Mechanical Boundary Conditions Research Articles

Electro-thermo-mechanical multiple fields coupled guided wave in a piezoelectric semiconductor plate

Based on a fully coupled model of electro-thermo-mechanical multiple physical fields, the guided wave in a piezoelectric semiconductor (PSC) plate is studied. The non-Fourier heat conduction (G-N III-type) and non-Fick diffusion are included in the model. By the appropriate definition of the extended displacements and stresses, the system of second-order differential coupled equations degrades to a first-order matrix differential equation. The transfer matrix of the PSC plate is derived from the first-order matrix differential equation. Some combinations of mechanical, electric and carrier boundary conditions are applied to obtain the dispersion relations. The influences of various boundary conditions and typical parameters on the dispersion and attenuation are investigated numerically. The gradient profiles of the inhomogeneous plate are also discussed. Based on the numerical results, it is found that the dispersion characteristics are mainly affected by the mechanical boundary conditions, while the attenuation characteristics are mainly affected by the transport of carrier and heat. In addition, the gradient profiles of inhomogeneous plates affect both dispersion and attenuation characteristics.

Thermal Stress Formation in a Functionally Graded Al2O3-Adhesive Single Lap Joint Subjected to a Uniform Temperature Field

This study investigates the strain and stress states in an aluminum single lap joint bonded with a functionally graded Al2O3 micro particle reinforced adhesive layer subjected to a uniform temperature field. Navier equations of elasticity theory were designated by considering the spatial derivatives of Lamé constants and the coefficient of thermal expansion for local material composition. The set of partial differential equations and mechanical boundary conditions for a two-dimensional model was reduced to a set of linear equations by means of the central finite difference approximation at each grid point of a discretized joint. The through-thickness Al2O3-adhesive composition was tailored by the functional grading concept, and the mechanical and thermal properties of local adhesive composition were predicted by Mori–Tanaka’s homogenization approach. The adherend–adhesive interfaces exhibited sharp discontinuous thermal stresses, whereas the discontinuous nature of thermal strains along bi-material interfaces can be moderated by the gradient power index, which controls the through-thickness variation of particle amount in the local adhesive composition. The free edges of the adhesive layer were also critical due to the occurrence of high normal and shear strains and stresses. The gradient power index can influence the distribution and levels of strain and stress components only for a sufficiently high volume fraction of particles. The grading direction of particles in the adhesive layer was not influential because the temperature field is uniform; namely, it can only upturn the low and high strain and stress regions so that the neat adhesive–adherend interface and the particle-rich adhesive–adherend interface can be relocated.

Experimental investigations on early crack development in fully restrained reinforced concrete members with active zero-displacement control

The development of restraint-induced stress in reinforced concrete members at an early age can be tested using setups applying either passive or active displacement control. To provide results relevant for the practical design of minimum reinforcement to limit the crack width, an actively controlled test setup for large specimens was developed to perform a systematic experimental campaign focusing on the early crack development under well-defined mechanical and thermal boundary conditions. The full degree of restraint achieved by actively preventing any displacement of the member allowed for the theoretical interpretation of early restraint-induced stress evolution, deformation behaviour in the cracked and uncracked regions, and reinforcement stress for a variety of parameters, including hydration rates, member height, bar diameters and subsequently reinforcement ratio. The cracking process, starting with primary cracks accompanied by secondary cracks within the effective reinforcement area, is monitored in combination with the development of the tensile strength and Young’s modulus of the concrete. Furthermore, the time of cracking and the forces released upon crack occurrence, considering the effect of self-equilibrating stresses, are determined, analysed, and compared with the results of crack-force based design rules to determine the minimum amount of reinforcement that ensures the crack width limitation.

Dynamic Control of Contractile Force in Engineered Heart Tissue

Here, we demonstrate a technique to control afterload while monitoring contractile force exerted by EHTs. We developed an apparatus that can regulate EHT boundary conditions using real-time feedback control. The system is comprised of a pair of piezoelectric actuators that can strain the scaffold and a microscope that can measure EHT force and length. Closed loop control allows dynamic regulation of effective EHT boundary stiffness. When controlled to switch instantaneously from auxotonic to isometric boundary conditions, EHT twitch force immediately doubled. Changes in EHT twitch force as a function of effective boundary stiffness were characterized and compared to twitch force in auxotonic conditions. EHT contractility can be regulated dynamically through feedback control of effective boundary stiffness. The capacity to alter the mechanical boundary conditions of an engineered tissue dynamically offers a new way to probe tissue mechanics. This could be used to mimic afterload changes that occur naturally in disease, or to improve mechanical techniques for EHT maturation.

Comparative analysis of dynamic stability of cylindrical and conical shells under periodic axial compression

A comparative analysis of the dynamic stability of cylindrical and conical shells with the same geometric and mechanical characteristics under periodic uniformly distributed axial compression was presented. The study of the stability of steady periodic vibrations of thin elastic shells was based on the joint use of the method of curvilinear grids, the projection method and the parameter continuation method combined with the Newton–Kantorovich method. Geometrically nonlinear relations of the thin elastic shells theory are formulated on the basis of the vector approximation of the displacements function in the general curvilinear coordinate system in tensor form and satisfy the Kirchhoff-Love hypothesis. The discretization of the differential equations of the steady forced vibrations in the direction of the generating shells using the method of curvilinear grids was carried out. The components of the elements displacement vectors of the shells middle surface in the circular directionare approximated by trigonometric series. Reduction of the number of generalized coordinates of the discrete dynamic model of shells steady forced vibrations was performed by the Bubnov-Galerkin basis reduction method. A transition from vector ordinary differential equations to a nonlinear system of algebraic equations was made. The construction of a mathematical model of the dynamic stability of steady forced nonlinear vibrations of thin elastic shells was performed according to Floquet's theory using the projection method. The criterion for the loss of stability was the equality to zero of the determinant of the matrix of linearized equations of steady forced nonlinear vibrations of shells according to the Lyapunov theorem. A comparative analysis of frequencies and modes of natural vibrations of cylindrical and conical shells with the same geometric and mechanical characteristics and boundary conditions was performed. Nonlinear steady vibrations of the shells due to periodic axial compression were studied. The critical values of the dynamic load and the corresponding forms of loss of shell stability in the range of lower frequencies of their natural vibrations were obtained.

Calculation of thermal-hydraulic-mechanical response of a deep geological repository for radioactive used fuel in granite

The NWMO is considering siting a deep geological repository (DGR) for used fuel in a crystalline or sedimentary rock setting. One aspect of the long-term performance is the Thermal-Hydraulic-Mechanical (THM) response of the rock caused by the heat released from the high-level radioactive used fuel placed in the repository. In this paper, a method was proposed by building an integrated coupled THM model (Integrated model) to explore the importance of scale in modelling the host rock THM response in a crystalline rock setting. This integrated model included a site-scale domain of rock, with embedded large fracture zones (FZ). The full DGR geometry was represented by using large flat blocks to represent the bulk of the repository panels, with five placement rooms modelled explicitly. In one of these five placement rooms, a 9.24-m-long placement room section with 14 containers was modelled in detail. A preliminary study of the THM response of the rock mass around the proposed repository was performed using this model and its results were validated by comparing with accurate thermal response. The results from this integrated model were compared with the results from a near-field coupled THM model; and the comparison proves that the near-field model cannot give the correct THM response in a repository. To accurately model the THM response in a DGR, the proposed integrated model approach can be used to avoid the influence of the thermal, hydraulic and mechanical boundary conditions applied on near-field models.

Analysis and Design of the High Current Rising Rate Hybrid DC Current Limiting Circuit Breaker

To solve the problem of the high rising rate and large peak value of the expected current of the short-circuit current in marine DC power system faults, a hybrid DC current limiting circuit breaker scheme based on a high-speed electromagnetic repulsion mechanism is proposed. A parameter selection model is constructed by comprehensively considering the short-time withstand of the thyristor, the volume of the commutation circuit, and capacitor energy, and the optimal value of the commutation circuit parameters at a certain voltage level is obtained. The finite element mathematical model of the high-speed electromagnetic repulsion mechanism is established by coupling the electromagnetic force field, which enables the deformation process of the mechanism under the condition of high acceleration to be considered. The von Mises yield criterion is adopted as the mechanical boundary condition in the design of a high-speed electromagnetic repulsion mechanism, which solves the problem of the long inherent time of opening. The experiment platform is built, and the experiment under the fault condition with a current rising rate of 20 A/μs is completed. The arcing time, commutation time, zero-voltage recovery time, and contact movement characteristics are obtained, which meet the design requirements, verify the effectiveness of the analysis, and lay a solid foundation for further research and development of the current limiting circuit breakers for medium voltage DC systems.

Hybrid fire testing using FMI-based co-simulation

The failure mode of a structural component subjected to fire loading is sensitive to its mechanical boundary conditions. It follows that controlling boundary conditions is crucial to obtain a realistic loading scenario with a fire experiment. Hybrid fire testing has been developed for this purpose. Specifically, a structural component under test is loaded using actuators and exposed to fire loading. A coordination algorithm updates actuator setpoints on the fly so that the tested structural component experiences the sought mechanical boundary conditions, e.g., associated with a virtual assembly that is simulated numerically.To enable interoperability of simulation tools and control systems utilized in hybrid testing, a number of middlewares have been proposed, some of which have been adapted to hybrid fire testing. Middlewares facilitate the implementation of hybrid fire testing within the same laboratory. However, the portability of hybrid models from one laboratory to another is critical when different middleware and simulation tools are adopted. A consequence of that is that round-robin verification cannot be easily deployed for hybrid fire testing.In response to this limitation, this paper proposes to craft hybrid fire testing experiments using the Co-Simulation paradigm supported by the Functional Mock-up Interface standard. The methodology is demonstrated experimentally.

Enhanced Electrocaloric Response in Electrostatically Frustrated Ferroelectrics

The electrocaloric effect (i.e., the temperature change of a material upon the adiabatic application of an electric field) can be large in ferroelectric compounds, to the extent of offering an eco‐friendly alternative to current, polluting and noisy, refrigeration technologies. So far, ferroelectric‐based electrocaloric devices employ materials (from perovskite oxides to polymers) that have been known for decades. At the same time, recent studies have shown that the functional response of a ferroelectric can be drastically enhanced by controlling its mechanical and electric boundary conditions. Here, we use atomistic second‐principles simulations to quantify how much the electrocaloric effect benefits from such an enhancement. Using ferroelectric/dielectric superlattices as representative model systems, we predict that temperature changes can indeed be increased significantly (more than two times at room temperature), suggesting electrostatic engineering—to induce very reactive frustrated ferroelectric states—as a promising route for electrocaloric optimization.

Opto-mechanical self-adjustment model of the human eye.

The eye has specific optical and biomechanical properties that jointly regulate the eye's quality of vision, shape, and elasticity. These two characteristics are interdependent and correlated. Contrary to most currently available computational models of the human eye that only focus on biomechanical or optical aspects, the current study explores the inter-relationships between biomechanics, structure, and optical properties. Possible combinations of mechanical properties, boundary conditions, and biometrics were specified to ensure the opto-mechanical (OM) integrity to compensate for physiological changes in intraocular pressure (IOP) without compromising image acuity. This study evaluated the quality of the vision by analyzing the minimum spot diameters formed on the retina and drew how the self-adjustment mechanism affects the eye globe shape by adopting a finite element (FE) model of the eyeball. The model was verified by a water drinking test with biometric measurement (OCT Revo NX, Optopol) and tonometry (Corvis ST, Oculus).

Vibration characteristics of smart laminated carbon nanotube-reinforced composite cylindrical shells resting on elastic foundations with open circuit

This study analyzes the vibration of smart laminated carbon nanotube (CNT)-reinforced composite cylindrical shells integrated with piezoelectric materials and resting on elastic foundations. The effect of open circuit electrical boundary condition on vibration characteristics is investigated when a quadratic variation is considered for the electrical potential. Furthermore, the elastic foundations are included in the mathematical modeling to see their effects when the open circuit electrical boundary condition is applied. The first-order shear deformation shell theory and Maxwell’s static electricity equation are employed to derive the governing equations, and natural frequencies are calculated by solving an eigenvalue problem. Vibration characteristics with the open circuit electrical boundary condition are compared with those of closed (short) circuit electrical boundary condition for various mechanical boundary conditions and shell and piezoelectric geometrical parameters. The results demonstrate that the open circuit electrical boundary condition leads to higher frequencies than the closed circuit condition and it must be noticed in the design of smart composite structures with embedded or surface-bonded piezoelectric materials.

Thermodynamics of Light-Induced Nanoscale Polar Structures in Ferroelectric Superlattices.

We study the thermodynamics of nanoscale polar structures in PbTiO3/SrTiO3 ferroelectric superlattices induced by above-bandgap optical excitation using a phase-field model explicitly considering both structural and electronic processes. We demonstrate that the light-excited carriers provide the charge compensation of polarization bound charges and the lattice thermal energy, both of which are key to the thermodynamic stabilization of a previously observed supercrystal, a three-dimensionally periodic nanostructure, within a window of substrate strains, while different mechanical and electrical boundary conditions can stabilize a number of other nanoscale polar structures by balancing the competing short-range exchange interactions responsible for the domain wall energy and long-range electrostatic and elastic interactions. The insights into the light-induced formation and richness of nanoscale structures from this work offer theoretical guidance for exploring and manipulating the thermodynamic stability of nanoscale polar structures employing a combination of thermal, mechanical, and electrical stimuli as well as light.

Application of GDQ method to large amplitude response of FGM joined spherical-conical shells under rapid surface heating

Geometrically non-linear thermally induced vibrations of functionally graded material (FGM) joined spherical-conical shells are analyzed in the current research. Thermo-mechanical properties of the shells are assumed to be temperature and position dependent. The system of joined spherical-conical shells is subjected to thermal shock on the ceramic-rich surface, whereas the metal-rich one is kept at reference temperature. The one-dimensional transient heat conduction equation is established and solved via the generalized differential quadrature (GDQ) and Crank-Nicolson methods. This equation is non-linear since the thermo-mechanical properties of the shells are temperature dependent. The total functional of the shells is obtained under the assumptions of uncoupled thermoelasticity laws, first order shear deformation shell theory, and the von Kármán type of geometrical non-linearity. Non-linear coupled equations of motion are solved via the iterative Picard method accompanied with the β-Newmark time approximation technique. Numerical results are well-validated with the available data for the case of single FGM spherical shell. Parametric studies are conducted to examine the influences of conical and spherical shell geometries, material composition, temperature dependence, in-plane and out-of-plane mechanical boundary conditions, various configuration of conical/spherical shell system, and thermal boundary conditions. It is highlighted that thermally induced vibrations indeed exists.

Verification of deflections and cracking of RC flat slabs with numerical and analytical approaches

The Serviceability Limit States (SLS) of reinforced concrete structures, where cracking and deflection limits are verified in the design process, are directly related to time-dependent properties of concrete. In this study, the effects of creep and shrinkage of concrete, implemented in a Finite Element Model for Non-Linear Analysis, are in-deeply investigated for reinforced concrete flat slabs at SLS and used as basis for the development of an analytical framework. In particular, the paper focuses on: (i) the description of the FE model, (ii) the comparison between the numerical and experimental results of some reinforced concrete (RC) flat slab tests from literature characterized by different mechanical properties, span-to-depth ratios, boundary conditions and loading histories and (iii) the development of an analytical framework for the evaluation of long term-term deflections and crack openings in hogging regions of flat slabs near columns. Based on the findings, useful insights for the evaluation of the long term-term deflections and crack opening in the context of Serviceability Limit States are provided for practical purposes.

The Study of the Acoustic Characteristics of Chitosan Acetate Film Using a Radial Electric Field Excited Resonator.

Currently, the lateral electric field excited resonators are used for the creation of various sensors. We have recently proposed a new type of acoustic resonator called radial electric field excited disk acoustic resonator. The advantage of this type of resonator is its high sensitivity to mechanical and electrical boundary conditions on its free surface. This makes it possible to determine both the acoustic and electrical properties of a thin layer of material deposited on the free end of the resonator. In this work, we used a radial electric field excited disk acoustic resonator of Russian-made barium plumbum zirconate titanate (BPZT) piezoceramics. With the help of this resonator, the material constants for the piezoceramic sample were refined, and their temperature dependencies were determined. Then, this resonator was used to determine the elastic modulus, viscosity, and conductivity of the chitosan acetate film in air and ammonia vapors of various concentrations. It was shown that the chitosan acetate film under the influence of ammonia vapor significantly changes its mechanical properties and increases its electrical conductivity thousands of times, and then completely restores its properties.

Microtemperature photothermal excitation of semiconductor material influenced by electromagnetic Hall current and variable thermal conductivity

The effects of fluctuating thermal conductivity and Hall current in an elastic semiconductor material is studied. When the thermal conductivity is assumed to be a linear function of temperature, the medium is exposed to a very high magnetic field. It is investigated how the elastic medium's microstructure and the external magnetic field interact. The photothermal excitation mechanisms are the setting in which the microtemperature effect is produced. For a semiconductor rod, the overlapping of elastic, magnetic, plasma, and thermal waves is addressed in one dimension (1D). The governing equations of the microtemperature field are solved using the Laplace transform and a specific map in 1D. Analytically and in the physical time domain, the principal physical quantities in this phenomenon have their exact solutions. In the framework of the recombination plasma process, some mechanical boundary conditions with thermal shock are applied at the free surface of the elastic medium. To acquire the complete solutions of the major physical quantities, the Laplace transform is inverted and employed as a numerical method. Under the influence of fluctuating thermal conductivity in Hall current and the microtemperature effect, certain comparisons are done for the physical values that are shown graphically and theoretically addressed.

Mechanical boundary of motor proteins dictates dynamic cell functions.

Dynamic behaviors in cell functions are determined by the harmonized movement of cytoskeletal filaments and motor proteins. For example, actin filaments and motor protein myosin are arranged in the inner leaflet of the cell membrane in a two-dimensional ring shape in the process of cell division. Shrinkage of the ring divides a mother cell into two daughter cells. Many studies demonstrated how the contraction characteristics of actin-myosin assembly in terms of magnitude, speed, and direction are altered by proteins concentrations and their arrangement pattern. However, it remains unclear how the mechanical boundary condition determined at attachment site of protein affect the contractile behavior of actin-myosin assembly. In this study, we investigated the effect of mechanical boundary of motor protein myosin on contraction of actin network organized by actin binding protein. Using the agent-based simulation, Cytosim, we showed how the contraction speed and degree depend on the mechanical boundary condition between myosin and substrate. When myosin motility increased, both the contraction speed and effective area increased. The simulation results were compared with results of experiments where purified actin and myosin were organized on the membrane-coated substrate to control the motility of myosin. Finally, we developed a model to replace the dynamic process of actin structure occurred during cell division and muscle contraction by controlling myosin mechanical boundary conditions. We showed that the actin-myosin contraction regulated by the boundary condition of myosin is critical for proper functions of cell. Our results and methods will be beneficial in understanding the underlying mechanism of dynamics in cell functions and related diseases.

Novel electromechanical coupling theory of GaN HEMT structure under mechanical clamping

As known, the piezoelectric polarization effect makes GaN HEMT have unique electrical characteristics and wider applications compared with other III-V semiconductor devices. In this letter, a novel electromechanical coupling theory is proposed and verified for the first time for the GaN HEMT heterojunction structure by applying the mechanical clamping boundary conditions in piezoelectric constitutive equations. The total piezoelectric polarization defined as P couple in this new electromechanical coupling theory consist of three parts: the spontaneous polarization P sp, the strain-induced piezoelectric polarization P strain due to lattice mismatch in growth, and the bias-dependent piezoelectric polarization . The output and transfer characteristics of GaN HEMT have been simulated based on the new electromechanical coupling theory and verified in agreement with the experimental results in trend. This theory helps to extend the understanding of the basic physical mechanism of the AlGaN/GaN interface and is of great significance to device design and application for GaN-based materials.

Modeling How Interface Geometry and Mechanical Stress Affect Li Metal/Solid Electrolyte Current Distributions

We develop a coupled electrochemical-mechanical model to assess the current distributions at Li/single-ion conducting solid ceramic electrolyte interfaces containing a parameterized interfacial geometric asperity, and carefully distinguish between the thermodynamic and kinetic effects of interfacial mechanics on the current distribution. We find that with an elastic-perfectly plastic model for Li metal, and experimentally relevant mechanical initial and boundary conditions, the stress variations along the interface for experimentally relevant stack pressures and interfacial geometries are small (e.g., <1 MPa), resulting in a small or negligible influence of the interfacial mechanical state on the interfacial current distribution for both plating and stripping. However, we find that the current distribution is sensitive to interface geometry, with sharper (i.e., smaller tip radius of curvature) asperities experiencing greater current focusing. In addition, the effect on the current distribution of an identically sized lithium peak vs valley geometry is not the same. These interfacial geometry effects may lead to void formation on both stripping and plating and at both Li peaks and valleys. The presence of high-curvature interface geometry asperities provides an additional perspective on the superior cycling performance of flat, film-based separators (e.g., sputtered LiPON) versus particle-based separators (e.g., polycrystalline LLZO) in some conditions.

Monoclinic distortion in strain relaxed [111] PbTiO3 epitaxial films

Stabilization of monoclinic distortion in [111]-oriented strained and strain-relaxed PbTiO3 epitaxial films are investigated using a phenomenological model. Under the effect of biaxial strain due to SrTiO3, temperature stability of monoclinic structure induced by vertical compressive stress is investigated. Due to relaxation of substrate constraint in-plane misfit of the epitaxial system changes from isotropic to anisotropic. Strain relaxation mediated formation of monoclinic structure, polarization, volume and shear strain, and dielectric and piezoelectric constants are investigated and compared with experimental data in the literature. For anisotropically strained [111] epitaxial films mechanical boundary conditions that facilitate enhancement of physical constants are explored.