Laser-induced orientational nonlinear phenomena and new vortex structures

Theoretical study of the third-type thermomechanical effect in a twist cell of nematic liquid crystal is presented. The solution of director equation is obtained and optical nonlinearity is evaluated. It is proved that the nonlinearity is of the same order as in case of the direct orientational optical nonlinearity. Possibility of observing thermomechanical effect in non-uniform vortex structures is also discussed.     

Thermomechanical modeling of regressing heterogeneous solid propellants

A numerical framework based on the generalized finite element method (GFEM) is developed to capture the coupled effects of thermomechanical deformations and thermal gradients on the regression rate of a heterogeneous solid propellant. The thermomechanical formulation is based on a multiplicative split of the deformation gradient and regression of the heterogeneous solid propellant is simulated using the level set method. A spatial mesh convergence study is performed on a non-regressing solid heterogeneous propellant system to examine the consistency of the coupled thermomechanical GFEM solver. The overall accuracy (spatial and temporal) of the coupled thermomechanical solver for regressing solid propellants is obtained from a periodic sandwich propellant configuration, where the effects of thermomechanical deformations on its regression rate is investigated. Finally, the effects of thermomechanical deformations in a regressing two-dimensional heterogeneous propellant pack are studied and time-average regression rates are reported.     

Effect of ballistic electrons on ultrafast thermomechanical responses of a thin metal film

The ultrafast thermomechanical coupling problem in a thin gold film irradiated by ultrashort laser pulses with different electron ballistic depths is investigated via the ultrafast thermoelasticity model. The solution of the problem is obtained by solving finite element governing equations. The comparison between the results of ultrafast thermomechanical coupling responses with different electron ballistic depths is made to show the ballistic electron effect. It is found that the ballistic electrons have a significant influence on the ultrafast thermomechanical coupling behaviors of the gold thin film and the best laser micromachining results can be achieved by choosing the specific laser technology(large or small ballistic range).In addition, the influence of simplification of the ultrashort laser pulse source on the results is studied, and it is found that the simplification has a great influence on the thermomechanical responses, which implies that care should be taken when the simplified form of the laser source term is applied as the Gaussian heat source.     

Thermal expansion of SOFC materials

A short overview is given for the thermal expansion of solid oxide fuel cell materials. The thermomechanical compatibility of state-of-the-art materials is compared with alternative, new materials. With these alternatives a better adjustment of the thermal expansion coefficients of the various fuel cell components is possible and fuel cells based on the newly developed materials are proposed.     

The influence of particle distribution and interphase on the thermal expansion coefficient of particulate composites by the use of a new model

In this work, the thermal expansion coefficient (CTE) of a composite containing spherical particles surrounded by an inhomogeneous interphase embedded in an isotropic matrix is evaluated by means of a new model. The thermomechanical properties of the interphase are formulated as continuous radial functions. It is assumed that this third phase developed between the polymeric matrix and the filler particles contains both areas of absorption interaction in polymer surface layers onto filler particles as well as areas of mechanical imperfections. It can be said that the concept of boundary interphase is a useful tool to describe quantitatively the adhesion efficiency between matrix and particles and that there is an effect of this phase on the thermomechanical properties of the composite. The thickness and volume fraction of this phase were determined from heat capacity measurements for various filler contents. On the other hand, it is assumed that the particle arrangement (distribution) which can be considered as an influence of neighboring inclusions and their interaction should affect the thermomechanical constants of the composite. The theoretical predictions were compared with experimental results as well as with theoretical values from expressions obtained from other workers and they were found to be in satisfactory agreement.     

A molecular dynamics investigation of the deformation mechanism and shape memory effect of epoxy shape memory polymers

Following deformation, thermally induced shape memory polymers(SMPs) have the ability to recover their original shape with a change in temperature. In this work, the thermomechanical properties and shape memory behaviors of three types of epoxy SMPs with varying curing agent contents were investigated using a molecular dynamics(MD) method. The mechanical properties under uniaxial tension at different temperatures were obtained, and the simulation results compared reasonably with experimental data. In addition, in a thermomechanical cycle, ideal shape memory effects for the three types of SMPs were revealed through the shape frozen and shape recovery responses at low and high temperatures, respectively, indicating that the recovery time is strongly influenced by the ratio of E-51 to 4,4'-Methylenedianiline.     

A new analytical method for the optimization of thermomechanical conditions in zone drawing and its application to polyethylene

An original method of zone drawing of polymers at constant load and a procedure for the optimization of thermomechanical conditions (stress, temperature) are suggested, allowing high draw ratios and favorable strength values to be obtained. The temperature (or stress) range of necking has been determined in a nonisothermal and in an isothermal regime. The advantage of the method consists in that the increasing orientation in the neck starting from the initiation point and up to fracture allows the morphology and properties to be quickly examined, depending on the varying thermomechanical conditions in different regions of the neck. At a high temperature and low load the mechanism of oriented crystallization from melt becomes operative; in opposite cases, orientation of the solid state takes place. It is shown that in the nonisothermal regime an increased rate of heating allows extreme draw ratios (up to ca. 150) to be obtained, approximately twice as high as those obtained in the isothermal regime. This is interpreted as a gradual improvement of the oriented structure by recrystallization during extension and by a quick fixation on cooling below the zone.     

Microscopic vs. macroscopic origin of the Lehmann effect in cholesteric liquid crystals

In a recent letter (EPL 97, 36006 (2012)), we have shown that the Leslie thermomechanical coupling cannot alone explain the Lehmann effect (namely the rotation of cholesteric droplets when they are subjected to a temperature gradient). This result was obtained by measuring in a compensated cholesteric mixture the “Lehmann coefficient” as a function of temperature both below and at the transition to the isotropic liquid. In this article, we detail these experiments and present new ones performed with other compensated mixtures and a diluted cholesteric mixture. The new results confirm the macroscopic origin of the Lehmann effect, in contrast to the Leslie thermomechanical effect that is clearly of microscopic origin.     

Hydrodynamic Flows in Microsized Liquid Crystal Cells with Orientational Defects

The effect of the orientational defect (OD) on the formation process of a vortical flow v(t, r), emerging in a microsized liquid crystal (LC) cell under the action of a focused laser radiation, was studied within the nonlinear generalization of the classical Ericksen–Leslie theory by numerical methods, considering the thermomechanical contributions to both the stress tensor and viscous torque, that acts on the unit volume of the liquid crystal phase (LC phase). The analysis of the obtained results showed that the vortical flow, rotating clockwise, is generated in a “defect” LC cell close to the OD, with the OD, placed on the lower bounding surface, on which the laser radiation was focused. The rotational velocity of this flow is two orders of magnitude greater than the rotational velocity of the vortex, which is generated in a “pure” LC cell at the same conditions and rotates anticlockwise.

     

A digital spatial carrier for wavelet phase extraction

In this paper, we present a new method to digitally add a high-frequency spatial carrier in order to use the wavelet phase extraction algorithm, which leads directly to the phase without the unwrapping process. The method needs a fringe pattern and its π/2 shifted version. The application is performed with two shifted fringe patterns obtained from the phase shifting images of the thermomechanical study of an MOS power transistor. A comparison with the phase shifting results is made.     

Modeling the thermomechanical processes in the output window of a high-power СО<Subscript>2</Subscript> laser

A mathematical model of thermomechanical processes in the output window of a high-power continuous gas laser is developed and used to examine the windows of a СО₂ laser. The dependence of the maximum allowed output radiation power on the beam diameter and the distributions of temperatures and mechanical stresses are obtained, and the divergence of radiation is studied for windows made from ZnSe, KCl, and polycrystalline diamond (PD). In addition, the damage threshold of a composite output window made from PD with a single-crystalline region at the center is considered.     

Global Existence of Smooth Solution to¶Non-Linear Thermoviscoelastic System with¶Clamped Boundary Conditions in Solid-like Materials

In this paper, we prove the global existence and uniqueness of smooth solution to a system of one-dimensional non-linear thermoviscoelasticity which describes the thermomechanical processes in a class of solid-like materials, such as rubber, etc. The materials satisfy that both ends of the rod are fixed. This assumption was proposed by Dafermos in 1982 (see [6]). A new approach is developed to obtain the crucial estimate of the L^X-norm of the strain u.     

Elastic displacement of surface due to laser picosecond pulse heating of gold

Laser picosecond pulse heating initiates nonequilibrium energy transport in the surface vicinity of the metallic substrates. In this case, electron temperature rises rapidly while lattice site temperature is slow during the heating period. Although the rise of lattice site temperature is low, the temperature gradient is high. This results in elastic displacement of the surface. To model the heating process thermomechanical coupling needs to be introduced in the analysis. In the present study, picosecond pulse heating of gold surface is considered. Three-dimensional analysis of electron kinetic theory is introduced when formulating the electron and the lattice site temperatures. The analysis is extended to include the thermomechanical coupling due to mechanical response of the substrate material. The lattice site temperatures obtained from the electron kinetic theory are compared with the predictions of the two-equation model. It is found that both models predict almost identical temperature profiles in the surface vicinity of the substrate material. In addition, the surface displacement on the order of 10⁻¹¹ m is predicted.     

Equilibrium and nonequilibrium thermomechanics for an effective pair potential used in smooth particle applied mechanics

The smooth-particle weighting functions used in numerical solutions of the thermomechanical continuum equations can be interpreted as weak pair potentials from the standpoint of statistical physics. We examine both equilibrium and nonequilibrium thermomechanical properties of many-body systems using a typical smooth particle potential, Lucy's, and discuss the implications for macroscopic continuum simulations.     

Thermomechanical effect in a planar nematic induced by a quasistatic electric field

A thermomechanical flow of uniformly oriented nematic liquid crystal induced by a quasistatic electric field is observed. This flow occurs when the electric field strength exceeds the static Fréedericksz transition threshold. The effect is attributed to an electric-field-induced nonuniformity of the director orientation which is required for the onset of the thermomechanical effect. The experimental results show good agreement with the theoretical estimates. Zh. Tekh. Fiz. 69, 122–124 (April 1999)     

Bone and joint changes in pneumatic drillers : M. J. Burke,E. C. Fear and V. Wright 1977 Annals of Rheumatic Diseases36, 276–279. (4 pages, 2 figures, 13 references)

Propagation of waves in linear anisotropic non-homogeneous thermoviscoelastic media is investigated by employing the basic ideas of the theory of singular surfaces and of ray theory. The characteristic equation governing the wave velocities, and the decay and growth equations describing the change of the strength of the discontinuity as the wave front moves in the medium are obtained. The results then are reduced to the case of isotropic materials. The decay and growth equations for this case are integrated along the rays and the general solutions are obtained. The factors affecting the decay and growth, namely the effects of inhomogeneity, geometry of the wave front, material internal friction and thermomechanical coupling, are discussed.     

A New Form of the Crystal Potential in BCC Iron

By approximating the electron density in the crystal as a sum of spherically symmetrical atomic densities using the Green's function, a new exact relation for the Coulomb potential is derived. The relation allows calculation of its values at any point of a unit cell. An attractive potential of the Coulomb hole that screens the electron long-range action is obtained. The method is applied in practice to the bcc iron for the [100], [110], and [111] directions.     

The effect of technological microdefects of structure on service life of polymer materials

Within the framework of the kinetic theory of strength, relations for estimation of durability of thermoplastic materials under stationary and non-stationary thermomechanical impacts are obtained It is shown that durability decreases linearly with increase in the natural (Napierian) logarithm of the mean size of microcracks formed during manufacturing of polymer products. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–6, July, 2007.     

Laser emission with low quantum defect in Yb: CaGdAlO4

It is demonstrated that 2% Yb: CaGdAlO4 (called CAlGO) presents favorable thermomechanical properties with a high measured thermal conductivity (Kc = 6.3 and Kc = 6.9 W m(-1) K(-1). A laser oscillation in this material at 1016 nm is demonstrated for the first time to our knowledge while pumping at 979 nm. This implies a very small quantum defect (3.5%). A simple new figure of merit that takes into account thermomechanical properties and quantum defects is proposed here to compare the resistance of materials under high-power laser pumping. Consequently, Yb:CAlGO is similar to garnets and sesquioxides in regard to laser power resistance.     

Quasistatic and dynamic functional properties of thin superelastic NiTi wires

2D/3D structures made from thin NiTi wires (d < 100μm) are currently considered for engineering applications in textile, medical or robotics fields. The development of such novel applications requires the knowledge of the thermomechanical behaviour of NiTi ultra thin wires in tension, torsion bending and combined loads, which might differ from that of thicker wires due to influence of texture, cold work and higher aspect ratio. To deal with this challenge, a new experimental testing approach presented in this paper has been developed. It includes thermomechanical tensile testing, combined tension-torsion testing and forced tensile vibrational testing realized on self-made testing rigs equipped with Peltier furnaces and in-situ electric measurement systems. The collected experimental datasets provide a basis upon which FEM implementable SMA models describing functional thermomechanical behaviours of 2D/3D NiTi wire structures (quasistatic and dynamic) are presently being constructed