Mesoscopic Finite Element Research Articles

Damage analysis of shear pre-deformed 3D angle-interlock woven composites using experiment and non-orthogonal finite element model

The shear pre-deformed 3D angle-interlock woven composites under tension load is studied using experimental and numerical methods. A novel non-orthogonal mesoscopic finite element (FE) model taking into consideration non-orthogonal periodic boundary conditions and elasto-plastic damage model is developed to predict the effective mechanical properties and reveal the damage and failure mechanisms. The predicted stress-strain curve is agreement with that of experiment, and the failure mechanisms are verified by Scanning Electron Microscopy (SEM). The effects of shear pre-deformed angle on the stiffness and strength of 3D angle-interlock woven composites are discussed based on simulation results. This study can be helpful of providing reliable guidance for analyzing and designing the woven composite complex structures with pre-deformation.

Mesoscopic finite element analysis on dynamic direct tensile failure of lightweight aggregate concrete and corresponding size effect

A comprehensive finite element analysis at the mesoscopic level has been conducted into the complex topic of size effect coupling dynamic strain-rates. Taking the lightweight aggregate concrete (LWAC) dumbbell-shaped samples as the object of numerical investigation, the influence of strain-rate (with the range of 10−5/s ∼ 100/s) on direct-tensile failure of LWAC (including different lightweight aggregate volume fractions [Formula: see text] = 40%, 30% and 20%) was discussed. Subsequently, the structure size of LWAC samples was further expanded (width W = 100, 200 and 300 mm) and the dynamic size effect on direct-tensile strength was investigated. Numerical results show that both the direct-tensile strength and its corresponding size effect of LWAC exhibit a strain-rate dependent behaviour. The increasing strain-rate can gradually weaken the size effect of LWAC and direct-tensile strength would be independent to the structure size as the strain-rate reaches the critical strain-rate. The increasing lightweight aggregate volume fraction can reduce direct-tensile strength. Furthermore, a dynamic size effect model establishing the direct link between the strain-rate effect and size effect was proposed, which can quantitatively predict the dynamic direct-tensile strength of LWAC.

Experimental-Numerical Analysis of Bending Behavior of Reinforced Concrete Beam with Electrochemical Chloride Extraction-Strengthening

The magnesium phosphate cement (MPC)-carbon fiber reinforced plastics (CFRP) strengthening and electrochemical chloride extraction (ECE) integration experiment was carried out for reinforced concrete beams corroded by sodium chloride. Then the bending performance of the reinforced concrete beams was analyzed by test and numerical simulation. The reinforced concrete beams of the control group were energized to accelerate corrosion until the theoretical corrosion rate of the steel bars reached 10%. The second group of the beams were strengthened, and the other three groups of the beams were strengthened and the inside chloride was removed by electrochemistry method with different dechlorination current densities. The bending test results showed that the bending bearing capacity of the strengthened concrete beam increased by 18.22%. The bending bearing capacity of the strengthened and dechlorinated beam increased by 15.11%, 13.25% and 9.76%, respectively. The chloride ion content at the interface between steel bar and concrete reduced by 68.68%–82.64%. In addition, the numerical simulation method of “standard cube test block-central pull out test block-reinforced concrete beam” was proposed. A 3D mesoscopic finite element plastic damage model of the reinforced concrete beams strengthened by MPC-CFRP was established. The numerical results were in good agreement with the experimental results.

Systematic approach for finite element analysis of thermoplastic impregnated 3D filament winding structures – General concept and first validation results

This work presents a systematic procedure for the detailed, mesoscopic Finite Element simulation of 3D filament wound fiber skeletons with thermoplastic impregnation. First, relevant structural constituents of thermoplastic fiber skeletons are identified and mechanically characterized by means of specially adapted test methods and specimens. In the next step, the mechanical behavior of the structural constituents is simulated in separate FE models, so-called sub-models. This includes the selection, implementation and parametrization of suitable material models. After that, a Finite Element model for a simple demonstrator fiber skeleton is created, the so-called main model, into which the sub-models are integrated. Finally, the simulation results of the main model are compared to mechanical tests of the demonstrator fiber skeleton. The main model developed in this work allows a precise calculation of the maximum bearable load and a good representation of the delamination process occurring before rupture.

Enhanced Breakdown Strength of BN/ER by the 2D Ordinal Tiled Structure of Fillers

The objective of this paper is to propose constructing a 2D ordinal fillers structure to improve the breakdown strength of BN/ER. By loading ferrite on the fillers and applying a rotating magnetic field during curing process, epoxy composites with tiled structure of plate-type BN are obtained. Considering the side effect of ferrite on the insulation properties, its minimum load under the premise of implementing the tiled structure is determined, accordingly the composite materials with breakdown strength about 30% higher than that of traditional BN/ER is obtained. Furthermore, the technological parameters of magnetic treatment is analyzed quantitatively, and a meso-scopic finite element model of BN/ER is built to further discuss the enhancing mechanism of breakdown strength brought by the filler tiled structure.

Axial compression mesoscale modelling of RC columns after reinforcement-electrochemical chloride extraction

The reinforced concrete column after corrosion of chloride salt was subjected to magnesium phosphate cement (MPC)-carbon fiber reinforced plastics (CFRP) reinforcement and electrochemical chloride extraction (ECE) integration experiment. The axial compression behavior of the reinforced concrete column was tested, and then the numerical analysis was carried out. Two types of the corroded reinforced concrete columns were prepared for the test. The columns in type 1 were naturally corroded, and the columns in type 2 were electrified to accelerate the corrosion until the theoretical mass corrosion rate of steel bars reached 15%. Afterward, the MPC-CFRP was used to strengthen the corroded reinforced concrete columns and to remove the inside chlorine by electrochemistry method. The test results showed that the ultimate compressive capacity of the reinforced concrete columns strengthened by the MPC-CFRP increased by more than 22.63%. However, after ECE (current density, 3 A/m2), the ultimate compressive capacity of the reinforced concrete columns strengthened by MPC-CFRP reduced by 3.07% − 3.66%, and the chloride ion content at the interface between steel bar and concrete reduced by 78.52%, which postponed the corrosion of the steel bars. In addition, considering the complex nonlinear bond slip relationship between the steel bar and the concrete interface, a 3D mesoscopic finite element plastic damage model of the reinforced concrete columns strengthened by the MPC-CFRP was established. The numerical results were in good agreement with the test results, which indicated the 3D mesoscopic finite element numerical model can predict the compressive behavior of the reinforced concrete columns adequately.

Finite element analysis of orthogonal cutting of cellular metals: influence of cutting conditions on chip formation and surface damage

Cellular metals are highly porous materials with very distinctive properties. Although it is possible to manufacture near-net-shape parts of these materials with some manufacturing processes, additional machining operations are usually required. However, as conventional machining causes surface damage and poor precision, finishing operations are often performed using alternative, cost-intensive methods. In the present work, a mesoscopic finite element model was used to analyze the influence of cutting conditions and tool geometry in the orthogonal cutting process of cellular metals. The study considered the variability of the mesostructure, as the behavior of multiple cell arrangements under cutting conditions was evaluated. The influence of cutting speed, depth of cut, and tool rake angle on chip formation and surface damage was explored. Chip formation at higher cutting speed was different, as chip fragmentation was visible. Tools with a positive rake angle and a higher cutting speed led to a decrease in subsurface damage. Nevertheless, surface quality is markedly dependent on the stochastic nature of the mesostructure, as material separation is defined by the response of the cell array. Hence, a set of cutting parameters for optimal surface quality could not be identified.

Experimental study on the collapse behavior of graded Ti-6Al-4V micro-lattice structures printed by selective laser melting under high speed impact

Functionally graded Ti-6Al-4V micro-lattice structures were manufactured by selective laser melting (SLM) method. By controlling the cell size along the building direction, two density distribution modes including step-wise gradient and continuous gradient were designed. The density distributions of the specimens were evaluated according to the geometric model reconstructed from the X-ray tomography images. Afterwards, the high speed impact experiments on the graded specimens were conducted by direct Hopkinson Pressure Bar (DHPB) system. Through the high speed photography and digital image correlation (DIC) method, the effects of gradient on the global deformation evolution of the graded Ti-6Al-4V micro-lattice structures under high velocity loading were revealed. Meanwhile, cell assembly based 3D mesoscopic finite element (FE) models were respectively created based on perfect beam elements and X-ray tomography. Accordingly, the dynamic response of the 3D-printed graded structures were simulated by LSDYNA. A more detail vision to the local deformation of the specimens under impact loading was provided. Finally, the protective abilities of the two graded structures under high speed collision were compared and analyzed. It was concluded that the continuously graded micro-lattice structures with negative gradient could provide better protection for the object behind, while the stepwise graded structures might lead to secondary shock which should be avoided.

Mesoscopic damage evolution on bonding interface and its influence on macroscopic performance deterioration of reinforced concrete member

Slip or debonding of bonding interface is the key cause of the performance degradation or failure of the reinforced concrete (RC) member. In this paper, based on Monte Carlo method, a mesoscopic finite element model composed of mortar, coarse aggregate and steel rebar was established to consider the mesoscopic damage on the bonding interface and its influence on macroscopic performance deterioration of RC specimen. The results show that the simulation results fit well with experimental data. Higher initial interfacial damage results in lower bonding strength and smaller final displacement. Higher mortar modulus could greatly improve the initial bonding property and bonding strength, but slightly increase the final damage. Compared with the RC specimen model with plain steel bar, the model with deformed steel bar shows a longer duration of nonlinear increase for drawing force and lower bonding strength. When confinement is applied, the coalescence of damage zones is prevented due to the effect of thread. Therefore, the application of confinement could increase the bonding strength and the initial bonding property.

The fixed-strut-angle finite element (FSAFE) model for reinforced concrete structural walls

This paper evaluates the capabilities and limitations of a relatively simple mesoscopic finite element modeling approach, referred to as the Fixed Strut Angle Finite Element (FSAFE) Model, in simulating the hysteretic lateral load behavior of reinforced concrete structural walls designed to yield in flexure. The behavioral characteristics of the constitutive panel (membrane) elements incorporated in the model are based on a simple fixed-crack-angle formulation, where normal stresses in concrete are calculated along fixed crack directions using a uniaxial stress–strain relationship, with modifications to represent biaxial softening effects. The constitutive panel model formulation also incorporates behavioral models for the shear-aggregate-interlock effects in concrete and dowel action on reinforcing bars, constituting the shear stress transfer mechanisms across cracks. Model predictions are compared with experimentally-measured responses of benchmark wall specimens with a variety of configurations and response characteristics. Accurate predictions are obtained for important global response attributes of the walls prior to failure, including their lateral load capacity, stiffness, ductility, and hysteretic response characteristics; although instability failures related to buckling of reinforcement and/or out-of-plane instability of the wall boundary region are not captured. The model provides accurate estimates of the relative contribution of nonlinear flexural and shear deformations to wall lateral displacements, as well as local response characteristics including the distribution of curvatures, strains, and shear deformations on the walls, prior to failure. Based on the response comparisons presented, model capabilities are assessed and possible model improvements are identified. Overall, the FSAFE model is shown to be a practical yet reliable modeling approach for simulating nonlinear wall behavior, which can be used within the framework of performance-based seismic design and assessment of building structures.

Effects of coarse aggregates on chloride diffusion coefficients of concrete and interfacial transition zone under experimental drying-wetting cycles

Chloride ions diffusion into concrete is one of the most significant factors for reinforced concrete (RC) structures exposed to marine environment, and this can deteriorate reinforcing steel in concrete and cause steel corrosion, ultimately, shorten the durability and serviceability of RC structures. In this paper, the in-door physical experiment method was adopted to explore the effects of natural coarse aggregate on chloride concentration profiles and diffusion behaviors for concrete specimens under man-made exposure condition of drying-wetting cycles. All investigations were based on 10 concrete mixture designs containing variable volume fractions and maximum sizes of natural coarse aggregate for maximum exposure period of 170 days. The chloride profiles versus exposure time, coarse aggregate volume fraction and maximum size were discussed in this paper. The time-dependent model of surface chloride concentration, and the multifactor model of chloride diffusion coefficient of concrete specimens included exposure time, volume fraction, and maximum size of coarse aggregate were developed based on the experimental measurements. In addition, an empirical prediction model for estimating the chloride diffusion coefficient of interfacial transition zone (ITZ) by the consideration of coarse aggregate was determined. Finally, the accuracy and reasonability of the two models of chloride diffusion coefficients for concrete and ITZ considering the effects of coarse aggregate were validated in terms of the meso-scopic finite element numerical simulation method for chloride diffusion into concrete.

Geometrical scaling effect for penetration depth of hard projectiles into concrete targets

Since penetration tests of concrete targets against rigid projectiles are commonly conducted in reduced geometrical scale, whether the replica scaling law holds or not is very important for extending the knowledge based on the small-scale experiments to the large-scale or prototype penetration scenarios. In this paper, based on the available experimental data for depth of penetration (DOP) and discussions on the empirical formulae, it is verified that the replica scaling law is satisfied for DOP in rigid projectile penetrations, as long as the scaling is done strictly for both projectiles and concrete targets including the coarse aggregates. And the coarse aggregates with invariant size (not replica-scaled) could account for the non-scaling effect in DOP found in tests and empirical formulae. To explore the non-scaling effect in DOP caused by aggregates, a 3D mesoscopic finite element model for concrete target is developed. Based on the parametric analyses, it indicates that, the magnitude of the non-scaling effect decreases with the increasing of the cement strength when the aggregate strength is fixed. While the influence of the aggregate strength on the non-scaling effect is not so obvious comparing with the influence of cement strength. Besides, the magnitude increases with the increasing of the volume fraction of aggregates. These conclusions imply that, the non-scaling effect in DOP for different concrete targets with the same magnitude implied by the empirical formulae, and the always held scaling law in the (semi-)analytical models, may be unreasonable. Finally, based on the numerical results, a semi-analytical model for predicting DOP is proposed, which improved our previous model by further considering the non-scaling effect.

Mesoscopic Finite Element Method of the Effective Thermal Conductivity of Concrete with Arbitrary Gradation

The effective thermal conductivity (ETC) of concrete is the most important parameter in determining the temperature field and thermal stresses. A 2D random polygonal aggregate model and its modified model considering porosity were established in this paper in order to partially replace the experiment for parametric analysis on the ETC of concrete and to save the experiment cost. A mesoscopic finite element method for the ETC of concrete with arbitrary gradation was also proposed. In addition, the influence factors (thermal conductivity of coarse aggregate, cement mortar, and volume fraction of coarse aggregate) of the effective thermal conductivity of concrete were analyzed. The results show that the 2D gradation curve of coarse aggregates is proved to exist, and there is a corresponding relationship between the 2D and 3D gradation curves of coarse aggregates. The effective thermal conductivity of concrete has a positive exponential relationship with the volume fraction of coarse aggregates, a positive logarithm relationship with the thermal conductivity of coarse aggregates, and a positive linear correlation with the thermal conductivity of cement mortar. The most practical way to improve the effective thermal conductivity of concrete is to increase the ETC of the cement mortar, but the most effective way is to replace the aggregate with a material with a high thermal conductivity.

Prediction model of long-term chloride diffusion into plain concrete considering the effect of the heterogeneity of materials exposed to marine tidal zone

Long-term chloride-induced corrosion of steel reinforcement would make concrete structures vulnerable to deterioration and as a result shorten their service life. However, physical experiment of in-situ or laboratory may not be an effective way to determine long-term chloride profiles and diffusion behaviors into concrete attributed to its time-consuming and implementary difficulty. This study adopted the results of chloride concentration into mortar and corresponding concrete specimens from a filed experiment in marine tidal zone at exposure times of 60, 120, 240, 360, 480, and 600 d reported in literatures, and the meso-scopic finite element numerical simulation method, in which the concrete was modeled as a three-phase composite material composed of mortar, coarse aggregate, and interfacial transition zone (ITZ), was combined to develop a simplified empirical model for predicting long-term chloride diffusion into plain concrete considering the effect of the heterogeneity of materials exposed to marine tidal zone (with exposure limited to 20 years). Certainly, the accuracy, reasonability and reliability of the evaluated chloride profile results associated with this empirical prediction model were validated as well. The proposed empirical model is expected to be useful in realistically predicting the long-term diffusion of chloride into concrete.

Mesoscopic simulation of thermo-mechanical behaviors in concrete under frost action

This work presents a comprehensive coupled thermal-hydro-mechanical model to simulate both freezing and thawing processes in concrete while considering liquid overpressure and the deterioration of material characteristics. The proposed model is validated through a comparison with available experimental data obtained from the literature. A random polygons meso-structure of concrete is established, within which the mortar, interfacial transition zones (ITZs) and aggregates are distinctly represented. Mesoscopic finite element simulations are conducted to assess the effects of the permeability of mortar and the ITZs, the thermal expansion property of the ITZs, aggregate volume fraction and aggregate gradation. The outcomes indicate that the permeability of the ITZs and the aggregate volume fraction play a significant role in the freezing/thawing behavior of concrete. Furthermore, the impacts of the thermal expansion property of ITZs and the aggregate gradation should not be overlooked.

Mesoscale experimental procedure and finite element analysis for an indirect tensile test of asphalt concrete

This paper describes the experimental procedures used and finite element analyses performed at the mesoscale for an indirect tensile (IDT) strength test of an Asphalt Concrete-25 Marshall specimen. The mesoscale test specimen was a three-phase composite material composed of aggregate, asphalt mastic, and void. The test results demonstrate that digital image technology combined with a finite element numerical analysis model can be used to simulate the mesoscopic damage process of asphalt concrete in an IDT test. A contactless optical system with a strain measurement accuracy of 50 was used and was a suitable method for experimentally verifying the mesoscopic strain measurements. Although the optical method provided results that were always slightly higher than those of the mesoscopic finite element method (FEM) analysis, the optical microstructure test results showed reasonably good agreement with those obtained from the FEM. Both the experimental and FEM results demonstrate that the mastic binder and interface are locations of weakness during the growth and propagation of crack damage in asphalt concrete. The study results demonstrate that the visual mesoscopic FEMs used can be an effective means for further research on the strength mechanisms of asphalt concrete at the mesoscale.

Strain Rate Effect in the Mesoscopic Modeling of High-Strength Steel Fiber-Reinforced Concrete

The research presented in this paper aims to investigate the behavior of a high-strength steel fiber-reinforced concrete (HSSFRC) mesoscopic finite element model, at compressive high strain rates. In order to produce a three-dimensional meso-scale finite element model, a computer code is developed to randomly produce mesoscopic models of SFRC specimen. The specimen is assumed to be reinforced by 0.6 percent volume fraction of hooked steel fibers (Dramix RC-65/35-BN) with random positions and orientations. Aggregates of the compound are assumed to have spherical shape and are produced according to Fuller grading curve. Based on the initial mesoscopic model, a finite element model is produced and used in the explicit dynamic simulation. The contribution of inertial confinement in the dynamic strength enhancement of concrete at high strain rates was investigated and its effective role was observed. Accordingly defining a Dynamic Increase Factor (DIF) for mortar matrix led to overestimation; nevertheless, the inertial confinement by itself could not justify the increment of specimen strength under the dynamic loading. Obtained results also show that steel fibers have a negligible influence on the strength, strength enhancement ratio (DIF) and post peak behavior of the model at high strain rates.

Different strain distributions of cement-emulsified asphalt concrete pavement between the macro- and meso-scale

The purpose of this paper is to study the different strain distributions of CEAC (cement-emulsified asphalt concrete) pavement between the macro- and meso-scale, using a three-dimensional macroscopic finite element ANSYS model of pavement composed of hot mix asphalt (HMA), CEAC, and stone mastic asphalt (SMA). Applying digital image processing technology, the composition of the CEAC material at the meso-scale is identified as a three-phase material that includes cement-emulsified asphalt mastics, aggregates, and air voids. A two-dimensional mesoscopic finite element submodel measuring 80 × 80 mm was identified in a local region and built. Boundary conditions from the global CEAC pavement model were applied to the submodel. Eight control points in the CEAC submodel were selected for analysis. A comparison of the calculated strains of the CEAC local region at the macro- and meso-scales shows that the strain distributions at the meso-scale are not as uniform as those at the macro-scale. The extreme strain at the meso-scale is larger than that at the macro- scale. The reserve strength factors of mastic and aggregates of CEAC at the meso-scale are 31.82% and 49.07%, respectively, compared with 93.2% at the macro-scale. These values indicate that analyses at the macro- and meso-scales yield results that are not only different but can also have a significant effect on the potential failure mechanism of the CEAC component in the pavement.

Random aggregate model for mesoscopic structures and mechanical analysis of fully-graded concrete

The realization of approximate practical aggregate models is a crucial link to the mesoscopic mechanical analysis for fully-graded concrete. This study systematically presents the establishment of mesostructure models of three-phase fully-graded concrete composed of anisotropic high-contented aggregates, cement paste, and interfaces bonding between them. A convex extension method is demonstrated in details to construct convex polygons and polyhedrons represented as crushed coarse aggregates in two- and three-dimensional spaces. The Walraven formula combined with the definition of an equivalent diameter is adopted to ensure the equivalence between the gradation and content of circle and spherical aggregates and that of the actual anisotropic geometric aggregates. We also propose a so-called “occupation and removal method (ORM)” to improve the efficiency of generating mesostructure models with a high aggregate content. Combining the ORM, the inter-particle overlap detection, and the random sequential packing scheme, random aggregate models (RAMs) of polygonal and polyhedral particles are realized to characterize the mesostructure models of concrete. Moreover, by the present RAMs applied, we implement a mesoscopic finite element simulation to investigate the flexural failure process of fully-graded concrete. The numerical results for dynamical flexural strength and dynamic increasing factors show a good in agreement with the experimental data reported in the literature. The proposed RAMs can simulate and explain qualitatively the effects of the mesoscopic structural properties on the macroscopic dynamic characteristics of fully-graded concrete.

Experimental Investigation and Simulation of the Fatigue Mechanisms of a Duplex Stainless Steel under HCF and VHCF Loading Conditions

High frequency push-pull fatigue experiments on the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318LN) revealed that crack nucleation and crack propagation through the first grain determine significantly the lifetime of the material. Only in very few cases it was observed that fatigue samples which endured one billion load cycles without failure (run-out samples) contain microcracks which reached or overcame the first microstructural barrier (phase or grain boundary). This leads to the conclusion that in most cases the highest macroscopic stress or strain amplitude which does not lead to fatigue crack propagation through the entire first grain can be considered as the fatigue limit of the material. The present study documents that the experimentally identified fatigue mechanisms can be represented in mesoscopic finite element simulations by taking into account the effects of anisotropic elasticity, crystal plasticity, macro and micro residual stresses, plastic strain concentration in form of slip bands, crack nucleation and short crack propagation through the first grain. The current investigation shows that such simulations enable the determination of the fatigue limit of both real and synthetic microstructures. By means of real microstructures, containing slip traces and microcracks, the calculations can be verified and the required microstructural parameters can be determined.