Higher Length Scales Research Articles

Integrated testing and modelling of substructures using full-field imaging and data fusion

A new integrated testing and modelling paradigm based on full-field imaging and finite element (FE) analysis that utilises full-field data fusion is proposed for structural evaluation and model validation at substructure level. The approach is developed for the assessment of a composite wind turbine blade (WTB) substructure subjected to multiaxial loading, mimicking in-service conditions, using a new reconfigurable loading rig. A steel mock-up equivalent to the WTB substructure was used to demonstrate the new experimental, numerical, full-field imaging, and data fusion approaches. Digital Image Correlation (DIC) and Thermoelastic Stress Analysis (TSA) were used to obtain the complex load response of the substructure. Strains and displacements derived from DIC were fused with numerical predictions obtained using a FE-based stereo-DIC simulator, which provided unparalleled like-for-like data comparisons. A numerical FEA solution for TSA was also obtained that accounts for heat transfer and allowed an independent means of structural evaluation. The challenges of deploying full-field imaging on the substructure scale are highlighted alongside procedures for mitigating multiple deleterious effects that are concatenated in large structures testing. It is demonstrated that high quality and fidelity experimental data can be obtained and fused with numerical models to provide a comprehensive and quantitative structural assessment at the substructure scale. It is shown that the proposed full-field data fusion efficiently reveals uncertainties in both the models and experiments. The work provides important steps to support virtual testing at higher length scales and their integration into the design, development, and certification programs of next generation, high-performance structures.

Hierarchical deformation and anisotropic behavior of (α+β) Ti alloys: A microstructure-informed multiscale constitutive model study

In this study, the hierarchical deformation and anisotropic behavior of (α+β) Ti alloys are investigated using a novel microstructure-informed multiscale constitutive model. State-of-the-art crystal plasticity finite element (CPFE) models, due to their emphasis on a single length scale, are inadequate for capturing the complex hierarchical behavior of additively manufactured (AM) (α+β) Ti alloys, which are characterized by columnar grains and lamellar subgrain features at distinct length scales. To overcome this limitation, a decoupled multiscale framework was developed, integrating representative volume elements (RVEs) for both the columnar grain structure at the higher length scale and the lamellar subgrain microstructure at the lower length scale, with equal emphasis on each. The material behaviors at these scales were modeled using an anisotropic classical plasticity model and a mechanism-based CPFE model, respectively. The framework was experimentally validated for Directed Energy Deposition (DED) manufactured Ti-6Al-4V. It was then used to investigate microscopic stress/strain fields, deformation localizations at grain and subgrain levels, and stress partitioning among neighboring grains. From the insights gained a new theory of anisotropy for AM (α+β) Ti alloys is proposed.

Bone matrix properties in adults with osteogenesis imperfecta are not adversely affected by setrusumab-a sclerostin neutralizing antibody.

Osteogenesis imperfecta (OI) is a skeletal dysplasia characterized by low bone mass and frequent fractures. Children with OI are commonly treated with bisphosphonates to reduce fracture rate, but treatment options for adults are limited. In the Phase 2b ASTEROID trial, setrusumab (a sclerostin neutralizing antibody, SclAb) improved bone density and strength in adults with type I, III, and IV OI. Here, we investigate bone matrix material properties in tetracycline-labeled trans iliac biopsies from 3 groups: (1) control: individuals with no metabolic bone disease, (2) OI: individuals with OI, (3) SclAb-OI: individuals with OI after 6 mo of setrusumab treatment (as part of the ASTEROID trial). In addition to bone histomorphometry, bone mineral and matrix properties were evaluated with nanoindentation, Raman spectroscopy, second harmonic generation imaging, quantitative backscatter electron imaging, and small-angle X-ray scattering. Spatial locations of fluorochrome labels were identified to differentiate inter-label bone of the same tissue age and intra-cortical bone. No difference in collagen orientation was found between the groups. The bone mineral density distribution and analysis of Raman spectra indicate that OI groups have greater mean mineralization, greater relative mineral content, and lower crystallinity than the control group, which was not altered by SclAb treatment. Finally, a lower modulus and hardness were measured in the inter-label bone of the OI-SclAb group compared to the OI group. Previous studies suggest that even though bone from OI has a higher mineral content, the extracellular matrix (ECM) has comparable mechanical properties. Therefore, fragility in OI may stem from contributions from other yet unexplored aspects of bone organization at higher length scales. We conclude that SclAb treatment leads to increased bone mass while not adversely affecting bone matrix properties in individuals with OI.

Curli Amyloid Fibers in Escherichia coli Biofilms: The Influence of Water Availability on their Structure and Functional Properties.

Escherichia coli biofilms consist of bacteria embedded in a self-produced matrix mainly made of protein fibers and polysaccharides. The curli amyloid fibers found in the biofilm matrix are promising versatile building blocks to design sustainable bio-sourced materials. To exploit this potential, it is crucial to understand i) how environmental cues during biofilm growth influence the molecular structure of these amyloid fibers, and ii) how this translates at higher length scales. To explore these questions, the effect of water availability during biofilm growth on the conformation and functions of curli is studied. Microscopy and spectroscopy are used to characterize the amyloid fibers purified from biofilms grown on nutritive substrates with different water contents, and micro-indentation to measure the rigidity of the respective biofilms. The purified curli amyloid fibers present differences in the yield, structure, and functional properties upon biofilm growth conditions. Fiber packing and β-sheets content correlate with their hydrophobicity and chemical stability, and with the rigidity of the biofilms. This study highlights how E. coli biofilm growth conditions impact curli structure and functions contributing to macroscopic materials properties. These fundamental findings infer an alternative strategy to tune curli structure, which will ultimately benefit engineering hierarchical and functional curli-based materials.

Investigating the post-yield behavior of mineralized bone fibril arrays using a 3D non-linear finite element unit-cell model

In this study, we propose a 3D non-linear finite element (FE) unit-cell model to investigate the post-yield behavior of mineralized collagen fibril arrays (FAY). We then compare the predictions of the model with recent micro-tensile and micropillar compression tests in both axial and transverse directions. The unit cell consists of mineralized collagen fibrils (MCFs) embedded in an extrafibrillar matrix (EFM), and the FE mesh is equipped with cohesive interactions and a custom plasticity model. The simulation results confirm that MCF plays a dominant role in load bearing prior to yielding under axial tensile loading. Damage was initiated via debonding in shear and progressive sliding at the MCF/EFM interface, and resulted in MCF pull-out until brittle failure. In transverse tensile loading, EFM carried most of the load in pre-yield deformation, and then mixed normal/shear debonding between MCF and EFM began to form, which eventually produced brittle delamination of the two phases. The loading/unloading FE analysis in compression along both axial and transverse directions demonstrated perfect plasticity without any reduction in elastic modulus, i.e., damage due to the interfaces as seen in micropillar compression. Beyond the brittle and ductile nature of the stress–strain curves, in tensile and compressive loading, the simulated post-yield behavior and failure mechanism are in good quantitative agreement with the experimental observations. Our rather simple but efficient unit-cell FE model can reproduce qualitatively and quantitatively the mechanical behavior of bone ECM under tensile and compressive loading along the two main orientations. The model's integration into higher length scales may be useful in describing the macroscopic post-yield and failure behavior of trabecular and cortical bone in greater detail.

Hierarchically self-assembled homochiral helical microtoroids

Fabricating microscale helical structures from small molecules remains challenging due to the disfavoured torsion energy of twisted architectures and elusory chirality control at different hierarchical levels of assemblies. Here we report a combined solution–interface-directed assembly strategy for the formation of hierarchically self-assembled helical microtoroids with micrometre-scale lengths. A drop-evaporation assembly protocol on a solid substrate from pre-assembled intermediate colloids of enantiomeric binaphthalene bisurea compounds leads to microtoroids with preferred helicity, which depends on the molecular chirality of the starting enantiomers. Collective variable-temperature spectroscopic analyses, electron microscopy characterizations and theoretical simulations reveal a mechanism that simultaneously induces aggregation and cyclization to impart a favourable handedness to the final microtoroidal structures. We then use monodispersed luminescent helical toroids as chiral light-harvesting antenna and show excellent Förster resonance energy transfer ability to a co-hosted chiral acceptor dye, leading to unique circularly polarized luminescence. Our results shed light on the potential of the combined solution–interface-directed self-assembly approach in directing hierarchical chirality control and may advance the prospect of chiral superstructures at a higher length scale.

Boron Arsenate Scaled‐Up: An Enhanced Nano‐Mimicking Mechanical Metamaterial

Novel mechanical metamaterials inspired by single crystal boron arsenate built at the macroscale using conventional materials are presented and investigated through finite element simulations. It is shown that these mechanical metamaterials can mimic, to some extent, the properties of boron arsenate with the more robust rotating rigid‐units mechanism being excellently replicated and even made to operate more efficiently through modifications of the geometry of the mechanical metamaterial. It is also shown that not all mechanisms causing auxeticity at the nanoscale can be upscaled due to the absence of the appropriate interactions at higher length scales. This confirms that already existing materials, which have been adequately characterized and studied, can prove to be a very useful source of information for the design of novel mechanical metamaterials which can exhibit even more auxetic properties than the original material itself.

Aqueous Dispersion of Carbon Nanomaterials with Cellulose Nanocrystals: An Investigation of Molecular Interactions.

Dispersing carbon nanomaterials in solvents is effective in transferring their significant mechanical and functional properties to polymers and nanocomposites. However, poor dispersion of carbon nanomaterials impedes exploiting their full potential in nanocomposites. Cellulose nanocrystals (CNCs) are promising for dispersing and stabilizing pristine carbon nanotubes (pCNTs) and graphene nanoplatelets (pGnP) in protic media without functionalization. Here, the underlying mechanisms at the molecular level are investigated between CNC and pCNT/pGnP that stabilize their dispersion in polar solvents. Based on the spectroscopy and microscopy characterization of CNCpCNT/pGnP and density functional theory (DFT) calculations, an additional intermolecular mechanism is proposed between CNC and pCNT/pGnP that forms carbonoxygen covalent bonds between hydroxyl end groups of CNCs and the defected sites of pCNTs/pGnPs preventing re-agglomeration in polar solvents. This work's findings indicate that the CNC-assisted process enables new capabilities in harnessing nanostructures at the molecular level and tailoring the performance of nanocomposites at higher length scales.

De novo multiscale method for nonequilibrium molecular dynamics

In this work, we report a de novo multiscale method that can significantly reduce the number of atoms required to simulate an atomistic system for nonequilibrium molecular dynamics (NEMD) problems. In our approach, first, we form a dynamical matrix that breaks an atomistic system into real and virtual domains, so as real and virtual atoms, Then, the dynamical matrix corresponding to the virtual domain is transformed into the time-history kernel function (THKF), where all the dynamic behavior is just the output of the kernel function taking the behavior of the bonded real atoms as the input so that the virtual atoms no longer need to participate in the simulation. In the practical approach, the real atoms are kept for typical NEMD ensembles, such as thermal states, to retrieve physical properties, while the virtual atoms not bonded with real atoms can be neglected, or even removed. Furthermore, with our semi-analytical approach, the THKF can be derived from eigenvalue and eigenvectors on the dynamical matrix of the virtual domain in arbitrary geometries. To examine whether the dynamic behavior is properly reserved, we first introduce 1-D wave propagation of atom chains with our multiscale method. In addition, we adopt this method to simulate different 3-D silicon-based nanowires with or without twin boundaries and point defects. All the results via our multiscale method agree with the counterpart from fully atomistic systems, which indicates the ability to obtain physical properties with fewer atoms in simulation models than in NEMD. Overall, this method has great potential to perform simulations to collect physical properties with only a small number of atoms at higher length scales, which is important for future studies on the physical meanings over higher length scales.

Modelling articular cartilage: the relative motion of two adjacent poroviscoelastic layers.

In skeletal joints two layers of adjacent cartilage are often in relative motion. The individual cartilage layers are often modelled as a poroviscoelastic material. To model the relative motion, noting the separation of scales between the pore level and the macroscale, a homogenization based on multiple scale asymptotic analysis has been used in this study to derive a macroscale model for the relative translation of two poroviscoelastic layers separated by a very thin layer of fluid. In particular the fluid layer thickness is essentially zero at the macroscale so that the two poroviscoelastic layers are effectively in contact and their interaction is captured in the derived model via a set of interfacial conditions, including a generalization of the Beavers-Joseph condition at the interface between a viscous fluid and a porous medium. In the simplifying context of a uniform geometry, constant fixed charge density, a Newtonian interstitial fluid and a viscoelastic scaffold, modelled via finite deformation theory, we present preliminary simulations that may be used to highlight predictions for how oscillatory relative movement of cartilage under load influences the peak force the cartilage experiences and the extent of the associated deformations. In addition to highlighting such cartilage mechanics, the systematic derivation of the macroscale models will enable the study of how nanoscale cartilage physics, such as the swelling pressure induced by fixed charges, manifests in cartilage mechanics at much higher lengthscales.

Large-deposit non-linear chemical reactive flows in porous media: Identifiability and observability

This work discusses the use of additional pressure drop measurements in the characterisation of chemical reactive flow in porous media resulting in insoluble solid deposition. Previous studies of this problem have used outlet breakthrough concentration of one species and pressure drop across the sample during the commingled injection of reacting species, to determine model coefficients. In this work, we consider the introduction of additional measurements of pressure drop across subsections of the sample. The recently derived analytical model is compared to laboratory data to demonstrate the validity of both the model and additional measurement data. The effect of measuring different quantities is assessed by considering the identifiability of the model parameters. All parameters exhibit finite confidence intervals, however, datasets neglecting breakthrough concentration result in higher parameter uncertainty. Higher parameter uncertainty is shown to result in uncertainty in deposit profiles and prediction of pressure drop at higher length scales, demonstrating the importance of limiting parameter uncertainty through experimental design.

Electron catalysis expands the supramolecular chemist’s toolbox

Molecular recognition is at the heart of the noncovalent synthesis of supramolecular assemblies and, at higher length scales, supramolecular materials. In a recent publication in <i>Nature</i>, Stoddart and co-workers demonstrate that the formation of host-guest complexes can be catalyzed by one of the simplest possible catalysts: the electron.

Harnessing atomistic simulations to quantify activation parameters for dislocation nucleation from a grain boundary in Nickel

We perform atomistic simulations and nudged elastic band calculations to quantify activation energy barriers for dislocation nucleation from ∑3 grain boundary containing a pre-existing void in Ni. By changing the void size, we offer a phenomenological relationship between the activation free energy at zero stress and boundary porosity. Simulations at different temperatures are also conducted to gain some insights into the inherent intricacies of activation energy landscape. It is envisioned that the approach can be pushed forward to guide the flow rules of physics based crystal plasticity models at higher length scales that account for grain boundary effects.

Simulation of bicrystal deformation including grain boundary effects: Atomistic computations and crystal plasticity finite element analysis

Grain boundaries play a pivotal role in dictating the deformation behavior of crystalline materials. Modeling their effects within the framework of physically based crystal plasticity approaches demands a multiscale description of the underlying phenomena. This work puts-forth a two-scale atomistic to crystal plasticity approach for determining the deformation behavior of a ductile face-centered cubic material. The approach uses atomistic computations to quantify the activation energies for nucleation of partial dislocations from a grain boundary under tensile loading. To this end, embedded-atom method based atomistic simulations involving nudged elastic band method are utilized to compute stress-dependent activation parameters of the grain boundary. The extracted parameters are then used as input to the flow rule of crystal plasticity at higher length scale, which is based on transition state theory. At this scale, the grain boundaries are explicitly accounted for by assigning a finite thickness and are differentiated from their bulk counterparts by prescribing distinct flow parameters extracted from atomistic simulations. The predictive capabilities of the proposed methodology are then assessed by performing numerical simulations on uniaxial tensile behavior of Ni bicrystal and validating them from the published experiments and computations available in the literature. This research paradigm can be pushed forward by incorporating more complex grain boundary behaviors at higher length scales while preserving the richness provided by atomic scales.

Dislocation nucleation from damaged grain boundaries in face centered cubic metals – An atomistic study

Towards unraveling the underlying behavior of grain boundaries containing a pre-existing void, we here present a computational approach to quantify the resistance of voided grain boundaries to plasticity evolution in face centered cubic metals. This is realized by conducting an atomistic modeling survey on 104 distinct grain boundaries in both Ni and Cu bicrystals subjected to uniaxial tension perpendicular to the interface plane. Within the framework of embedded-atom method, molecular dynamics simulations are performed to evaluate the critical stress and energy barrier required for dislocation nucleation from the voided interfaces. The resulting set of data is then analyzed to gage correlations between the strength of voided boundaries and common grain boundary descriptors in an effort to inform plasticity models at higher length scales. It is found that macroscopic grain boundary descriptors fail to uniquely predict the strength of damaged grain boundaries, thus suggesting some other mechanistic rationale for the role of such boundaries in governing dislocation nucleation under uniaxial tension in metals. Simulations indicate a direct correlation between the strength and ability of a grain boundary to store energy during tensile deformation. Additionally, it is also observed that a simple statistical distribution can represent dislocation nucleation stresses in face centered cubic metals. These correlations can definitely benefit the materials community in formulating rules for multi-scale materials modeling.

Integrated Experimental-Modelling Strategy to Understand Dislocation–Defect Interactions During Hot Working of Face-Centred Cubic Alloys

The interactions of moving dislocations with various point, line and planar defects during hot working of polycrystalline face-centred cubic alloys are studied. Experimental parameters from three austenitic steels and a Ni-base superalloy are integrated with a dislocation model within a multi-scale framework. The combination of these inputs is used to explain the microstructure evolution and flow behaviour through atomic-scale phenomena and their manifestation at higher length scales.

Active control of three-phase CNT/resin/fiber piezoelectric polymeric nanocomposite porous sandwich microbeam based on sinusoidal shear deformation theory

Vibration control in mechanical equipments is an important problem where unwanted vibrations are vanish or at least diminished. In this paper, free vibration active control of the porous sandwich piezoelectric polymeric nanocomposite microbeam with microsensor and microactuater layers are investigated. The aim of this research is to reduce amplitude of vibration in micro beam based on linear quadratic regulator (LQR). Modified couple stress theory (MCST) according to sinusoidal shear deformation theory is presented. The porous sandwich microbeam is rested on elastic foundation. The core and face sheet are made of porous and three-phase carbon nanotubes/resin/fiber nanocomposite materials. The equations of motion are extracted by Hamilton's principle and then Navier's type solution are employed for solving them. The governing equations of motion are written in space state form and linear quadratic regulator (LQR) is used for active control approach. The various parameters are conducted to investigate on the frequency response function (FRF) of the sandwich microbeam for vibration active control. The results indicate that the higher length scale to the thickness, the face sheet thickness to total thickness and the considering microsensor and microactutor significantly affect LQR and uncontrolled FRF. Also, the porosity coefficient increasing, Skempton coefficient and Winkler spring constant shift the frequency response to higher frequencies. The obtained results can be useful for micro-electro-mechanical (MEMS) and nano-electro-mechanical (NEMS) systems.

Influence of turbulent flow characteristics on flame behaviour in diffuser combustors

The influence of turbulence on flame location in a diffuser-type combustor is the focus of this study. Two types of diffuser combustors were selected to study the influence of turbulence intensity and length scale on flame location. The first combustor was a cylindrical diffuser, and the second combustor had a conical insert. As turbulence intensity and length scale determine Taylor-scale Reynolds numbers, this parametric study also explored the effects of the latter. Flame moved towards the inlet of the diffuser with the increase in turbulence intensity and length scale in cases with and without a conical insert. At a high turbulent length scale, the flame rapidly dropped at the inlet of the diffuser with a conical insert as the turbulence intensity increased. By contrast, the flame dropped to an intermediate level for the diffuser without a conical insert. Results showed that the Taylor-scale Reynolds number is a parameter that influences flame location, as well as turbulence intensity and length scale. An increase in the Taylor-scale Reynolds number leads to the movement of flame location towards the combustor inlet. Flame drops to the inlet of the combustor at a high turbulent Taylor-scale Reynolds number.

Atomistically informed multiscale modeling of radially grown nanocomposite using a continuum damage mechanics approach

An atomistically-informed multiscale modeling framework to investigate damage evolution and failure in radially-grown carbon nanotube (CNT) architecture is detailed in this paper. Molecular dynamics (MD) simulations are performed to investigate the effects of nano-reinforcements on the elastic-plastic characteristics of the constituent interphase in the radially-grown nanocomposite. An interphase damage model is developed using the continuum damage mechanics approach with damage evolution equations derived using atomistic simulations. The developed damage model is integrated with a high-fidelity micromechanical analysis and captures the underlying physical behavior that could be attributed to the enhancement of the out-of-plane properties at higher length scales. The mechanical properties of the nanocomposite obtained from micromechanical simulations are compared to experimental values reported in the literature, to validate the developed modeling framework. Conclusions are presented by comparing the material response of radially-grown CNT architectures with the traditional fiber reinforced polymer (FRP) with dispersed CNT architecture.

Modification of poly(ethylene glycol) on the microstructure and mechanical properties of calcium silicate hydrates

With the aim of creating more sustainable building materials, calcium silicate hydrate (C-S-H), the primary binding phase in modern concrete, was integrated with poly(ethylene glycol) (PEG). At the crystal lattice scale, synchrotron radiation-based high-pressure X-ray diffraction (HP-XRD) reveals that the incorporation of PEG increases the ab-planar stiffness of C-S-H leading to a higher bulk modulus, while molecular simulation results indicate polymers are not likely intercalated into the interlayer region. At a higher length scale, nanoindentation measurements show the introduction of PEG lowers the packing density of C-S-H particles and, thus, decreases the indentation modulus. However, a high creep resistance of the C-S-H/PEG sample is still maintained, which suggests a meso-composite may form to restrict the rapid translation between neighboring calcium silicate sheets. Furthermore, 29Si nuclear magnetic resonance (NMR) shows that the presence of PEG shortens the mean chain length of C-S-H, making dimer the predominant structure.