Micromechanical Modeling Approach Research Articles

Micromechanical prediction of the elastic and plastic properties of sintered steels

One of the characteristic features of sintered steels is the porosity in their microstructure resulting from the compaction and sintering process. This porosity strongly influences the mechanical properties. To enhance the understanding for the structure–property relationship of sintered Astaloy®85Mo with 0.4 wt.%C, a micromechanical modelling approach based on face-centred cubic (fcc) representative volume elements (RVE) is proposed. The fcc-like periodic arrangement of the sintered particles in the RVE enables the consideration of a realistic non-spherical pore morphology. To compare the predictions with experimental results, accompanying uniaxial tensile tests are considered at different pore volume fractions after initial microstructure characterisation. In addition to the effect of pore volume fraction, the influence of sinter necks on the predicted overall strength is also systematically investigated. Despite the fairly simple nature of the underlying fcc structure, the RVE simulations are perfectly capable of reproducing the experimental trend, showing that the elasto-plastic properties decrease with increasing porosity. This is in contrast to analytical predictions, which underestimate the decrease in properties due to spherical pore assumptions. Moreover, the finite element-based simulations reveal a less pronounced influence of the sinter neck shape on the macroscopic behaviour, even though substantial differences in plastic strain localisation are discernible at the microscopic scale.

A micromechanical modeling approach for predicting the piezoelectric and mechanical properties of piezoelectric fiber-reinforced multiphase composites containing CNT/GNP hybrids

This paper focuses on the prediction of piezoelectric coefficients, Young’s moduli and shear moduli of unidirectional piezoelectric fiber/carbon nanotube (CNT)/graphene nanoplatelet (GNP)-reinforced polymer composites using a novel micromechanics modeling technique. The multiphase composite configuration is that piezoelectric fibers are embedded within a polymer matrix filled by CNT/GNP hybrids. Agglomeration of hybrid nanofillers as a significant factor is identified and incorporated in the micromechanical calculation. Parametric studies are carried out to understand the effect of volume fraction, size and dispersion type of carbonous nanofillers as well as the piezoelectric fiber volume fraction on the effective material constants of multiphase composites. It is found that in comparison with the piezoelectric fibrous composite, the piezoelectric fiber multiphase composite containing CNT/GNP hybrids shows better elastic and piezoelectric properties. Also, further improvement in effective properties can be obtained by increasing the length and percentage of CNTs and GNPs. But, degradation in the elastic and piezoelectric constants is observed by the agglomeration of CNT/GNP hybrids. The current results are found to be in close proximity with the experimental data and other numerical results available in the literature.

Modulus of recycled hot mix asphalt: Micromechanical modeling and experimental approaches

To improve the availability of reclaimed asphalt pavement (RAP) in recycled hot mix asphalt (HMA), this study is focused on the mechanical performance of recycled HMA, which is beneficial for environmental conservation and consistent with the concept of sustainability. To alleviate the heterogeneity of the material composition of recycled HMA and enhance the usability and applicability of research means, micromechanical modeling is used to examine the modulus of recycled HMA, which is a critical parameter for asphalt pavement analysis. In this investigation, a four-layer built-in micromechanical model was constructed and derived to characterize recycled HMA. The micromechanical model indicated that the modulus of recycled asphalt was the main factor influencing the modulus of recycled HMA. Thus, the following study took the modulus of recycled asphalt as the main study subject. The frequency sweep test and three-point flexure beam test for recycled asphalt were designed and conducted to clarify the change trend of influence factors and further verify the reasonability and feasibility of the micromechanical model. Considering the diffusion behavior of the rejuvenator, the change trends of the factors are explored. The curing time apparently influences the modulus of recycled asphalt, followed by the curing temperature and the rejuvenator proportion. Finally, analytical results from micromechanical model matched well with the experimental results. The results of this study may be valuable for predicting the dynamic moduli of recycled HMA.

A Micromechanical Modeling Approach for the Estimation of the Weathering-Induced Degradation of Wind Turbine Blades

Glass fiber reinforced polymers (GFRPs) are widely used as composite material for a variety of applications such as wind turbine blades (WTBs). During their operating time, these GFRP structures are exposed to natural weathering conditions, such as low and elevated temperatures, ultraviolet radiation, and moisture. These weathering phenomena influence the material’s mechanical properties due to material aging and the degradation of the composite’s mechanical properties. For a reliable lifetime assessment and the design of a repurposed application of WTBs, the quantification of GFRP’s degradation is required. For this reason, the aim of the current study is to numerically estimate the combined effects of weathering on the mechanical properties of GFRP. Therefore, the effective elastic properties of a unidirectional GFRP composite were determined considering representative volume elements. The required numerical modeling was performed using finite element analysis. The mechanical properties of glass fibers, epoxy resin and their relationship with individual natural aging phenomena were used based on the existing literature values. As a result of the micromechanical modeling, the change of temperature and moisture absorption have the highest effect on the elastic properties on the epoxy resin and thus also on the GFRP composite. The used numerical approach enables a preliminary estimation of environmental-based degradation phenomena of GFRP which can be used at an early stage of developments of composite structures, the reuse of composites or for planning experimental studies considering degradation of these composite materials.

Micromechanical modeling of the low-cycle fatigue behavior of additively manufactured AlSi10Mg

Factors such as high degrees of design freedom and flexibility in the production process contribute to a continued interest in the additive manufacturing (AM) process in industrial and academic research. The AM-processed components are widely used in industries; however, their use for production of parts under cyclic loading is still limited due to a significant variance in the cyclic behavior and the effect of the AM-associated defects, like porosity or unmelted powder, on the fatigue resistance. Micromechanical modeling approaches can be used to understand the relationship between the microstructure and the cyclic properties of the AM-processed material and thus expedite its employment in safety-critical applications. By using experiments and microstructure-sensitive models, this study aims to examine and to predict the low-cycle fatigue (LCF) behavior of AlSi10Mg parts produced by laser-based powder bed fusion in both the as-built and the direct-aged condition. Fatigue testing reveals higher stress amplitudes and cyclic hardening capabilities for the as-built condition. Direct-aged specimens demonstrate a higher number of cycles to failure at the highest strain amplitude, albeit at lower stress amplitudes when compared to the as-built condition. Both conditions show significant mean stress relaxation during LCF testing at a load ratio of R = 0. The applied modeling framework consists of a J2-plasticity model for the Si-rich phase and a crystal plasticity model for the Al phase. This hybrid model accurately predicts the LCF behavior under various strain amplitudes and ratios for both the as-built and direct-aged conditions of the AlSi10Mg.

Uncertainty analysis for prediction of elastic properties of unidirectional bamboo fiber-reinforced composites

This paper presents a virtual framework to obtain the elastic response of unidirectional (UD) continuous bamboo fiber-reinforced composites based on the micromechanical modeling approach. The virtual framework involves the development of an algorithm for the generation of stochastic Representative Volume Elements (RVEs), taking into account the micro-structural uncertainty and variability of the bio-based bamboo fibers and their composites. The developed algorithm and model are validated with experimental characterizations and theoretical models. Different statistical micro-structural realizations with random fiber packing, fiber geometries, and variable RVE sizes are compared to investigate the effects of these parameters on the accuracy of the prediction of the elastic properties of bamboo fiber-reinforced composites. All models and loading scenarios are applied automatically through Python scripting in the Abaqus finite element (FE) solver. The uncertainty analysis of input parameters is carried out through parametric studies. This virtual micromechanical framework can be further linked in the future to macroscale models of bamboo or other natural fiber composites for multiscale prediction of the behavior of biocomposite structures under in-service loading.

Modelling of 3D-printed bio-inspired Bouligand cementitious structures reinforced with steel fibres

Latencies and limitations exist in the simulation of bio-inspired structures or highly complex geometries manufactured by digital concrete fabrication. This study develops a computational framework to portray the anisotropic material attributes and predict the crack patterns of hardened 3D printed fiber-reinforced concrete beams. Bio-inspired Bouligand structures printed with four pitch angles of 0˚, 15˚, 30˚, and 45˚ are adopted to validate the numerical model. Extensive experiments, including uniaxial compressive tests, splitting tensile tests, and three-point bending tests, are conducted. The material model is attained from the samples extracted from the printed concrete blocks with and without steel fibres after a 28-day curing age for an accurate representation of the design and process. The micromechanical modelling approach is employed to obtain the homogeneous materials for the macro-scale model. The finite element (FE) analysis provides sound agreement with the experiment results of both stress–strain curves and crack patterns. In addition, under bending loading, the 3D printed beams with a unidirectional printing pattern or a small pitch angle perform better than those with larger pitch angles.

Integrated multi-scale modeling of variant selection during stress-induced martensite formation in metastable β Ti-alloys

Stress-induced martensite formation is the predominant mechanism during the deformation of various functional and structural Ti- alloys. To predict the performance of these alloys, stress-induced martensite formation was modeled as a function of crystal orientation and stress state. We present an integrated micromechanical modeling approach using finite element (FE) analysis and an elastic spectral solver based on Fast Fourier Transforms (EFFT), which allows direct correlation of the available work from martensite formation under complex stress-states in an in-situ characterized microstructure. The model is applied as a virtual analogue of an experimental 3-point bending test of a metastable β Ti–10V–2Fe–3Al alloy containing 5% α. The FE model incorporates the experimental β microstructure from electron backscattering diffraction (EBSD) with anisotropic elastic behavior. The EFFT solver uses strain fields in two local regions from the FE model to predict local stresses in experimental microstructures containing β and α phases. The stress and orientation data are used to predict the available work from stress-induced α″ martensite formation of the six different α″ variants. It was found that stress and available work concentrated around the tips of lamellar α, making these regions preferred nucleation sites for α″ formation, which is in excellent agreement with the in-situ experimental observations. This suggests that the local stress conditions in the β phase play a more important role in triggering α″ formation than the compositional inhomogeneity in the β phase at α-β interfaces. Using the integrated model, the first variants forming at low levels of deformation could be confidently predicted. However, the transformation behavior at elevated levels of deformation could not be properly captured. This indicates the need of further model improvement, including Dirichlet boundary conditions for the EFFT solver and accounting for the role of plasticity induced by phase transformation.

Genetic programming-assisted micromechanical models of graphene origami-enabled metal metamaterials

Graphene origami (GOri) enabled metallic metamaterials are novel nanomaterials simultaneously possessing negative Poisson's ratio (NPR) and enhanced mechanical properties that are independent of the topology/architecture of the structure. Predicting their material properties via existing micromechanical models, however, is a great challenge. In this paper, a highly efficient micromechanical modeling approach based on molecular dynamics (MD) simulation and genetic programming (GP) algorithm is developed to address this key issue. The GP-based Halpin-Tsai model is extensively trained from MD simulation data to accurately predict the Young's modulus of GOri/Cu metamaterials with various GOri folding degrees, graphene contents and temperatures with a high coefficient of determination (R2) of ∼0.95. Meanwhile, the well-trained GP-based rule of mixture can accurately predict the coefficient of thermal expansion (CTE), Poisson's ratio and density of metamaterials with R2 of ∼0.95, ∼0.93 and ∼0.99, respectively. The excellent agreement between our estimated results and experimental data shows that the models developed herein are highly efficient and accurate in predicting mechanical properties that are essential for the analysis and design of functionally graded metal metamaterial composite structures. The theoretical results demonstrate that the proposed functionally graded metamaterial beam achieves significantly improved bending performance.

Understanding the Rheological Transitions in Semi-Solid Alloys by a Combined <i>In Situ</i> Imaging and Granular Micromechanics Modeling Approach

To gain better understanding of rheological transitions from suspension flow to granular deformation and shear cracking, this research conducted shear-deformation on globular semi-solid Al-Cu alloys to study the rheological behavior of semi-solid as a function of solid fraction (38% - 85%) and shear rate (10-4 – 10-1 s-1) under real-time synchrotron radiography observation. By analyzing 17 X-ray imaging datasets, we define three rheological transitions: (i) the critical solid fraction from a suspension to a loosely percolating assembly; (ii) from the net contraction of a loose assembly to the net dilation of a densely packed assembly, and (iii) to shear cracking at high solid fraction and shear rate. Inspired by in-situ observations of semi-solid deformation showing a disordered assembly of percolating crystals in partially-cohesive contact with liquid flow, we reproduced a two-phase sample using the coupled lattice Boltzmann method-discrete element method (LBM-DEM) simulation approach for granular micromechanical modeling. In DEM, each globular Al grain is represented by a discrete element, and the flow of interstitial liquid is solved by LBM. The LBM-DEM simulations show quantitative agreement of semi-solid strain localization with the experiments and are used to explore the components involved in the shear rate dependence of the transitions, and the role of liquid pressure on the initiation of shear cracking.

Predicting the master curves of bituminous mastics with micromechanical modelling

The performance of asphalt mixtures is significantly affected by the viscoelastic properties of their mastic phase. The analytical approaches used to predict the properties of mastics from their constituents’ properties are limited in their accuracy and potential to handle non-linear material behaviour. An alternative micromechanical finite element modelling approach to calculate the master curves of mastics from the binder and filler phase properties is presented, where the representative volume elements of mastics consist of linear-viscoelastic bitumen matrices and elastic spherical filler particles. For validation, shear relaxation moduli of bitumen and bitumen-filler mastics are measured at °C°C. Additionally, the model is evaluated and compared with the existing analytical solutions. The results indicate that the proposed approach is advantageous as compared to the analytical solutions, as it allows predicting the mastics’ properties over wider temperature, frequency and material ranges at better agreement with the measurements while giving insight into the micromechanical behaviour.

Deep insights on the creep behavior and mechanism of a novel G115 steel: Micromechanical modeling and experimental validation

Ultra-supercritical (USC) power plants with enhanced thermal efficiency have gained significant attention for reducing carbon dioxide (CO2) emissions. A new tempered martensitic heat-resistant steel, so-called the G115 steel, shows great potential as the candidate for next-generation USC power plant components. However, so far, the intrinsic relationship between microstructure and creep response of this steel still remains ambiguous. In this study, we examined the microstructure-creep behavior relationship of the G115 steel by utilizing a combined experimental and micromechanical modeling approach. More specifically, the modified Orowan's equation was applied to evaluate the creep strain since dislocation motion was a basis of creep deformation. Three kinds of dislocation configurations were incorporated to model the evolution of the substructures, and meanwhile, the kinetic equations were also employed here to describe the damage evolution. The creep curves of G115 steel simulated by this model are reasonably good for up to thousands of hours compared with our experimental data. Moreover, the model outputs, i.e., dislocation densities and width of tempered laths, also agree well with the microstructural parameters achieved from the interrupted creep tests at 923 K and 140 MPa. Regarding the microstructural evolution during the creep process, internal dislocation density (mobile + dipole dislocation densities) first decreases rapidly along the steep slope, and then the rate of decline becomes slower with increasing creep time. In contrast, boundary dislocation density first increases whereas subsequently decreases due to the dominance of tempered lath growth in the later stage of creep. Our results suggest that the micromechanical creep model can fairly elucidate the relationship of microstructural parameters and creep response for G115 steel, which greatly enhances the fundamental understanding of its microstructural features for governing the creep responses.

Micromechanical Study of Porosity Effects on Coupled Moisture-Mechanical Responses of Viscoelastic Asphalt Concrete

AbstractThe present paper offers a micromechanical modeling approach to evaluate the impact of the porosity structure on the response of asphalt concrete as a viscoelastic multiphase material under...

Nonlinear multiscale analysis of coronary atherosclerotic vulnerable plaque artery: fluid-structural modeling with micromechanics.

A unique three-dimensional (3D) computational multiscale modeling approach is proposed to investigate the influence of presence of microcalcification particles on the stress field distribution in the thin cap layer of a coronary atherosclerotic vulnerable plaque system. A nested 3D modeling analysis framework spanning the multiscale nature of a coronary atherosclerotic vulnerable plaque is presented. At the microscale level, a micromechanical modeling approach, which is based on computational finite-element (FE) representative unit cell, is applied to obtain the homogenized nonlinear response of the calcified tissue. This equivalent response effectively allows the integration of extremely small microcalcification inclusions in a global biomechanical FE model. Next, at the macroscale level, a 3D patient-based fluid-structure interaction FE model, reconstructing a refined coronary artery geometry with calcified plaque lesion, is generated to study the mechanical behavior of such multi-component biomechanical system. It is shown that the proposed multiscale modeling approach can generate a higher resolution of stress and strain field distributions within the coronary atherosclerotic vulnerable plaque system and allow the assessment of the local concentration stress around the microcalcifications in plaque cap layers. A comparison of stress field distributions within cap layers with and without inclusion of microcalcifications is also presented.

Micromechanical Modeling of Unreinforced Masonry Arches Accounting for Flexural Hinges and Shear Slidings

ABSTRACT This paper investigates the structural response of unreinforced masonry arches subjected to asymmetrical vertical concentrated loads, by studying global force-displacement response curves and collapse mechanisms. The micromechanical modeling approach is adopted, which describes the detailed arrangement, geometry and mechanical properties of the masonry constituents. This is introduced in a 2D finite element procedure modified and enriched to describe masonry curved geometry and the occurring damage and friction mechanisms. Linear elastic behavior is assumed for bricks, whereas a damage-friction law is considered for the mortar to describe flexural and shear failure mechanisms. First, to prove the efficiency of the adopted model, the response of experimentally tested arches is numerically reproduced. Then, a parametric analysis is performed to analyze the effect of the geometric parameters on the global structural response. It is proved that, as the thickness/internal radius ratio increases, the failure mechanism moves from the formation of flexural hinges to shear sliding, leading to a more brittle response. Moreover, influence of relevant material parameters is investigated and the numerical outcomes are compared with those obtained by the classical limit analysis approach. It appears that the maximum predicted load generally exceeds the limit analysis collapse value, depending on the material properties mostly influencing the failure mechanisms.

Thermal Post-Buckling Strength Prediction and Improvement of Shape Memory Alloy Bonded Carbon Nanotube-Reinforced Shallow Shell Panel: A Nonlinear Finite Element Micromechanical Approach

Abstract This article reported first-time the post-buckling temperature load parameter values of nanotube-reinforced polymeric composite panel and their improvement by introducing the functional material (shape memory alloy, SMA) fiber. The temperature load values of nanotube composite and SMA activation are modeled using the single-layer type higher-order kinematic model in association with isoparametric finite element technique. To ensure the effective properties of SMA bonded nanotube composite under the elevated temperature, a hybrid micromechanical material modeling approach is adopted (Mori–Tanaka scheme and rule of mixture). The present structural geometry distortion under elevated temperature is modeled through the nonlinear strain kinematics (Green–Lagrange), whereas the strain reversal achieved with the help of marching technique (inclusion of material nonlinearity). Owing to the importance of geometrical distortion of the polymeric structure, the current model includes all of the nonlinear strain terms to accomplish the exact deformation. Further, to compute the post-buckling responses, the governing nonlinear eigenvalue equations are derived by Hamilton's principle. The numerical solution accuracy is verified with adequate confirmation of model consistency. The material model applicability for different structural configurations including important individual/combined parameter tested through a series of examples. Moreover, the final understanding relevant to the post-buckling characteristics of the polymeric structure and SMA influences is highlighted in details considering the prestrain, recovery stress, and their volume fractions.

Micromechanical modeling approach with simplified boundary conditions to compute electromechanical properties of architected piezoelectric composites

Architected piezoelectric composites (PCs) have recently gained interest in designing transducers and nondestructive testing devices. The current analytical modeling approach cannot be readily applied to design architected periodic PCs exhibiting elastic anisotropy and piezoelectric activity. This study presents a micromechanical (MM)-model based finite element (FE) modeling framework to predict the electromechanical properties (EMPs) of the architected PCs. As an example, the microstructure with one-dimensional (1–3 PCs) connectivity is considered with different cross-sections of fibers. 3D FE models are developed. The intrinsic symmetry of architected composite is used to derive boundary conditions (BCs) equivalent to periodic BCs (PBCs). The proposed approach is simple and eliminates the need for a tedious mesh generation process on opposite boundary faces on the MM model of architected PCs. The EMPs of 1–3 PCs calculated from the proposed micromechanics-FE models were compared with those obtained from analytical solutions (i.e. based on micromechanics theories), and FE homogenization (i.e. obtained by employing the PBCs available in the literature). A quite good agreement between the proposed modeling approach and the ones obtained using the analytical model was observed. However, an excellent agreement is observed with the MM results that employed PBCs. Hence, we have concluded that the proposed MM modeling approach is equivalent to MM models that employed PBCs. The computed enhanced effective elastic, piezoelectric, and dielectric properties and corresponding figure of merit revealed that 1–3 PCs are suitable in transducer applications.

Structure-property relationship of a nickel-based honeycomb sealing composite

Since brittle intermetallic compounds can form during soldering of nickel-based alloys, the microhardness in the area of the solder material is increased. This can be observed when joining thin-walled honeycomb structures. They are used as stationary component in sealing systems of gas turbine engines. Both the increase in the microhardness and the volume fraction of the solder material are assumed to threaten the integrity of the rotating sealing component if contact (rubbing) between the two components occurs during engine operation. To predict the impact of the solder material on the structure-property relationship of the nickel-based honeycomb structure, a micromechanical modelling approach is presented in this work. The three-dimensional micromechanical modelling approach comprises a composite of two distinct materials, the cell-wall and the solder material. The constitutive behaviour of the ductile cell-wall material is modelled with a crystal plasticity approach. A purely elastic material behaviour is assumed for the brittle solder material. For an efficient simulation, the investigated domain is restricted to the unit cell of the honeycomb composite. The studies show that with increasing volume fraction of the solder material, both thermal and mechanical loads of the honeycomb composite increase significantly, raising the likelihood of damage to the material of the rotating sealing component.

Micromechanical modeling approach to single track deformation, phase transformation and residual stress evolution during selective laser melting using crystal plasticity

Single track scanning is a widely used method to evaluate the effects of rapid solidification of metals and to analyze their printability. Microstructure level stresses play a dominant role in causing material failure during deposition or poor performance on the finished product. This work formulates a thermomechanical crystal plasticity model capable of presenting microscale level evolution and residual state of stresses and strains in a single track event of selective laser melting. The present novel thermomechanical model is a vital piece of an overall workflow to analyze material properties and more complex performance inherent and dependent on the microstructure scale phenomena. The results show effectiveness of the model in addressing microscale residual stress heterogeneities dependent on the melt pool area thermal and microstructural evolution, including micromechanical phase transformations, and their interaction with the surrounding matrix on the studied H13 tool steel. The method is found exceptionally robust in terms of predicting microstructural residual stresses and deformation, while its greatest limiting feature is the requirement of prior solidified microstructure as an input for the computations.

Failure Process of Modeled Recycled Aggregate Concrete under Uniaxial Compression.

This paper investigates the failure processes of recycled aggregate concrete by a model test and numerical simulations. A micromechanical numerical modeling approach to simulate the progressive cracking behavior of the modeled recycled aggregate concrete, considering its actual meso-structures, is established based on the discrete element method (DEM). The determination procedure of contact microparameters is analyzed, and a series of microscopic contact parameters for different components of modeled recycled aggregate concrete (MRAC) is calibrated using nanoindentation test results. The complete stress–strain curves, cracking process, and failure pattern of the numerical model are verified by the experimental results, proving their accuracy and validation. The initiation, growth, interaction, coalescence of microcracks, and subsequent macroscopic failure of the MRAC specimen are captured through DEM numerical simulations and compared with digital image correlation (DIC) results. The typical cracking modes controlled by meso-structures of MRAC are concluded according to numerical observations. A parameter study indicates the dominant influence of the macroscopic mechanical behaviors from the shear strength of the interfacial transition zones (ITZs).