Mechanical Stress Field Research Articles

Analysis of laminated composites and sandwich structures by trigonometric, exponential and miscellaneous polynomials and a MITC9 plate element

This paper proposes some advanced plate theories obtained by expanding the unknown displacement variables along the thickness direction using trigonometric series, exponential functions and miscellaneous polynomials. The used refined models are Equivalent Single Layer (ESL) theories. They are obtained by means of the Unified Formulation by Carrera (CUF), and they accurately describe the displacement field and the stress distributions along the thickness of the multilayered plate. The governing equations are derived from the Principle of Virtual Displacement (PVD), and the Finite Element Method (FEM) is employed to solve them. The plate element has nine nodes, and the Mixed Interpolation of Tensorial Components (MITC) method is used to contrast the membrane and shear locking phenomenon. Cross-ply plates with simply-supported edges and subjected to a bi-sinusoidal load, and sandwich plates with simply-supported edges and subjected to a constant transverse uniform pressure are analyzed. Various thickness ratios are considered. The results, obtained with different theories within CUF, are compared with the elasticity solutions given in the literature and the layer-wise solution. It is shown that refined kinematic theories employing trigonometric or exponential terms are able to accurately describe the displacement field and the mechanical stress fields. In some cases, the reduction of computational costs is particularly relevant respect to the layer-wise solution.

Non-local behavior of two collinear mixed-mode limited-permeable cracks in a functionally graded piezoelectric medium

In this paper, the problem of two collinear mixed-mode limited-permeable cracks embedded in an infinite medium made of a functionally graded piezoelectric material (FGPM) with crack surfaces subjected to electro-mechanical loadings is investigated. Eringen's non-local theory of elasticity is adopted to formulate the governing electro-elastic equations. The properties of the piezoelectric material are assumed to vary exponentially along a perpendicular plane to the crack. Employing the Fourier transform, the mixed-boundary value problem is converted into three integral equations for each crack, with the unknown variables being the jumps of mechanical displacements and electric potentials across the crack surfaces. To solve the integral equations, the unknowns are directly expanded as a series of Jacobi polynomials, and the resulting equations solved using the Schmidt method. In contrast to the classical solutions based on the local theory, it is found that no mechanical stress and electric displacement singularities are present at the crack tips when nonlocal theory is employed to investigate the problem. A direct benefit is the ability to use the calculated maximum stress as a fracture criterion. The primary objective of this study is to investigate the effects of the interaction of two cracks, material gradient parameter describing functionally graded piezoelectric materials and lattice parameter on the mechanical stress and electric displacement field near crack tips.

Chemically driven tetragonal-to-monoclinic polymorphic transformation in retrieved ZTA femoral heads from dual mobility hip implants

Two short-term (two and nine months) retrieved zirconia-toughened alumina (ZTA) femoral heads and nine pristine femoral heads from the same manufacturer have been investigated with respect to their surface stability by means of confocal Raman spectroscopy. Quantitative estimations of monoclinic volume fraction have been carried out in both non-wear and main wear zones of the retrieved heads, which invariantly showed high volume fractions of monoclinic polymorph. In-depth (sub-surface) profiles, non-destructively collected in the main wear zones with the Raman probe in confocal configuration, indeed confirmed that polymorphic transformation was extended down to 100μm below the bearing surface of the femoral heads. Acceleration of tetragonal-to-monoclinic transformation rate leads to unexpectedly high fractions of monoclinic phase within very short-term in-vivo exposures. Phase transformation in-vivo is much more marked than what one could actually predict according to simply simulating a hydrothermal environment in-vitro and could not be simply ascribed to the mechanical stress fields generated during normal service at the bearing surface. Instead, the chemical consequences of metal contamination on the ZTA femoral head surface are shown to play the most detrimental role in phase destabilization.

Development of a method for evaluation of the life curve of the HV and EHV cable polymeric insulation: The first results and fields of application

A test procedure and an experimental setup are presented that enable derivation of the “life curve,” i.e., the electrical tree induction period vs. applied voltage dependence for the low-density crosslinked polyethylene insulation. The developed equipment allows testing samples of materials both made in a laboratory and cut out from commercially manufactured cables at room temperature and at long-term permissible and overload temperatures. The fact of the electrical tree inception is established by light microscopy directly during simultaneous high-voltage test of ten samples. The optical system and the applied micromanipulation technique provide spatial resolution of approximately 1.5 μm, high-precision delivery of the microelectrode to the area of interest inside the sample, a permanent and reliable contact between the microelectrode and the material, and preservation of the residual mechanical stress field. The obtained results can be applied to development of new high-voltage cables with polymer insulation. The derived “life curve” enabled—using the corresponding mathematical model—calculation of the electric insulation thickness, which reaches 19–20 mm for the 220-kV cables.

A Biological Model of Acupuncture and its Derived Mathematical Modeling and Simulations

Abstract(Aims) Acupuncture was employed since 2 millenaries, but the underlying mechanisms are not globally handled. The present study is aimed at proposing an explanation by pointing out involved processes and a convincing modeling to demonstrate its efficiency when carried out by trained practitioners.(Method) In the absence of global knowledge of any mechanism explaining the acupuncture process, a biological model is first developed, based on stimulation in a given domain around the needle tip of a proper mastocyte population by a mechanical stress, electrical, electromagnetic, or heat field. Whatever the type of mechanical or physical stimuli, mastocytes degranulate. Released messengers either facilitate the transfer of main mediators, or target their cognate receptors of local nerve terminals or after being conveyed by blood their receptors on cerebral cells. Signaling to the brain is fast by nervous impulses and delayed by circulating messengers that nevertheless distribute preferentially in the brain region of interest due to hyperemia. The process is self-sustained due to mastocyte chemotaxis from the nearby dense microcirculatory circuit and surrounding mastocyte pools, which are inadequate for acupuncture, but serve as a signal relay. A simple mathematical model is solved analytically. Numerical simulations are also carried out using the finite element method with mesh adaptivity.(Results) The analytical solution of the simple mathematical model demonstrates the conditions filled by a mastocyte population to operate efficiently. A theorem gives the blow-up condition. This analytical solution serves for validation of numerical experiments. Numerical simulations show that when the needle is positioned in the periphery of the acupoint or outside it, the response is too weak. This explains why a long training is necessary as the needle implantation requires a precision with a magnitude of the order of 1mm.(Conclusion) The acupoint must contain a highly concentrated population of mastocytes (e.g., very-high–amplitude, small-width Gaussian distribution) to get an initial proper response. Permanent signaling is provided by chemotaxis and continuous recruitment of mastocytes. Therefore, the density and distribution of mastocytes are crucial factors for efficient acupuncture as well as availability of circulating and neighboring pools of mastocytes.

Structural changes in Bi-43 wt % Sn eutectic alloy under superplastic deformation

Methods of scanning electron microscopy have been used to study the microstructure of superplastically deformed samples of eutectic alloy Bi-43 wt % Sn. The observed specific features of the deformation relief of the samples reveal the active development of the viscous dislocation-diffusion flow under superplasticity conditions. The manifestation of the hydrodynamic mode of deformation has been revealed under these conditions. The opportunity of the realization of viscous mechanisms of the transport of substance and of the manifestation of the effect of superplasticity are explained by the appearance in the material of a state that is characterized by a high dislocation density and low strength properties. An additional increase in the dislocation density and softening under superplasticity conditions are attributed to the occurrence of structural and phase transformations stimulated by deformation, the relaxation of significant internal elastic stresses, and the instability of the structural state of the initially nonequilibrium alloy in the field of mechanical stresses. Factors responsible for the appearance of significant internal elastic stresses in the alloy are analyzed.

Processing of energy materials in electromagnetic field

This paper presents the research results of complex impact of mechanical stress and electromagnetic field on the defect structure of energy materials. As the object of research quite a typical energy material - silver azide was chosen, being a model in chemistry of solids. According to the experiments co-effect of magnetic field and mechanical stress in silver azide crystals furthers multiplication, stopper breakaway, shift of dislocations, and generation of superlattice dislocations - micro-cracks. A method of mechanical and electric strengthening has been developed and involves changing the density of dislocations in whiskers.

Self-assembly of nanostructures and nanomaterials.

Presently, most microelectronic devices are fabricated using top-down approaches. According to Moore’s law, with the predicted ultimate reduction in size over the next years, these processes will reach the limits of technological instrument resolution. In addition, a major bottleneck in top-down approaches is the prohibitive cost due to the large number of technological steps required to reduce the device size. Furthermore, a broad range of applications require ultrasmall, complex devices that cannot be produced using top-down methods. New processes building on the natural self-organization of matter should therefore be conceived and developed, along with adequate characterization methods in order to allow for their application in innovative devices. Such self-organization processes are already implemented in various materials such as biological materials, soft matter, metals and semiconductors. Self-assembly is a process that builds an ordered structure, brick-by-brick, starting from disordered building blocks, using simple key ingredients. Self-assembly is commonly controlled by certain intrinsic material parameters (e.g., composition, strain, thickness, phase transformation, structural changes) and results from the interaction between different factors (e.g., deposit/substrate, liquid/gas/solid phases, crystals). Besides these intrinsic parameters, a number of extrinsic factors, including thermal treatment, chemical and electrochemical reactions, mechanical stress, electric or magnetic fields, can strongly influence the self-assembled morphologies. Self-assembly processes are generally low cost, large-scale techniques, which can be suitable for various industrial environments. Identified as one of the key topics in nanoscience with potential to shape future scientific research, self-assembly is the most promising path to breakthroughs in nanoelectronics, optoelectronics, spintronics, molecular nanotechnology, biology, materials science, software, robotics, manufacturing, transportation, energy harvesting, infrastructure and construction. Research on self-assembled nanostructures encompasses fundamental issues in chemical synthesis, crystal growth and self-organization of 0D, 1D and 2D nanostructures, nanopatterning, lithographic techniques, nanocharacterization, scaling of materials properties down to molecular dimensions, quantum properties and applications of nanoscale assemblies to advanced devices. The main topics of interest involve 2D nanomaterials such as nanomembranes, graphene, silicene and ordered mesoporous oxides, 1D nanomaterials such as nanowires and nanotubes, and 0D nanomaterials such as quantum dots, nanocrystals and Q-bits. Focusing on this dynamic new field, self-assembly “the science of things that put themselves together” explores the physics of nanostructures, new synthesis approaches, in addition to size-, shape- and composition-dependent properties. The major obstacles concern the reproducibility and control of the basic mechanisms in order to predict and produce patterns with tunable size, periodicity and position and the new physical properties resulting from low dimensionality. The goal of this Thematic Series is to bring together studies in the broad field of research on self-assembled nanostructures, which may seem far away, but are expected to promote an extraordinary exchange of ideas appropriate to conceive multidisciplinary processes suitable for various materials with the aim to produce exceptional new nanomaterials with revolutionary properties. This Thematic Series covers the physics of nanostructures at the nanoscale including: large-scale patterning obtained by spontaneous structuring as well as local probe nanopatterning for nanostructure size and position control; theoretical and experimental efforts dedicated to a better understanding of the formation, evolution, and organization of nanoscale systems; fundamental and new issues in nucleation, crystal growth, surface and interface atomistic mechanisms; and new optical, electrical, magnetic, and mechanical properties of self-assembled systems. Isabelle Berbezier and Maurizio De Crescenzi Marseille and Rome, June 2015

Wide Compositional RangeIn SituElectric Field Investigations on Lead-FreeBa(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3Piezoceramic

Applications ranging from microphones to AFM transducers exploit the interplay between mechanical stress and electric field arising from structural transitions in certain perovskite oxides. The classic material contains lead, however, and efficient, environmentally friendly alternatives have long been sought. The authors use a challenging technique to visualize the real-time microstructural evolution of a lead-free ferroelectric system, and the multidomain states and transitions they find are expected to inform the search for additional systems, as well as the applications that will rely on them.

Pulmonary Atelectrauma. Biophysical Insights into Mechanisms of Damage and Prevention

Mechanical ventilation is inarguably a necessary life-sustaining medical treatment for acute respiratory distress syndrome. However, mechanical stresses and strains exerted on the delicate tissues of the airways and alveoli may result in ventilation-induced lung injury (VILI). This presentation describes the coupling between fluid flows, mechanical stresses, and surfactant biophysical interactions that occurs during the recruitment of obstructed airways and alveoli that can result in atelectrauma. Our investigations couple computational simulation with in vitro experimental investigations. We idealize the system as an airway that is lined with epithelial cells, with reopening occurring through the migration of a finger of air that displaces the obstruction. Mechanical stress fields and surfactant transport processes are assessed by computational fluid dynamic simulation, flow visualization is completed with microparticle image velocimetry, and cell damage is evaluated using biomimetic airway constructs of respiratory bronchioles. These studies demonstrate the strong coupling between interfacial flows and atelectrauma. Finally, we asked whether it is possible to enhance the surface activity of endogenous surfactant, using unsteady flows to reduce atelectrauma. Our investigations demonstrate that the use of pulsatile flow to enhance surfactant function provides an opportunity for “endogenous surfactant delivery” that diminishes epithelial damage and thus may reduce VILI.

Mechanical stress field assisted charge de-trapping in carbon doped oxides

Leakage current and dielectric breakdown effects are conventionally studied under electrical fields alone, with little regard for mechanical stresses. In this letter, we demonstrate that mechanical stress can influence the reliability of dielectrics even at lower field strengths. We applied tensile stress (up to 8MPa) to a 33% porous, 504nm thick carbon doped oxide thin film and measured the leakage current at constant electrical fields (up to 2.5MV/cm). The observed increase in leakage current at relatively low electric fields suggests that mechanical stress assists in trap/defect mediated conduction by reducing the energy barrier potential to de-trap charges in the dielectric.

Caloric and Multicaloric Effects in Shape Memory Alloys

Caloric properties rely on the reversible thermal response of a given material to changes induced by an applied field. In magnetic shape-memory materials, thanks to a strong interplay between magnetism and structure, caloric effects can be induced by application of both magnetic and mechanical (stress) fields. The study of such effects is an expanding field of research opening novel opportunities in solid state refrigeration and energy harvesting applications. In this paper we present an overview of the present state of the art of the subject. After a brief discussion of the thermodynamics of systems with magneto-structural interplay, we survey the caloric and multicaloric properties of NiMn-based Heusler materials. The conditions required for optimal caloric efficiency in cycling processes are considered. Finally, we outline some present challenges and future perspectives in this topic.

Analytic and numeric solution of a magneto-mechanical inclusion problem

In this paper, exact solutions for the primary field variables and their first derivatives are derived for the coupled magneto-mechanical problem of a circular inclusion embedded in an infinite surrounding. Both material domains possess linear and isotropic material properties. The assumed planarity of all fields enables and recommends the description with analytic functions depending on the complex variable z. The well-known technique of a complex stress potential satisfying the bipotential equation is used and adapted to coupled magneto-mechanical problems. The provided analytic expressions are of special interest for the validation of numeric solutions of coupled magneto-mechanical boundary value problems. In this contribution, they are applied to analyze the convergence behavior of an extended finite element formulation for coupled magneto-mechanical problems. Based on the analytic results, proper boundary conditions are prescribed to the numeric model. The convergence in terms of polynomial degree and mesh refinement of the implemented element formulation is proved by computing the L2 and the energy norm which requires the knowledge of the exact solution. The generality of the proposed formulas allows for the deduction of some other kind of problems, e.g., the pure mechanical displacement and stress field of a bimaterial setting or the stress concentration around a circular hole.

High temperature oxidation of MCrAlY coatings modified by Al2O3 PVD overlay

In this study the high temperature oxidation behavior of NiCoCrAlTaY coatings modified by a PVD (physical vapor deposition) surface treatment was evaluated: the coatings, produced by HVOF (high velocity oxygen fuel) technique, were protected by an Al2O3 PVD sputtered overlay prior to top coating with an YSZ ceramic layer, and were exposed in air at 1100°C up to 400h.The oxidation kinetics were evaluated by measuring the thickness of the TGO (thermally grown oxide) scale at several exposure times and the microstructural evolution induced by the heat treatments was investigated. The effect of the presence of the Al2O3 PVD overlay, in terms of microstructure and oxidation resistance, was assessed by comparing the performance of modified TBCs with unprotected NiCoCrAlTaY systems. Experimental evidence was gathered showing that the presence of an Al2O3 PVD overlay enhances the oxidation resistance of NiCoCrAlTaY coatings and promotes the adhesion between bond and top coat.A thermo-mechanical finite element model of the modified TBC system was also developed: the growth of the oxide scale was modeled and the previously assessed oxidation kinetics were implemented in order to estimate the effects of the PVD film on the mechanical stress field of the TBC system.

Quantitative Characterization of Mechanical Stress Field and Fracture Strength in Isotropic Brittle Materials During Crack Tip Propagation

A generalized method for characterizing mechanical stresses in brittle materials during crack tip propagation is presented. This approach was derived from classical fracture mechanics and is therefore mechanism independent and applies to any isotropic brittle material, including glasses, fine grained ceramics or metals, and high stiffness polymers. A practical implementation demonstrates the merits of this technique: the fracture strength can be determined by characterizing the angle between the free surface of a flexural overload fracture and stress intensity factor loci. The accuracy of this method was phenomenologically validated using flexural strength tests on glass as a model material system. In addition, such fractographic measurements can also be used to characterize an inhomogeneous internal stress field, and thereby, for example, help discriminate whether the sample failed due to pure bending loads alone, or whether membrane stresses were also present.

Oxygen gettering in low‐energy arsenic or antimony ion implanted Cz‐silicon

AbstractLow energy As or Sb ion implantation followed by furnace annealing was used to create ultra shallow junctions. It was found that a significant amount of oxygen was redistributed from the Si bulk to the As implanted layer leading even to an increase of the screening oxide film thickness. Using a marker layer created by implantation of 18O ions, it was confirmed that a large number of interstitial oxygen atoms are transferred from the bulk of the Si wafer to the wafer surface during implanted As activation annealing. Estimation of the O diffusivity in Si during the 950 °C activation anneal, yields a value of about 10–10 cm2 /s which is more than an order of magnitude larger than the literature value which is close to 7 × 10–12 cm2/s. In the case of Sb implantation, the oxygen gettering effect is much reduced. This difference in oxygen behavior can be attributed to the influence of mechanical stress in the near surface highly doped layer. The configuration of the mechanical stress field is different for the cases of As and Sb implantation, with tensile stress dominating for the latter. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of the behavior of a mixed-mode crack in a functionally graded magneto–electro-elastic material by use of the non-local theory

In this paper, we consider the problem of a mixed-mode crack embedded in an infinite medium made of a functionally graded magneto–electro-elastic material (FGMEEM) with the crack surfaces subjected to magneto–electro-mechanical loadings. Eringen’s non-local theory of elasticity is applied to obtain the governing magneto–electro-elastic equations. To make the analysis tractable, it is assumed that the magneto–electro-elastic material properties vary exponentially along a perpendicular plane to the crack. Using Fourier transform, the resulting mixed-boundary value problem is converted into four integral equations, in which the unknown variables are the jumps of mechanical displacements, electric and magnetic potentials across the crack surfaces. To solve the integral equations, the jumps of displacements and electric and magnetic potential across crack surfaces are directly expanded in a series of Jacobi polynomials and the resulting equations are solved using the Schmidt method. Unlike classical magnetic, electric and elasticity solutions, it is found that no mechanical stress, electric displacement and magnetic flux singularities are present at the crack tips. This enables the use of the maximum stress as a fracture criterion. The primary objective of this study is to investigate the effects of crack length, material gradient parameter describing functionally graded materials and lattice parameter on the mechanical stress, magnetic flux and electric displacement field near crack tips.

Mechanical and Electrical Control of Charged Domain Walls in Ferroelectric Materials

Charged domain walls (CDWs) in ferroelectrics, as a result of “head-to-head” or “tail-to-tail” polarization configurations, are of significant scientific and technological importance, as they have been shown to play a critical role in affecting the atomic structures and controlling the electric, photoelectric and piezoelectric properties of ferroelectric materials. An understanding of the local response and underlying dynamic mechanism of CDWs under external excitations is necessary to engineer reliable ferroelectric devices. In this work, we present the nanoscale behavior of individual CDWs under applied mechanical stress and electric fields in a BiFeO3 thin film using in situ transmission electron microscopy (TEM).

Localizing sources of acoustic emission during the martensitic transformation

Acoustic avalanches are a general feature of solids under stress, e.g., evoked by external compression or arising from internal processes like martensitic phase transformations. From integral measurements, it is usually concluded that nucleation, phase boundary pinning, or interface incompatibilities during this first-order phase transition all may generate acoustic emission. This paper studies the local sources of acoustic emission to enlight the microscopic mechanisms. From two-dimensional spatially resolved acoustic emission measurement and simultaneous optical observation of the surface, we can identify microstructural events at the phase boundary that lead to acoustic emission. A resolution in the $100\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$ range was reached for the location of acoustic emission sources on a coarse-grained Ni-Mn-Ga polycrystal. Both, the acoustic activity and the size distribution of the microstructural transformation events, exhibit power-law behavior. The origin of the acoustic emission are elastically incompatible areas, such as differently oriented martensitic plates that meet each other, lamellae growing up to grain boundaries, and grain boundaries in proximity to transforming grains. Using this result, we propose a model to explain the decrease of the critical exponent under a mechanical stress or magnetic field.

The effect of a magnetic field on the rheological properties of iron-aerosil-glycerol suspensions

The viscosity of a suspension consisting of iron nanoparticles, aerosil, and glycerol has been studied under the conditions of a rotational flow in the presence and absence of a magnetic field. At low shear rates, the viscosity is governed by the magnetic field and increases 40 times with a rise in the field strength; at high shear rates, the effect of the mechanical stress field prevails. In this case, the relative viscosity increases by 1.4 times, and its concentration dependence in the magnetic field is described by a curve passing through a maximum.