristic of irregualar shaped particles and spatial distribution charateristics are evaluated and accessed;(2)presents a new approach that uses 3D printed models based on of a manually prepared concrete to replicate their complex aggregate structure in a transparent matrix;a commercially available photopolymer material VeroClear,which possess both transparent and stress-visible properties,is used to fabricate testing samples by PolyJet 3D printer;the mechanical properties,the optical-stress sensitivity of VeroClear,including residual birefringence,stress-visualized property and stress-frozen performance are characterized and evaluated;(3)Uniaxial compression tests incorporated with photoelastic techniques were performed on the transparent models to acquire and visualise the stress distribution of the aggregated models at various loading stages.The effect of randomly distributed aggregates on the stress-field characteristics of the models,occurrence of plastic zones,and fracture initiation were analysed.The stress-field characteristics of the aggregated models were analysed using the finite element method(FEM).The failure process was simulated using the distinct element method(DEM).Both FEM and DEM results were compared with the experimental observations.(4)The associated three-dimensional stress field is visually characterized at a mesoscale through uniaxial compression tests and photo-elastic techniques that incorporate a three-dimensional frozen-stress test to analyse the effects of randomly distributed aggregates.These results are used to validate the accuracy of simulated data created using the finite element method,which allows a comparison to be made between two-dimensional and three-dimensional homogenous models and heterogeneous aggregated models;(5)the principal compositions of this printable material are investigated by integrating advanced techniques of infrared spectroscopy,X-ray diffraction,pyrolysis,gas chromatography and mass spectrometry,etc.Meanwhile,the effect of build-up orientation and post-processing temperature on the mechanical properties of printed samples are quantified.Some meaningful conclusions have been concluded through these researches:(1)the complex mesoscale features of heterogeneous aggregated models could be accurately characterized by CT imaging.CT scanning coupled with 3D printing appeared to be an efficient approach for producing aggregated models and visualizing their complex mesoscopic structures.(2)In terms of stress visualization property,the printing material we used is not only exhibit a good birefringence and photoelasticity performance at room temperature,it can also show stress-frozen property when exposure to temperature higher than its critical temperature(Tc).It is clear that VeroClear exhibits prominent optical-stress sensitivity.(3)The fringes around the voids and particles became denser with an increase in the applied compressive stress,revealing the presence of highly concentrated stresses.The stress field adjacent to the voids showed a higher gradient compared to those close to the particles because the strength difference between the matrix and the voids was greater than that between the matrix and the particles.The high stress concentration and stress field around the crack tips created plastic zones,which could be clearly observed and were captured.The spatial distribution of the particles significantly affected the mesoscale mechanical response of the particle materials during the loading process.The three models showed unique characteristics in terms of the distribution and evolution of stress,plastic pattern,crack-propagation path,etc.These results are extremely helpful for identifying the mechanisms governing the deformation and failure of heterogeneous aggregated geomaterials;the study indicates that meso-heterogeneity has more influence on the stress state of localized areas than the whole field.Compared to 2D structures,3D structures possess a lower stress concentration due to the lateral inertial confinement effect.The difference in stress field between the two is therefore attributable to a combination of structure heterogeneity and lateral inertial confinement.(4)Printing material VeroClear is a kind of acrylate-based polymers.Acrylate polymers are noted for their transparency,resistance to breakage,and elasticity.Building orientations shall produce a certain influences on the mechanical behaviors of printed samples.Adopted material and modelling precision can significantly reduce the effect of layered structure on the macroscopic response of printed samples.The UV exposure time shall be the main factor that affects the mechanical properties of printed samples.The mechanical properties of specimen made of VeroClear show a significant increase from 120? to 150?.A higher heat treatment temperature above the glass transition temperature can improve the mechanical properties of tested polymer materials.