Mesoscopic modelling of UHPCC material under dynamic tensile loadings

This paper presents a new 3D mesoscopic model of ultra-high performance cement-based composite(UHPCC)to investigate its dynamic tensile behavior.In this model,the UHPCC is regarded as a two-phase material composed of cementitious matrix and randomly distributed fibers.The model is established using the commercial software LS-DYNA and involves generating the randomly distributed fiber elements with considerations of diameter,length,orientation and volume fraction,and then fully constraining them with the matrix.In particular,to capture the slipping effect between fibers and matrix that has a strong influence on the dynamic tensile behavior,the fibers are modelled by a fictitious material represented by the load-slip relation.The strain-rate effect of slipping force neglected in most of previous studies is considered by calibrating constitutive parameters of the fictitious material under different strain-rates based on the single fiber pullout tests.Finally,the 3D mesoscopic model is validated against three sets of tension-dominated experiments covered a wide range of loading intensity.Numerical predictions demonstrate that strain-rate effect of slipping force must be considered,and the neglect of it may lead to a great underestimation of the dynamic tensile strength of UHPCC material and would unavoidably underestimate the blast resistance of UHPCC components.

Preface： theory, methods, and applications of mesoscopic modeling

With increasing attention to complex fluids and soft matter, we have witnessed a fast- growing research in mesoscopic modeling and simulation in the past decades. The development of mesoscopic methods offers many potential opportunities as well as challenges in modeling of complex materials for diverse applications. Despite significant progress in the past decade, mesoscopic methods are still under development. New formulation in the models, novel theo- retical interpretations, and innovative numerical algorithms often appear in literature. These mesoscopic methods have been already applied to a large number of problems, including poly- mer and colloidal suspensions, multiphase fluids, biological materials, and blood rheology. New applications of mesoscopic modeling in different areas are still emerging.

Selective Laser Melting under Variable Ambient Pressure: A Mesoscopic Model and Transport Phenomena

Recent reports on the selective laser melting(SLM)process under a vacuum or low ambient pressure have shown fewer defects and better surface quality of the as-printed products.Although the physical process of SLM in a vacuum has been investigated by high-speed imaging,the underlying mechanisms governing the heat transfer and molten flow are still not well understood.Herein,we first developed a mesoscopic model of SLM under variable ambient pressure based on our recent laser-welding studies.We simulated the transport phenomena of SLM 316L stainless steel powders under atmospheric and 100 Pa ambient pressure.For typical process parameters(laser power:200W;scanning speed:2m∙s^(-1);powder diameter:27 lm),the average surface temperature of the cavity approached 2800 K under atmospheric pressure,while it came close to 2300 K under 100 Pa pressure.More vigorous fluid flow(average speed:4m∙s^(-1))was observed under 100 Pa ambient pressure,because the pressure difference between the evaporation-induced surface pressure and the ambient pressure was relatively larger and drives the flow under lower pressure.It was also shown that there are periodical ripple flows(period:14ls)affecting the surface roughness of the as-printed track.Moreover,the molten flow was shown to be laminar because the Reynolds number is less than 400 and is far below the critical value of turbulence;thus,the viscous dissipation is significant.It was demonstrated that under a vacuum or lower ambient pressure,the ripple flow can be dissipated more easily by the viscous effect because the trajectory length of the ripple is longer;thus,the surface quality of the tracks is improved.To summarize,our model elucidates the physical mechanisms of the interesting transport phenomena that have been observed in independent experimental studies of the SLM process under variable ambient pressure,which could be a powerful tool for optimizing the SLM process in the future.

Improved similarity criterion for seepage erosion using mesoscopic coupled PFC–CFD model

Conventional model tests and centrifuge tests are frequently used to investigate seepage erosion. However, the centrifugal test method may not be efficient according to the results of hydraulic conductivity tests and piping erosion tests. The reason why seepage deformation in model tests may deviate from similarity was first discussed in this work. Then, the similarity criterion for seepage deformation in porous media was improved based on the extended Darcy-Brinkman-Forchheimer equation. Finally, the coupled particle flow code–computational fluid dynamics(PFC-CFD) model at the mesoscopic level was proposed to verify the derived similarity criterion. The proposed model maximizes its potential to simulate seepage erosion via the discrete element method and satisfy the similarity criterion by adjusting particle size. The numerical simulations achieved identical results with the prototype, thus indicating that the PFC-CFD model that satisfies the improved similarity criterion can accurately reproduce the processes of seepage erosion at the mesoscopic level.

Mesoscopic model for binary fluids

We propose a model for studying binary fluids based on the mesoscopic molecular simulation technique known as multiparticle collision, where the space and state variables are continuous, and time is discrete. We include a repulsion rule to simulate segregation processes that does not require calculation of the interaction forces between particles, so binary fluids can be described on a mesoscopic scale. The model is conceptually simple and computationally efficient; it maintains Galilean invariance and conserves the mass and energy in the system at the micro- and macro-scale, whereas momentum is conserved globally. For a wide range of temperatures and densities, the model yields results in good agreement with the known properties of binary fluids, such as the density profile, interface width, phase separation, and phase growth. We also apply the model to the study of binary fluids in crowded environments with consistent results.

Mesoscopic characterization and modeling of microcracking in cementitious materials by the extended finite element method

This study develops a mesoscopic framework and methodology for the modeling of microcracks in concrete. A new algorithm is first proposed for the generation of random concrete meso-structure including microcracks and then coupled with the extended finite element method to simulate the heterogeneities and discontinuities present in the meso-structure of concrete. The proposed procedure is verified and exemplified by a series of numerical simulations. The simulation results show that microcracks can exert considerable impact on the fracture performance of concrete. More broadly, this work provides valuable insight into the initiation and propagation mechanism of microcracks in concrete and helps to foster a better understanding of the micro-mechanical behavior of cementitious materials.

A melt-cast Duan-Zhang-Kim mesoscopic reaction rate model and experiment for shock initiation of melt-cast explosives

A melt-cast Duan-Zhang-Kim(DZK)mesoscopic reaction rate model is developed for the shock initiation of melt-cast explosives based on the pore collapse hot-spot ignition mechanism.A series of shock initiation experiments was performed for the Comp B melt-cast explosive to estimate effects of the loading pressure and the particle size of granular explosive component,and the mesoscopic model is validated against the experimental data.Further numerical simulations indicate that the initial density and formula proportion greatly affect the hot-spot ignition of melt-cast explosives.

Simulation of Fracture Process of Lightweight Aggregate Concrete Based on Digital Image Processing Technology

The mechanical properties and failure mechanism of lightweight aggregate concrete(LWAC)is a hot topic in the engineering field,and the relationship between its microstructure and macroscopic mechanical properties is also a frontier research topic in the academic field.In this study,the image processing technology is used to establish a micro-structure model of lightweight aggregate concrete.Through the information extraction and processing of the section image of actual light aggregate concrete specimens,the mesostructural model of light aggregate concrete with real aggregate characteristics is established.The numerical simulation of uniaxial tensile test,uniaxial compression test and three-point bending test of lightweight aggregate concrete are carried out using a new finite element method-the base force element method respectively.Firstly,the image processing technology is used to produce beam specimens,uniaxial compression specimens and uniaxial tensile specimens of light aggregate concrete,which can better simulate the aggregate shape and random distribution of real light aggregate concrete.Secondly,the three-point bending test is numerically simulated.Thirdly,the uniaxial compression specimen generated by image processing technology is numerically simulated.Fourth,the uniaxial tensile specimen generated by image processing technology is numerically simulated.The mechanical behavior and damage mode of the specimen during loading were analyzed.The results of numerical simulation are compared and analyzed with those of relevant experiments.The feasibility and correctness of the micromodel established in this study for analyzing the micromechanics of lightweight aggregate concrete materials are verified.Image processing technology has a broad application prospect in the field of concrete mesoscopic damage analysis.

Three-Dimensional Mesoscopic Investigation on the Impact of Specimen Geometry and Bearing Strip Size on the Splitting-Tensile Properties of Coral Aggregate Concrete

The use of coral aggregate concrete(CAC)as a novel construction material has attracted significant attention for the construction of reef engineering structures.To investigate the static splitting-tensile behaviors of CAC under the influence of two factors,namely specimen geometry and bearing strip size,a three-dimensional(3D)mesoscale modeling approach with consideration for aggregate randomness in shape and distribution was adopted in this study.We established 12 different specimen models with two specimen shapes(i.e.,a cube with an edge length of 150 mm and a cylinder with dimensions ofφ150 mm×300 mm)and six strip widths(i.e.,6,9,12,15,18,and 20 mm)for calculation.The effects of specimen geometry and strip width on the splitting-tensile properties of CAC,such as failure processes,final failure patterns,and splitting-tensile strength(fst),are analyzed and discussed systematically.The results indicate the high reliability of the developed mesoscale modeling approach and reveal the optimal computational parameters for simulating and predicting the splitting-tensile properties of CAC.The fstvalues of CAC are associated with both the specimen geometry and width of the bearing strip.The fstvalues of the cube model are slightly higher than those of the cylinder model for the same bearing strip size,representing geometry effects that can be explained by differences in fracture area.Additionally,the fstvalue of CAC gradually increases with the relative width of the bearing strip ranging from 0.04 to 0.13.Based on the elastic solution theory,the variation area of CAC fstvalues with the relative width of the bearing strip was determined preliminarily,which has great significance for studying the tensile performance of CAC.

Lattice Boltzmann simulations of high-order statistics in isotropic turbulent flows

The lattice Boltzmann method （LBM） is coupled with the multiple-relaxation- time （MRT） collision model and the three-dimensional 19-discrete-velocity （D3Q19） model to resolve intermittent behaviors on small scales in isotropic turbulent flows. The high- order scaling exponents of the velocity structure functions, the probability distribution functions of Lagrangian accelerations, and the local energy dissipation rates are investi- gated. The self-similarity of the space-time velocity structure functions is explored using the extended self-similarity （ESS） method, which was originally developed for velocity spatial structure functions. The scaling exponents of spatial structure functions at up to ten orders are consistent with the experimental measurements and theoretical results, implying that the LBM can accurately resolve the intermittent behaviors. This valida~ tion provides a solid basis for using the LBM to study more complex processes that are sensitive to small scales in turbulent flows, such as the relative dispersion of pollutants and mesoscale structures of preferential concentration of heavy particles suspended in turbulent flows.

Experimental and mesoscopic investigation on the dynamic properties of coral aggregate concrete in compression

To investigate the dynamic responses and comprehending the damage mechanism of coral aggregate concrete(CAC) in compression, both the experimental and numerical investigations were implemented in the present work. Firstly, the dynamic mechanical properties of CAC at different strain rates were tested through the Split-Hopkinson pressure bar(SHPB) tests.Moreover, the effects of concrete strength grade and strain rate on CAC were discussed and analyzed. Subsequently, we developed the three-dimensional(3D) random mesoscale model considering the randomness of aggregate shape, size and distribution at meso-level, which was validated and employed in the numerical simulation of CAC in compression. The results indicate that the splitting failure passing through the coral aggregate is CAC’S primary failure mode. It has been found that the failure pattern, deformation process, and dynamic increasing factor of CAC are associated with both the strain rate and concrete strength grade. Furthermore, by comparing the experimental and mesoscopic results, it has been proven to be reliable to employ the developed 3D mesoscale modelling method to simulate CAC’s dynamic performances, which has enormous potential in future research of CAC under intense dynamic loadings.

A Multi-phase Field Model for Static Recrystallization of Hot Deformed Austenite in a C–Mn Steel

A multi-phase-field model has been developed to simulate the microstructure evolution and kinetics of the austenite static recrystallization（SRX） in a C–Mn steel. In this model, the bulk free energy that coupling the deformation stored energy with a special interpolation function is incorporated. Both the deformed grain topology and the deformation stored energy have been included in order to investigate the influence of pre-deformation on the subsequent austenite SRX at different hot deformation levels. Diverse scenarios of microstructure evolution show different deformation-dependent recrystallized grain sizes. The transformation kinetics is then discussed by analyzing the overall SRX fraction and the average interface velocity on the recrystallization front.

Microstructural Depictions of Austenite Dynamic Recrystallization in a Low-Carbon Steel:A Cellular Automaton Model

A mesoscopic cellular automaton model that takes into account grain deformation during hot deformation has been developed to quantitatively depict the microstructural evolution of the austenite dynamic recrystallization （DRX） in a low-carbon steel. Both the grain deformation and the concept of DRX cycle are introduced, allowing accurate depictions of the grain structures, the overall microstructural properties and the flow stress evolutions that involving in the austenite DRX. The simulation results are compared with the experimental results and the predictions by the macroscopic DRX model and are found to be in good agreement.

Experimental and numerical study on surface roughness of magnetorheological elastomer for controllable friction

Magnetorheological elastomer (MRE) is a type of smart material of which mechanical and electrical properties can be reversibly controlled by the magnetic field. In this study, the influence of the magnetic field on the surface roughness of MRE was studied by the microscopic modeling method, and the influence of controllable characteristics of the MRE surface on its friction properties was analyzed by the macroscopic experimental method. First, on the basis of existing studies, an improved mesoscopic model based on magneto-mechanical coupling analysis was proposed. The initial surface morphology of MRE was characterized by the W–M fractal function, and the change process of the surface microstructures of MRE, induced by the magnetic interaction between particles, was studied. Then, after analyzing the simulation results, it is found that with the increase in the magnetic field and decrease in the modulus of rubber matrix, the surface of MRE changes more significantly, and the best particle volume fraction is within 7.5%–9%. Furthermore, through experimental observation, it is found that the height of the convex peak on the surface of MRE decreases significantly with the action of the magnetic field, resulting in a reduction in the surface roughness. Consistent with the simulation results, a particle volume fraction of 10% corresponds to a maximum change of 14%. Finally, the macroscopic friction experiment results show that the friction coefficients of MREs with different particle volume fractions all decrease with the decrease in surface roughness under the magnetic field. When the particle volume fraction is 10%, the friction coefficient can decrease by 24.7% under a magnetic field of 400 mT, which is consistent with the trend of surface roughness changes. This shows that the change in surface morphology with the effect of the magnetic field is an important factor in the control of MRE friction properties by magnetic field.