Macroscale Properties Research Articles

Molecular dynamics simulation on bulk bitumen systems and its potential connections to macroscale performance: Review and discussion

• The application cases of MD simulations on bulk bitumen systems were reviewed. • Bitumen model, Forcefield, validation and prediction parameter were summarized. • The effects of aging, modification, and rejuvenation on bitumen were overviewed. • The potential connections between nano-and-macroscales properties were discussed. Molecular dynamics (MD) simulation plays an effective role in predicting the critical properties and explaining the macroscale phenomenon at the nanoscale. This review summarized the application cases of MD simulations in various bitumen systems, considering aging, modification, and rejuvenation factors. Meanwhile, the potential relationships between the nanoscale parameters predicted from MD simulations and macroscale properties measured from experimental tests were discussed for the first time. Different molecular models of virgin bitumen, commonly-used Forcefields, and validation parameters for MD simulations on bituminous materials were summarized. Based on the reactive MD simulation outputs, the oxidative aging reaction path at the atomic scale of bitumen molecules was reviewed. Furthermore, the influence of aging (short-term and long-term), modification (polymers, fillers, and bio-bitumen), and rejuvenation (various rejuvenators) on the molecular-scale properties of virgin bitumen models would be evaluated through MD simulations. This review could help us further explore the main functions of MD simulations in different bulk bitumen systems and build an integral multi-scale research method from the molecular design and performance prediction to material optimization and synthesis of bituminous materials without lots of experimental attempts.

Nonlinear fiber-bundle-cells-based phenomenological modeling of human tissue samples

Certain assemblies of fibers, called fiber bundles, play a crucial role in the statistical macroscale properties of fibrous structures like natural or artificial materials. Based on the concept of using idealized statistical fiber bundle cells (FBCs) as model elements, the software named FiberSpace was developed by us earlier for the phenomenological modeling of the tensile test process of real fibrous structures. The model fibers of these FBCs had been considered linear elastic, which was suitable for modeling certain textiles and composites. However, the biological tissues are multilevel structures with fiber-like building elements on every structural level where the fiber elements on the dominant level are statistical bundles of elementary fibers. Hence, their modeling required us to introduce model fibers of nonlinear mechanical behavior and derive the proper mathematical formulas for the calculation of the expected tensile force processes of the FBCs. Accordingly, we developed a new version of FiberSpace. The proposed nonlinear FBCs-based modeling method is essentially phenomenological that decomposes the measured and averaged stress–strain curve into the weighted sum of the responses of different idealized nonlinear FBCs. However, this decomposition can give certain information about the fibrous structure and some details of its damage and failure sub-processes. A special application of nonlinear E-bundles, where the measured stress–strain curve is expanded into a product-function series, may give another type of description for the failure process and can be applied to single measurements of structured failure process containing significant peaks and drops as well. The fitted phenomenological FBC models provide a decomposition of the measured force–strain curve, which enables to construct informative damage and failure maps. The applicability of the phenomenological modeling method and the fitting procedure is demonstrated with the tensile test data of some human and animal tissues, such as facial nerves and tendons.

Characterisation of the Contact between Cross-Country Skis and Snow: A Macro-Scale Investigation of the Apparent Contact

In a cross-country skiing competition, the time difference between the winner and the skier coming in at second place is typically very small. Since the skier spends much of the energy on overcoming resistive forces, a relatively small reduction in these forces can have a significant impact on the results. The resistive forces come partly from the friction, at the tribological interface between the ski and the snow, and as with many tribological processes, the characterisation of its origin plays an important role in determining the frictional properties. Furthermore, in cross-country ski friction, there are several scales impacting the frictional performance, with the major contributors being the ski-camber profile and ski-base structure. Macro-scale measurements of the ski-camber profile under loading are often used to determine how adequate the ski is for use under specific conditions. The characteristic properties usually assessed are the force required to collapse the ski in order to obtain a certain camber height, the topography of the kick-wax zone, and the length (determined by simple means) of the frictional interfaces associated with the rear- and front glide zones, i.e., the apparent contact length. These measurements are, however, commonly performed by loading the ski against a much stiffer counter surface than snow and this affects the quantification of the characteristic properties. To date, some mathematical models have been proposed, but there is no reliable approach for determining the macro-scale properties of the contact between a cross-country ski and a counter surface using simulations. In the present paper, an Artificial Neural Network (ANN) has been trained to predict the ski-camber profile for various loads applied at different positions. A well-established deterministic approach has been employed to simulate the contact between the ANN-predicted ski-camber profile and a linearly elastic body with a flat upper surface, representing the snow. Our findings indicate that this method is feasible for the determination of relevant macro-scale contact characteristics of different skis with snow. Moreover, we show that the apparent contact area does not linearly depend on the load and that the material properties of the counter surface also exert a large impact when quantifying the apparent contact area and the average apparent contact pressure.

A Coarse-Grained Interaction Model for Sodium Dominant Montmorillonite.

Montmorillonite is the main crystalline mineral present in bentonite. It is an absorbent, swelling material; the physical chemistry underlying its ability to absorb water and swell occurs at the nanoscale, governed by electrical double-layer interactions. In turn, absorption and swelling lead to important changes in the macroscopic transport properties of the clay. Mesoscale models can help us establish a link between these nanoscale processes and macroscale properties, notably by providing a detailed description of its pore network. Models on the scale of hundreds to thousands of nanometers are required, which cannot realistically be handled using traditional all-atom molecular dynamics simulations. This work presents a coarse-grained (CG) mesoscale model of sodium montmorillonite. In our model, montmorillonite platelets are represented by two types of particles: central nonhydrogen-bonded particles and edge hydrogen-bonding particles. The particle interactions are described by two-body potentials, which were optimized based on all-atom molecular dynamics simulations. Specifically, several potential mean force calculations involving dry and hydrated montmorillonite were performed, using the ClayFF potential to calculate interatomic forces. The CG model was validated by testing the scalability of the model, testing its ability to reproduce potentials of mean force reported elsewhere in the literature, and by comparing the calculated elastic properties of a system containing 1000 Na montmorillonite platelets to experimentally measured elastic properties of bentonite. The simulated elastic properties obtained using our mesoscale model agree with these experimental values.

Tuning the Mechanical Behavior of Density-Graded Elastomeric Foam Structures via Interlayer Properties.

The concept of density-graded foams has been proposed to simultaneously enhance strain energy dissipation and the load-bearing capacities at a reduced structural weight. From a practical perspective, the fabrication of density-graded foams is often achieved by stacking different foam densities. Under such conditions, the adhesive interlayer significantly affects the mechanical performance and failure modes of the structure. This work investigates the role of different adhesive layers on the mechanical and energy absorption behaviors of graded flexible foams with distinct density layers. Three adhesive candidates with different chemical, physical, and mechanical characteristics are used to assemble density-graded polyurea foam structures. The mechanical load-bearing and energy absorption performances of the structures are evaluated under quasi-static and dynamic loading conditions. Mechanical tests are accompanied by digital image correlation (DIC) analyses to study the local strain fields developed in the vicinity of the interface. Experimental measurements are also supplemented by model predictions that reveal the interplay between the mechanical properties of an adhesive interlayer and the macroscale mechanical performance of the graded foam structures. The results obtained herein demonstrate that the deformation patterns and macroscale properties of graded foam composites can be tuned by selecting different bonding agents. It is also shown that the proper selection of an adhesive can be a practical way to address the strength–energy dissipation dichotomy in graded structures.

Nano-vascularized polymers: how nanochannels impact the mechanical behaviour at the macroscale

In recent decades, nature has inspired the engineering of many structural and smart materials. Nano-vascularization has been stimulating research on advanced materials for novel biomedical, orthopaedic, industrial, and aeronautical applications. The continuous development of nano-vascularized materials requires a more accurate understanding of their mechanical behaviour. This work provides a multiscale methodology to predict the macroscale properties of nano-vascularized materials. The methodology is experimentally validated for the case of a nano-vascularized epoxy resin manufactured using sacrificial electrospun nanofibers. It is based on the development of representative volume elements (RVEs) that encompass information about both the nanochannels distribution and on the mechanical properties of the material at different dimensional scales. The RVEs simulations allowed obtaining a homogenized model describing the nano-vascularized material properties and studying the most intimate failure mechanisms. A virtual stress tomographic investigation on the RVEs was adopted as a digital twin to reveal the damage evolution and the actual failure mechanisms of the nano-vascularized material: damage occurred mainly at the nanochannels intersections, particularly where the intersections become dense. Interestingly, the simulations revealed a correlation between the stress state and the formation of feather markings as well as local failures on the nanochannels linking directions, as evidenced by SEM analysis.

Chickpea flour and soy protein isolate interacted with κ-carrageenan via electrostatic interactions to form egg omelets analogue

In recent years, demand for the plant-based egg substitutes has increased significantly, especially in Singapore, a country seeking for innovative food sources imminently. In the current study, chickpea flour, soy protein isolate, shortening, baking powder, mono, diglyceride, transglutaminase, potassium chloride, flour, and hydrocolloids (κ-carrageenan (κ-C) or gellan gum (GG)) were used to develop the eggless omelets. A formulation comprising 0.3% κ-C (0.3κ-C) best matched the physiochemical properties of egg, in terms of hardness (4437 vs. 4614 g), specific volume (1.24 vs. 1.19 cm 3 /g), and gel strength (19.3 vs. 17.5 kPa). This could be attributed to the highest synergistic κ-C-protein interactions in 0.3κ-C, along with the most homogeneous gel structure observed under confocal laser scanning microscopy (CLSM). The addition of 0.1% κ-C induced more κ-C-protein interactions than the one without hydrocolloids, but such increase was not as dominant as 0.3κ-C. When the κ-C concentration reached 0.5%, however, the rheological synergism decreased while the electrostatic interactions increased; that signifies the increased κ-C-κ-C interactions. Contrastingly, a segregated GG-protein interaction occurred in all GG systems, as indicated from synergism and CLSM images. These differences in interactions and structures affected the macroscale properties of our plant-based egg products, explaining the different physiochemical properties among them. A schematic diagram was therefore proposed to build connections between physiochemical properties, interactions, and structure. • Plant-based egg with 0.3% κ-carrageenan (κ-C) matched the liquid egg best. • Synergy between 0.3% κ-C & proteins and the uniform gel structure caused the match. • Addition of 0.1% or 0.3% κ-C formed a 3D network with proteins filled by starch. • Adding 0.5% κ-C replaced part of κ-C-protein interactions by κ-C-κ-C. • Bi-continuous gel structure was observed in gellan gum gels.

Two-Scale Topology Optimization with Isotropic and Orthotropic Microstructures

Advances in additive manufacturing enable the fabrication of complex structures with intricate geometric details, which bring opportunities for high-resolution structure design and transform the potential of functional product development. However, the increasingly delicate designs bring computational challenges for structural optimization paradigms, such as topology optimization (TO), since the design dimensionality increases with the resolutions. Two-scale TO paves an avenue for high-resolution structural design to alleviate this challenge. This paper investigates the efficacy of introducing function-based microstructures into the two-scale TO. Both isotropic and orthotropic microstructure are considered to develop this TO framework. Implicit functions are exploited to model the two classes of cellular materials, including triply periodic minimal surfaces (TPMS) and Fourier series-based functions (FSF). The elasticity tensor of microstructures is computed with numerical homogenization. Then, a two-scale TO paradigm is formulated, and a gradient-based algorithm is proposed to simultaneously optimize the micro-scale structures and macro-scale material properties. Several engineering benchmark cases are tested with the proposed method, and experimental results reveal that using proposed microstructures leads to, at most, a 36% decrease in the compliance of optimal structures. The proposed framework provides achievable directionality and broader design flexibility for high-resolution product development.

Meso-structure-based thermomechanical modelling of thermoplastic-based laminates subjected to combined mechanical loading and severe thermal gradients

This paper presents a numerical modelling methodology to investigate the thermo-mechanics of carbon reinforced thermoplastic-based laminates in the situation of mechanical loading combined to thermal gradients. It concerns in particular the case of thermal irradiation on one face of the laminate, where temperature varies both from point to point of the laminate and through time, and ranges from the ambient to the temperature of matrix decomposition onset. Temperature is the key variable to determine the physical state and the properties of each phase of the composite. Particular attention is therefore paid to identifying the respective material properties according to temperature. The developed numerical model is based on an explicit representation of the yarn-matrix spatial arrangement within a representative volume element of the considered laminate. From this model the influence of the heterogeneous matrix thermal degradation on the macro-scale thermal dependent properties of the composite is discussed. Such a modelling requires the consideration of a wide range of phenomena (modification of physical and mechanical properties) and a comprehensive set of thermal boundary conditions (heterogeneous distribution of heat flux density on the sample surface, convection, radiation). The proposed model provides a very good prediction of the laminate’s stiffness evolution according to temperature up to the onset of thermal decomposition temperature. It further enables to analyze the gradual efforts take up between the plies when a constant mechanical loading is combined to a heat exposure.

A comparative study of micro-CT and mercury intrusion techniques for predicting permeability and surface area evolution during chemical dissolution

In this paper, we report on experimental and computational studies investigating the evolution of pore structure and permeability of a microporous carbonate rock during chemical dissolution using information provided from X-ray micro-computed tomography (µ-CT) and mercury intrusion porosimetry (MIP) techniques. We consider a chemical dissolution in a core sample by a nonacidic solution wherein a quasi-uniform modification of pore structure occurred. First, we conduct a comparative analysis on the capabilities and limitations of the µ-CT and MIP methods to quantify the evolution of pore size and pore surface area. In particular, we incorporate micropores into the calculations of the pore size-related parameters to highlight the uncertainties that ignoring microporosity causes. In the second part of the paper, we present predictive results on the fractional changes in the permeability of the rock using the Katz-Thompson (KT) and Kozeny-Carman (KC) models. The predictions are based on two characteristic parameters of the pore phase, i.e., the critical pore diameter and the specific pore surface area, and two characteristic macro-scale properties, i.e., porosity and formation factor, all calculated from both the MIP and µ-CT methods. The predicted changes in permeability are compared with those calculated directly on the images using the lattice-Boltzmann (LB) flow simulation on segmented images. The effect of changes in microporosity is also assessed to investigate its role in permeability evolution. The results showed that the KT model could reasonably predict the fractional changes in permeability in terms of the pore structure and macroscale parameters calculated from the MIP and µ-CT methods.

A multi-scale based fracture characterization in concrete under fatigue loading using critical energy dissipation

In this study, analytical formulations for energy dissipation rate and critical energy dissipation have been derived for concrete under cyclic loading conditions. Initially, the formulation for micro-crack growth rate has been derived adopting nano-mechanistic approach, which subsequently has been extended to express the critical energy dissipation. In order to eliminate the scale effect encountered between the micro and macro-scale stress properties, a multi-scale approach has been adopted in the present work. A new damage parameter has been proposed for the quantification of damage on the basis of critical energy dissipation.

Large deformation of shape-memory polymer-based lattice metamaterials

Through advanced computational analysis, this work demonstrates that metamaterial structures when made by shape-memory polymers (SMPs) can provide unique intelligent mechanical behavior. For the first time, geometrical effects, pertaining to low and ultra-low densities, on the thermomechanical behavior of SMP-based octet-truss lattice meta-structures are studied in this work. A reliable constitutive thermo-visco-hyperelastic model is applied to analyze the shape-memory behavior of several designed octet-truss lattices with ultra-low and low relative densities, ranging from 0.04 to 0.23. Different compressive strain values are tested to determine the effect of pre-straining. It is concluded that changing the relative density as a consequence of altering the diameter of struts in lattice meta-structures is a crucial factor affecting their shape-memory properties, deformation mechanism, and macro-scale mechanical properties compared to the bulk SMPs. The finite element results demonstrate that the relative density and pre-strain level provide two controlling parameters in programming the SMP-based lattice meta-structures and enhancing their stress recoverability.

Shape Memory Behaviour of PMMA-Coated NiTi Alloy under Thermal Cycle.

Both poly(methyl methacrylate) (PMMA) and NiTi possess shape memory and biocompatibility behavior. The macroscale properties of PMMA–NiTi composites depend immensely on the quality of the interaction between two components. NiTi shape memory alloy (SMA) and superelastic (SE) sheets were spin coated on one side with PMMA. The composite was prepared by the spin coating method with an alloy-to-polymer-thickness ratio of 1:3. The bending stiffness and radius of curvature were calculated by using numerical and experimental methods during thermal cycles. The experimental radius curvatures in actuation have good agreement with the model. The change in shape results from the difference in coefficients of thermal expansion between PMMA and NiTi. Actuation temperatures were between 0 and 100 °C for the SMA–PMMA composite with a change in curvature from 10 to 120 mm with fixed Young’s modulus of PMMA at 3 GPa, and a change in Young’s modulus of NiTi from 30 to 70 GPa. PMMA–NiTi composites are useful as actuators and sensor elements.

Multiscale Characterization of Fly Ash–Based Geopolymer and Type V Portland Cement Exposed to MgSO4

Fly ash–based geopolymer is an attractive supplemental cementitious material that has been receiving great attention from the community; however, its durability characteristics such as resistance to sulfate-related damage have not yet been fully examined. This study investigates the properties of fly ash–based geopolymer and its counterpart, Type V portland cement paste/mortar, when they are exposed to MgSO4 solution. Toward that end, a multiscale characterization was conducted. Changes in nanomechanical properties due to MgSO4 exposure were tracked and quantified by nanoindentation. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were coupled to characterize microstructural and chemical changes at different MgSO4 exposure levels. Moreover, the effect of MgSO4 solution on macroscale properties, including changes in mass and compressive strength, were examined. Results indicated that exposure to MgSO4 solution affected the two cementitious materials very differently. No significant sign of deterioration was observed in fly ash–based geopolymer, although MgSO4 changed the chemical compositions by increasing Mg content and decreasing Na in the original N-A-S-H gel. On the contrary, Type V portland cement presented the degradation of the main hydration product due to the decalcification process with increasing MgSO4 exposure. This resulted in a significant drop of Ca/Si ratio and compressive strength after 6 months of MgSO4 immersion. Test-analysis results in different length scales in this study imply that fly ash–based geopolymer can be a durable material under MgSO4 environments.

Relationship between Nano and Macroscale Properties of Postfire ASTM A36 Steels

Relationship between Nano and Macroscale Properties of Postfire ASTM A36 Steels

Influence of asymptotically-limiting micromechanical properties on the effective behaviour of fibre-supported composite materials

The macroscale tensile behaviour of slender fibre-supported composite bodies is examined via an asymptotic homogenisation approach. A series of semi-analytic three-dimensional models for linearly elastic fibre-reinforced materials under extreme, but realistic, limiting microscale mechanical properties are derived, and implemented using COMSOL Multiphysics. The key limits investigated are cases involving incompressibility of one component material, and those where dramatic differences in the shear moduli of the component materials exist within the composite body. Discrepancies are observed between the effective macroscale properties obtained from a standard model, based on the published literature, and those obtained from the models of micromechanical limiting behaviours derived here. Such discrepancies have significant implications when using such models to optimise the material properties of composite materials.

Metalation/Demetalation as a Postgelation Strategy To Tune the Mechanical Properties of Catenane-Crosslinked Gels.

Mechanically interlocked molecules (MIMs) possess unique architectures and nontraditional degrees of freedom that arise from well-defined topologies that are achieved through precise mechanical bonding. Incorporation of MIMs into materials can thus provide an avenue to discover new and emergent macroscale properties. Here, the synthesis of a phenanthroline-based [2]catenane crosslinker and its incorporation into polyacrylate organogels are described. Specifically, Cu(I) metalation and demetalation was used as a postgelation strategy to tune the mechanical properties of a gel by controlling the conformational motions of integrated MIMs. The organogels were prepared via thermally initiated free radical polymerization, and Cu(I) metal was added in MeOH to the pretreated, swollen gels. Demetalation of the gels was achieved by adding lithium cyanide and washing the gels. Changes in Young's and shear moduli, as well as tensile strength, were quantified through oscillatory shear rheology and tensile testing. The reported approach provides a general method for postgelation tuning of mechanical properties using metals and well-defined catenane topologies as part of a gel network architecture.

Enhancing the effectiveness of steam curing for cement paste incorporating fly ash based on long-term compressive strength and reaction degree of fly ash

Steam-cured concrete plays an imperative role in contemporary construction, particularly in the urban region where the progress of the project is one of the main keys for cost savings. However, steam curing can cause several adverse impacts on the mechanical and durability performances of concrete in long term. The macroscale properties of concrete are mainly controlled by the properties of cement paste. This research aims to find out the appropriate treatment stage of steam curing on blended cement paste and the optimum quantity of fly ash substituted in the paste that not only can achieve the design strength in short term but also be able to alleviate the detrimental impacts of heat treatment in long term. Cement pastes incorporating 0, 20, 35, and 50 wt% fly ash were mixed at a water/binder ratio of 0.28 and cast into the mold with a size of 50 × 50 × 50 mm. Then, these paste specimens were treated with the treatment stage at 55 °C, 70 °C, and 85 °C for various hours. The compressive strength and the hydration degree of fly ash were measured over 545 days. The results indicated that the treatment stage with the temperature of 70 °C applied for 4 h existed to be adequate to promote the early strength while the substitution of 35% fly ash happened to be the optimum quantity.

Influence of printing parameters on 3D printing engineered cementitious composites (3DP-ECC)

3D printing of engineered cementitious composites (3DP-ECC) serves as an effective means to enable digital and automated construction while removing the dependence of steel reinforcement typical in normal concrete construction. As generally recognized, process control in 3DP is as crucial as material constituents to the performance of the printed structure. Printing parameters pose direct influence on the micro-structure of material, which further impacts the macro-scale properties of printed ECC. In this paper, the effects of different nozzle standoff distances and nozzle travelling speeds are investigated. It is found that lowering the nozzle standoff distance within a certain range increases the in-plane tensile strength and strain capacity of 3DP-ECC by 39% and 30%, respectively. The tensile performance is also enhanced by a moderate printing speed. In addition, a 141% increase in interfacial fiber bridging force is found with elevated nozzle standoff distance. Furthermore, the micro-structure observed via μ-CT correlates well with the printing parameters, micro-structure and macro-scale properties of 3DP-ECC.

Refractory ceramics bonds from potassium-based inorganic polymer for advanced applications: Crystalline phase changes and descriptive microstructure

Understanding the structural and meso-scale properties of alkali activated matrices subjected to high temperature treatment, provides useful information on the stability of their self-life service or, fire resistance in extreme environments. The current project focuses on the transformation of phase and structural bonds within kyanite-reinforced refractory geopolymer. After being exposed to temperatures ranging from 1050 °C to 1250 °C, the micro-structural and macro-scale properties of the resulting matrices were evaluated using X-ray diffraction, chemical bond analysis, SEM-EDS, and resistance to flexural strengths. The fineness of the kyanite powder played an important role in the geopolymer composites' bonding system, as well as the crystallization of the cordierite phase during the sintering process. Further decomposition of kyanite minerals limited the thermal expansion to less than 0.8%, suggesting the formation of new and stronger bonds between the particles, which result in the formation of crystalline phases such as: cordierite, mullite, leucite and enstatite. These phases render the matrices compact and dense. Based on the findings, kyanite particles were discovered to be refractory components and fillers in the formation of refractory alkali-bonds ceramics, with environmental and eco-friendly benefits.