Mechanical Strain Field Research Articles

Mandrel diameter effect on ring-pull testing of nuclear fuel cladding

Inconsistencies in ring-pull testing methods for thin-walled tubes make it difficult to compare a material's mechanical properties in the hoop direction presented in past publications. The effect of test setup, specifically mandrel diameter, was investigated for nuclear fuel cladding tubes of the iron-chromium-aluminium (FeCrAl) alloy, C26M (Fe-12Cr-6Al-2Mo). Mandrel diameter for ring-pull testing can change how the ring deforms and friction effects, further convoluting the results from testing. To address the impact of mandrel size in relation to ring diameter, gaugeless ring-pull samples were tested across a range of mandrel diameters and compared to uniaxial tensile tests from the same tube of material. Two groups of samples were tested: an as-received, low ductility sample set and an annealed, high ductility sample set, both cut from the same extruded tube. By pairing systematic variation of mandrel geometry with analysis of local strain gradients using digital image correlation, this study investigates the effect of test fixture geometry on apparent mechanical properties and effective strain fields and provides guidance for comparing previously published data. Certain features in uniaxial stress-strain curves (namely the yield strength, ultimate tensile strength, and uniform elongation) appeared to have analogies in the effective stress-strain curves from ring-pull testing. These corresponding features are compared to expose biases in the analysis of material properties using gaugeless ring-pull and to provide novel guidance on test setup during experimental design.

Exploring electronic characteristics of bilayer HfS2 under mechanical strain and external electric field: A first-principles approach

This study employed the density functional theory (DFT) to explore the dynamic stability of different stacking configurations of bilayer HfS2 (2L-HfS2) and the influence of external conditions on its electronic properties. These results indicate that the AA configuration is the most stable among the various stacking configurations. Calculations of the carrier effective mass revealed that the artificially stacked 2L-HfS2 maintained a small effective mass of electrons and holes and exhibited high carrier mobility. Further investigation revealed diverse trends in the electronic properties of 2L-HfS2 under vertical strain, in-plane strain, shear strain, and an applied external electric field. These variations encompass bandgap size alterations, band structure evolution, and transitions in the metallic behavior. This systematic study elucidates the regulatory effects of different strains and external electric field conditions on the electronic properties of 2L-HfS2, thereby providing a vital theoretical foundation and experimental guidance for its application in electronics and optoelectronics.

Modeling mechanical waves propagation in flexoelectric solids

In this paper, the propagation of mechanical waves in flexoelectric solids with the consideration of both the direct and converse flexoelectric effects is studied via a collocation mixed finite element method (MFEM). The dynamic effects associated with mechanical waves propagation are accounted by introducing the kinetic energy in the Hamilton’s principle. In the proposed collocation MFEM, a quadratic polynomial is independently assumed for each component of the mechanical strain and electric field. The independently assumed mechanical strain and electric field are collocated with their counterparts computed from the displacement and electric potential at 9 Gaussian quadrature points. Thus, except for the fundamental field variables, no additional degrees of freedom (DOFs) are introduced. By performing the numerical experiments using the collocation MFEM, it is found that due to the direct flexoelectric effect, the propagation of mechanical waves can result in electric polarization in materials. Besides, the converse flexoelectric effect can induce mechanical waves when there are non-uniform transient electric field applied to the material. Numerical results indicate that by increasing the loading speed of the time varying mechanical displacement load, the direct flexoelectric effect associated with the mechanical strain gradient could be significantly enhanced.

Optical and Mechanical Properties of Si-Based Thin Films for Photonic Applications

In this talk, we will review recent advances in the fabrication and characterization of silicon-based thin film structures for photonic applications, including rare earth doped silicon oxynitrides and luminescent silicon carbonitrides.For Si-based materials to be used in solid-state lighting (SSL) and silicon photonics schemes it is necessary to have precise control of the optical emission from these materials. This can be accomplished using rare earth dopants such as Ce, Tb, and Eu to obtain blue, green, and red emissions, respectively. After a brief review of the latest developments in the field, this talk will focus on Eu-doped films, which are attractive for silicon photonic applications due to their intense emission in the visible spectral region [1]. The films are fabricated by electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD) and doping is accomplished using a novel hybrid radio frequency (RF) magnetron sputtering source in the ECR-PECVD reactor chamber [2]. This approach contrasts with traditional doping methods which use metal-organic precursors to introduce rare-earth dopant species into the host matrix. We will discuss the relationship between the photoluminescence, elasticity, and hardness of Eu doped silicon oxynitride (SiON) thin films.As the efficiency of photonic devices can be influenced by the mechanical properties of the deposited films, a good understanding of their mechanical properties is required for the integration of these films in photonic devices. SiNx thin films, for example, play an important role in strain engineering of active photonic devices, such as diode lasers fabricated on GaAs or InP substrates. We will discuss results of a collaboration between McMaster and the Université de Rennes in France, aimed at quantifying the effects of structured dielectric thin films on the generation of a mechanical strain field in the semiconductor through tuning the amount of mechanical stress present in the dielectric thin film, as a function of the details of the deposition process [3].[1] Fahmida Azmi et al., “Tunable Emission from Eu:SiOxNy Thin Films Prepared by Integrated Magnetron Sputtering and Plasma Enhanced Chemical Vapor Deposition”, J. Vac. Sci. Technol. A 40, 043402 (2022); https://doi.org/10.1116/6.0001761[2] J.W. Miller et al., “Integrated ECR-PECVD and Magnetron Sputtering System for Rare-Earth-Doped Si-Based Materials”, Surface and Coatings Technology 336, 99 (2018); doi: 10.1016/j.surfcoat.2017.08.051[3] Brahim Ahammou et al., “Strain engineering in III-V photonic components through structuration of SiNx films”, J. Vac. Sci. Technol. B 40, 012202 (2022); doi: 10.1116/6.0001352

Generative AI and image based numerical mechanics in wind blade adhesive composites

Numerical modelling of adhesive composites in wind energy is complicated in part due to material heterogeneity. Microstructural CT scan fibre composite patterns or representative elements, which play a major role in defining the mechanical behaviour of these adhesive structures, are both difficult to characterize as well as hard to numerically simulate. With advances in deep learning based generative AI, new ways of predicting the mechanical behaviour of heterogeneous materials is now possible. Here we put forward a data driven method to relate input composite adhesive microstructures with field data using deep learning and generative AI based methods. The prediction of mechanical stress or strain fields or other similar patterns and combining them as a function of boundary conditions, fibre composite microstructure and material models is achieved and the models are trained such that they closely approximate computationally expensive simulations based on numerical FE techniques and would have the ability to generalize. We also create a dataset of wind energy adhesives with their numerical mechanics based FE simulations subject to different boundary conditions and material models for further deep learning based composite studies.

Density functional study of atomic arrangements in CrMnFeCoNi high-entropy alloy and their impact on vacancy formation energy and segregation

Using the density functional theory-coupled Monte Carlo approach, we explored the chemical short-range order (SRO) and element segregation in equimolar CrMnFeCoNi alloy. We found that state-of-the-art approximation of random element distribution is only applicable at > 1100 K close to the melting temperature, while the Cr-Cr repulsion driving the system stabilization and accompanying the formation of cubic Cr sublattice, and mild Ni-Ni attraction are the most prominent pair interactions at lower temperatures. Chemical potential and vacancy formation energy calculations indicate that Cr is most sensitive to the local chemical environment, making Cr atoms most stabilized when the preferred SRO is introduced. While the vacancy formation is predicted equally probable among five constituting elements in the random solid solution, Cr and Ni atoms show the lowest vacancy formation energies in the structure with SRO. Furthermore, distinct element segregation was predicted in the vicinity of planar defects, including symmetric tilt grain boundary and stacking fault, which we correlated to the site- and chemistry-dependent atomic volume and bond lengths. It suggests that the local mechanical strain and bond energy induce the SRO development and element segregation: Namely, the system takes advantage of segregation of Ni atoms having large atomic volume or Cr-Cr pairs having elongated bond lengths to fill in the excess volume at defects that relaxes the mechanical strain field and optimizes bond energy distribution. The correlation between the SRO and properties of CrMnFeCoNi alloy needs further investigations, which is expected to greatly help understand and control the properties of high-entropy alloys.

Electronic structure and optical properties of GaTe/MoTe2 based vdW heterostructure under mechanical strain and external electric fields

In this work, we systematically designed a two-dimensional (2D) heterostructure based on GaTe and MoTe2 monolayers to circumvent the limitations of the constituents and enhance their performance for various optoelectronic and energy applications. All the calculations are performed using Hybrid-Density Functional Theory (DFT) calculations. We observed that the GaTe/MoTe2 heterostructure has an indirect bandgap (Eg) of 1.5 eV with type-I band alignment. Due to the type-I alignment, electrons and holes both reside in the MoTe2 monolayer, which restricts its use in various potential applications. Therefore, the modulation of electronic structure and optical spectra of 2D-GaTe/MoTe2 heterostructure are investigated in detail under external biaxial strain varying from −9% to +9%, and electric field values between ±1V/Å. We observed, a type II band alignment with high optical visible absorption of order 105 cm−1 within εxy > +2% and Eext between +0.4 V/Å to +1 V/Å, and −0.6 V/Å to −1 V/Å. This suggests that the GaTe/MoTe2 heterostructure has excellent potential for various next–generation optoelectronic and energy applications.

Tuning the magnetic interactions in van der Waals Fe3GeTe2 heterostructures: A comparative study of ab initio methods

We investigate the impact of mechanical strain, stacking order, and external electric fields on the magnetic interactions of a two-dimensional (2D) van der Waals (vdW) heterostructure in which a 2D ferromagnetic metallic Fe$_3$GeTe$_2$ monolayer is deposited on germanene. Three distinct computational approaches based on \textit{ab initio} methods are used, and a careful comparison is given: (i) The Green's function method, (ii) the generalized Bloch theorem, and (iii) the supercell approach. First, the shell-resolved exchange constants are calculated for the three Fe atoms within the unit cell of the freestanding Fe$_3$GeTe$_2$ monolayer. We find that the results between methods (i) and (ii) are in good qualitative agreement and also with previously reported values. An electric field of ${\cal E}= \pm 0.5$~V/{\AA} applied perpendicular to the Fe$_3$GeTe$_2$/germanene heterostructure leads to significant changes of the exchange constants. We show that the Dzyaloshinskii-Moriya interaction (DMI) in Fe$_3$GeTe$_2$/germanene is mainly dominated by the nearest neighbors, resulting in a good quantitative agreement between methods (i) and (ii). Furthermore, we demonstrate that the DMI is highly tunable by strain, stacking, and electric field, leading to a large DMI comparable to that of ferromagnetic/heavy metal (FM/HM) interfaces. The geometrical change and hybridization effect explain the origin of the high tunability of the DMI at the interface. The electric-field driven DMI obtained by method (iii) is in qualitative agreement with the more accurate \textit{ab initio} method used in approach (ii). However, the field-effect on the DMI is overestimated by method (iii) by about 50\%. The magnetocrystalline anisotropy energy can also be drastically changed by the application of compressive or tensile strain in the Fe$_3$GeTe$_2$/germanene heterostructure.

Straining to find the permeability

Characterizing fluid flow in a porous and permeable material is fundamental to energy and hydrological applications, yet direct measurements of permeability are very difficult to conduct in situ. However, attending fluid flow through a material are various mechanical responses, e.g., strain fields and acoustic emissions, and these mechanical responses may hold important clues to the fluid flow in the material, specifically the permeability. Here we report results from a numerical study of fluid flow through a channel, defined by confining side blocks, that contains a particle bed. For a range of inlet velocities, we study the strain and acoustic emission in the confining side blocks. The simulations are repeated for different configurations of the particle bed. We find a one-to-one correspondence between strain and acoustic emission and the quantities that determine the permeability. Thus, strain and acoustic emission may serve as ingredients in an unconventional scheme for remote monitoring to learn the permeability.

Second-Order Collocation-Based Mixed FEM for Flexoelectric Solids

Flexoelectricity is an electromechanical coupling between the electric field and the mechanical strain gradient, as well as between the mechanical strains and the electric field gradient, observed in all dielectric materials, including those with centrosymmetry. Flexoelectricity demands C1-continuity for straightforward numerical implementation as the governing equations in the gradient theory are fourth-order partial differential equations. In this work, an alternative collocation-based mixed finite element method for direct flexoelectricity is used, for which a newly developed quadratic element with a high capability of capturing gradients is introduced. In the collocation method, mechanical strains and electric field through independently assumed polynomials are collocated with the mechanical strains and electric field derived from the mechanical displacements and electric potential at collocation points inside a finite element. The mechanical strain gradient and electric field are obtained by taking the directional derivative of the independent mechanical strain and electric field gradients. However, an earlier proposed linear element is unable to capture all mechanical strain gradient components and, thus, simulate flexoelectricity correctly. This problem is solved in the present work by using quadratic shape functions for the mechanical displacements and electric potential with fewer degrees of freedom than the traditional mixed finite element method. A Fortran user-element code is developed by the authors: first, for the linear and, after that, for the quadratic element. After verifying the linear element with numerical results from the literature, both linear and quadratic elements’ behaviors are tested for different problems. It is shown that the proposed second-order collocation-based mixed FEM can capture the flexoelectric behavior better compared to the existing linear formulations.

Inverse identification of pipeline steel mechanical state by means of coupled magnetic measurements and artificial neural networks

Artificial neural networks are widely used to develop models able to predict properties of interest by learning and establishing relationships between inputs and outputs of a system. It is particularly relevant for the inversion process in the framework of Non-Destructive Testing (NDT) involving strongly non-linear and non-monotonic behavior and/or saturations. The case-study considered in this work is a magnetic material subjected to uniaxial mechanical stress, plastic strain and magnetic field. An Artificial Neural Networks (ANN) model is proposed to predict the corresponding remanent magnetization, coercive field and the maximum magnetization as target properties. A series of experimental data made of various magneto-mechanical measurements are used to train, evaluate and validate the ANN model. The proposed model suitably predicts the magnetic properties of a second specimen of the material in the same magnetic field, plastic strain and stress ranges as the first specimen. An inverse ANN is then proposed to evaluate the mechanical loading and the plastic strain from the magnetic signature. Unique and accurate solutions are found that proves the relevance of machine learning approach in such NDT application.

Microscopic deformations in MDF swelling: a unique 4D-CT characterization

Medium-density fiberboard (MDF), a wood-based material that consists of a tight random network of wood fibers, deforms more than wood when exposed to water. For the first time, the microscopic deformations of MDF were tracked during swelling. A hygroscopic swelling setup imposing the material to deform throughout tomographic acquisition was used coupled to X-ray microtomography. An advanced reconstruction algorithm enabled reconstruction of images free of motion artefacts, and state-of-the-art digital volume correlation was applied to determine the mechanical strain fields at high resolution. Wood fiber bundles were then segmented from single fibers with deep learning using the UNet3D architecture. Combined with the strain fields, this segmentation showed that wood fiber bundles were the drivers of MDF swelling. This contrasts with the hygroscopic behavior of wood, where structured wood swells less than single fibers, which might be caused by a difference in penetration and distribution of the adhesive, in and on the wood fiber cell wall. The unique characterization of MDF’s dynamic behavior can already be used to develop manufacturing strategies to improve water resistance, therefore widening the uses of natural fiber-based materials.

Mechanical strain and electric field tunable electronic structure of black/violet phosphorene van der Waals heterostructure: From type I to Z-scheme system

We reported a new van der Waals (vdW) heterostructure formed by monolayer black phosphorene (BP) and monolayer violet phosphorene, also known as Hittorf's phosphorene (HP). The First-principles calculation showed that the BP/HP heterostructure exhibits a direct band gap (1.43 eV) with distinct type-I band alignment. By applying tensile strains, the BP/HP heterostructure was modified from type-I junction to Z-scheme one, forming the built-in electric field directed from BP to HP which just promotes the separation of photogenerated holes and electrons, whereas compressive strains maintain the type-I band alignment but bring to the bandgap (direct to indirect) and the conductivity (semiconductor to metal) transitions. Moreover, negative electric fields could induce a more steady and robust Z-scheme band alignment over a wide range of the electric field intensities. Our present studies pointed out that this high tunability of the electronic structure in BP/HP vdW heterostructure is suitable for future nanoelectronics and photocatalysts.

Mechanical Strain and Electric Field Tunable Electronic Structure of Black/Violet Phosphorene Van Der Waals Heterostructure: From Type I to Z-Scheme System

Mechanical Strain and Electric Field Tunable Electronic Structure of Black/Violet Phosphorene Van Der Waals Heterostructure: From Type I to Z-Scheme System

Warm-up spectroscopy of quadrupole-split nuclear spins in n -GaAs epitaxial layers

The efficiency of the adiabatic demagnetization of the nuclear spin system (NSS) of a solid is limited, if quadrupole effects are present. Nevertheless, despite a considerable quadrupole interaction, recent experiments validated the thermodynamic description of the NSS in GaAs. This suggests that nuclear spin temperature can be used as a probe of nuclear magnetic resonances. We implement this idea by analyzing the modification of the NSS temperature in response to an oscillating magnetic field at various frequencies, an approach termed as the warm-up spectroscopy. It is tested in a $n$-GaAs sample where both mechanical strain and built-in electric field may contribute to the quadrupole splitting, yielding the parameters of electric field gradient tensors for $^{75}\mathrm{As}$ and both Ga isotopes, $^{69}\mathrm{Ga}$ and $^{71}\mathrm{Ga}$.

Biofidelic dynamic compression of human cortical spheroids reproduces neurotrauma phenotypes.

ABSTRACTFundamental questions about patient heterogeneity and human-specific pathophysiology currently obstruct progress towards a therapy for traumatic brain injury (TBI). Human in vitro models have the potential to address these questions. Three-dimensional spheroidal cell culture protocols for human-origin neural cells have several important advantages over their two-dimensional monolayer counterparts. Three-dimensional spheroidal cultures may mature more quickly, develop more biofidelic electrophysiological activity and/or reproduce some aspects of brain architecture. Here, we present the first human in vitro model of non-penetrating TBI employing three-dimensional spheroidal cultures. We used a custom-built device to traumatize these spheroids in a quantifiable, repeatable and biofidelic manner, and correlated the heterogeneous mechanical strain field with the injury phenotype. Trauma reduced cell viability, mitochondrial membrane potential and spontaneous synchronous electrophysiological activity in the spheroids. Electrophysiological deficits emerged at lower injury severities than changes in cell viability. Also, traumatized spheroids secreted lactate dehydrogenase, a marker of cell damage, and neurofilament light chain, a promising clinical biomarker of neurotrauma. These results demonstrate that three-dimensional human in vitro models can reproduce important phenotypes of neurotrauma in vitro.

Computational Analysis of Bone Remodeling in the Proximal Tibia Under Electrical Stimulation Considering the Piezoelectric Properties.

The piezoelectricity of bone is known to play a crucial role in bone adaptation and remodeling. The application of an external stimulus such as mechanical strain or electric field has the potential to enhance bone formation and implant osseointegration. Therefore, in the present study, the objective is to investigate bone remodeling under electromechanical stimulation as a step towards establishing therapeutic strategies. For the first time, piezoelectric bone remodeling in the human proximal tibia under electro-mechanical loads was analyzed using the finite element method in an open-source framework. The predicted bone density distributions were qualitatively and quantitatively assessed by comparing with the computed tomography (CT) scan and the bone mineral density (BMD) calculated from the CT, respectively. The effect of model parameters such as uniform initial bone density and reference stimulus on the final density distribution was investigated. Results of the parametric study showed that for different values of initial bone density the model predicted similar but not identical final density distribution. It was also shown that higher reference stimulus value yielded lower average bone density at the final time. The present study demonstrates an increase in bone density as a result of electrical stimulation. Thus, to minimize bone loss, for example, due to physical impairment or osteoporosis, mechanical loads during daily physical activities could be partially replaced by therapeutic electrical stimulation.

Hydro-chemo-mechanical phase field formulation for corrosion induced cracking in reinforced concrete

Steel corrosion induced cracking in reinforced concrete structures is often caused by volumetric straining at the steel-concrete interface. This volumetric strain is mainly controlled by the rate of generation of corrosion product, which depends on the diffusion of Cl− and dissolved O2 as well as the chemical reaction at the steel-concrete interface. While many numerical modeling techniques have been proposed to tackle this complex phenomenon, most have been limited to using staggered solution schemes, where the diffusion, chemical reaction, mechanical strain, and crack fields are solved for separately. Unfortunately, such approaches introduce numerical errors that can lead to unrealistic predictions. Therefore, a coupled solution, is needed to remove the formulation induced errors and understand this complex phenomenon.To provide a unifying solution framework for corrosion induced cracking in reinforced concrete, a phase field formulation is presented and implemented with FEM algorithms. The governing equations are derived from the system's free energy and Fick's law of diffusion. The proposed formulation was first validated in 1D by comparing the modeling results with the semi-analytical solution approximated via Laurent series to avoid singularities in the complex domain. 2D plane strain solutions to the embedded rebar undergoing corrosion induced cracking with pre-cracks are also presented and compared with the approximated solutions from linear elastic fracture mechanics theory (LEFM). The modeling of crack propagation enabled by the proposed formulation is also presented along with parametric studies to reveal the roles of the fracture toughness and permeability of the concrete on the corrosion induced cracking. A case study was also conducted on H-pile steel with concrete jackets to demonstrate the feasibility of modeling corrosion induced fracture in reinforced concrete structures with complex steel-concrete interfaces.

Mechanical Strain and Electric Field Tunable Electronic Structure of Black/Violet Phosphorene Van Der Waals Heterostructure: From Type I to Z-Scheme System

Mechanical Strain and Electric Field Tunable Electronic Structure of Black/Violet Phosphorene Van Der Waals Heterostructure: From Type I to Z-Scheme System

Strain and electric-field control of spin-spin interactions in monolayer CrI3

We investigate the impact of mechanical strains and a perpendicular electric field on the electronic and magnetic ground-state properties of two-dimensional monolayer CrI$_3$ using density functional theory. We propose a minimal spin model Hamiltonian, consisting of symmetric isotropic exchange interactions, magnetic anisotropy energy, and Dzyaloshinskii-Moriya (DM) interactions, to capture most pertinent magnetic properties of the system. We compute the mechanical strain and electric field dependence of various spin-spin interactions. Our results show that both the amplitudes and signs of the exchange interactions can be engineered by means of strain, while the electric field affects only their amplitudes. However, strain and electric fields affect both the directions and amplitudes of the DM vectors. The amplitude of the magnetic anisotropy energy can also be substantially modified by an applied strain. We show that in comparison with an electric field, strain can be more efficiently used to manipulate the magnetic and electronic properties of the system. Notably, such systematic tuning of the spin interactions is essential for the engineering of room-temperature spintronic nanodevices.