Thermo-mechanical Stress Fields Research Articles

Thermoacoustic Fano-based bistable energy converters: A new concept of thermoacoustic engine

In this work, we propose a highly efficient thermoacoustic energy converter based on the concept of Fano-based bistable systems. The proposed device, in its simplest version consists of a bilayer polymeric cylinder with harder core and softer and thinner shell. The heat source enforced at the core-shell interface induces thermo-mechanical stress fields. The stress field causes the occurrence of wrinkling patterns in the shell and the release of acoustic energy by a snap-through-buckling mechanism. We show that, when the shell is acoustically resonant at a specific frequency, the synergetic interplay of snap-through and Fano-resonances is capable of producing high amplitude and long-lasting pressure oscillations. In other words, under specifics conditions, the synergetic interaction between wrinkling instabilities and Fano resonance is able to extract mechanical energy from the heat source. The efficiency of the system in converting heat-into-work under resonant conditions is at least 6 times higher than that out of resonance. Notably, for a temperature difference of 1 K between the heat source (the hot thermal reservoir beneath the resonant shell) and the surrounding water (the cold thermal reservoir above the resonant shell), the system may reach a conversion efficiency above 70% of the theoretical Carnot efficiency. The so-generated mechanical oscillations can be converted in electrical energy for instance by using sound-to-energy converting systems such as conventional loudspeakers. The characteristics of the heat source have been selected to be representative of low-grade thermal sources (e.g. waste-heat, in particular intermittent sources) abundantly available in natural, and industrial contexts. Importantly, the systems may be adapted to many different geometries spanning from the proposed core-shell cylinders in which the heat source can be either inside the tube (for instance an exhaust gas/liquid outlet) or outside the tube, to plates (like a solar panel system).

Theoretical solution for thermo-mechanical crack-tip stress field for transversely graded materials

The present work deals with the structure of the thermo-mechanical stress field near the crack-tip in a graded material, where properties vary along the crack front in an exponential manner. With assumptions of steady-state conditions and insulated crack faces, expansion of the temperature field around the crack-tip is obtained. Results indicate that the heat flux in the radial and angular direction has inverse square root singularity near the crack-tip. The heat flux in the direction of gradation does not have any singularity. Navier’s equation for a thermo-mechanical system is solved using the Eigenfunction expansion approach to obtain the near crack-tip stress field. The explicit form of the first four terms in the expansion of the displacement field is obtained. It is observed that terms corresponding to in the stress field are directly affected by the temperature or heat flux. Effect of temperature is observed in out-of-plane normal stress only for Considering the first two terms in the asymptotic expansion, a parametric analysis, shows that thermal stresses help relax the constraint ahead of the crack-tip when the structure is constrained in the thickness direction. It helps in the suppression of crack growth mechanisms, which delays the failure of structures.

Analytical solution for rotating cylindrical FGM vessel subjected to thermomechanical loadings

In the present work, an exact closed-form analytical solution is derived from the governing equations in order to compute the thermomechanical stress field for a rotating hollow cylinder made of nonhomogeneous materials (functionally graded materials: FGMs) subjected to internal pressure and uniform temperature. We assume a radial variation along the cylinder wall of the thermomechanical properties namely: the Young's modulus and the thermal conductivity. The Poisson’s ratio is assumed constant. In order to provide an exact solution of the displacement and stress fields, the well-known Navier’s equation which is a second order partial differential equation derived from the equilibrium equation was solved. The results obtained reveal the influence of thermal loading, power-law constant and internal pressure on the thermal and mechanical stresses of the nonhomogeneous cylindrical vessel.

ВПЛИВ РЕЖИМНИХ І ЕКСПЛУАТАЦІЙНИХ ФАКТОРІВ НА ДЕМПФЕРНУ СИСТЕМУ РОТОРА ЯВНОПОЛЮСНОЇ СИНХРОННОЇ МАШИНИ

The physical processes in the damping system of the salient-pole synchronous machine rotor, which cause the gradual destruction of its structure, have been studied. In particular, the distributions of currents, temperatures and thermomechanical stresses in the damping system rods during its operation in asynchronous and asymmetric modes of operation, as well as in case of rotor eccentricity. A field mathematical model has been developed that takes into account the combined action of three physical fields of different nature: electromagnetic, temperaturic, and thermomechanical stress fields, and allows estimating heating and thermomechanical loads in the damping system of the rotor of the salient-pole synchronous machine. According to the results of the analysis, the heating and thermomechanical loads of the structural elements were determined and recommendations for its structural improvement were given. References 9, figures 9, tables 1.

A LN/Si-Based SAW Pressure Sensor.

Surface Acoustic Wave (SAW) sensors are small, passive and wireless devices. We present here the latest results obtained in a project aimed at developing a SAW-based implantable pressure sensor, equipped with a well-defined, 30 μm-thick, 4.7 mm-in-diameter, Lithium Niobate (LN) membrane. A novel fabrication process was used to solve the issue of accurate membrane etching in LN. LN/Si wafers were fabricated first, using wafer-bonding techniques. Grinding/polishing operations followed, to reduce the LN thickness to 30 μm. 2.45 GHz SAW Reflective Delay-Lines (R-DL) were then deposited on LN, using a combination of e-beam and optical lithography. The R-DL was designed in such a way as to allow for easy temperature compensation. Eventually, the membranes were etched in Si. A dedicated set-up was implemented, to characterize the sensors versus pressure and temperature. The achieved pressure accuracy is satisfactory (±0.56 mbar). However, discontinuities in the response curve and residual temperature sensitivity were observed. Further experiments, modeling and simulations were used to analyze the observed phenomena. They were shown to arise essentially from the presence of growing thermo-mechanical strain and stress fields, generated in the bimorph-like LN/Si structure, when the temperature changes. In particular, buckling effects explain the discontinuities, observed around 43 °C, in the response curves. Possible solutions are suggested and discussed.

In-phase thermomechanical fatigue damage evolution of long fiber-reinforced ceramic-matrix composites using fatigue hysteresis-based damage parameters

Under in-phase (IP) thermomechanical fatigue (TMF) loading, the thermal cyclic temperature changes with decreasing or increasing applied stress upon unloading or loading, and the fiber/matrix interface shear stress in the long fiber-reinforced ceramic-matrix composites (CMCs) changes with applied cycles. In this paper, the in-phase TMF damage evolution of CMCs has been investigated, considering the coupling effects of TMF loading stress level, thermal cyclic temperature and applied cycles. The interface debonding and sliding lengths are determined based the thermomechanical micromechanical stress field and the fracture mechanics approach. The relationships between the TMF hysteresis parameters, interface damage, thermal cyclic temperature and applied cycles have been developed. The damage accumulation processes subjected to in-phase TMF for different fibers volume fraction, peak stress, matrix crack spacing, interface properties and thermal cyclic temperature range have been analyzed. The distinction of the damage evolution between the in-phase TMF and isothermal fatigue (IF) loading at the same applied stress has been analyzed. The damage evolution of cross-ply CMCs subjected to the in-phase TMF and IF loading have been predicted.

Thermo-Mechanical Stress near Apex of a Bi-Material Wedge by a Novel Finite Element Analysis

A super wedge tip element for application to a bi-material wedge is develop utilizing the thermo-mechanical stress and displacement field solutions in which the singular parts are numerical solutions. Singular stresses near apex of an arbitrary bi-material wedge under mechanical and thermal loading can be obtained from the coupling between the super wedge tip element and conventional finite elements. The validity of this novel finite element method is established through existing asymptotic solutions and conventional detailed finite element analysis.

Transient thermo-mechanical analysis of dynamic curving cracks in functionally graded materials

Mixed-mode dynamic crack growth behavior along an arbitrarily smoothly varying path in functionally graded materials (FGMs) under transient thermo-mechanical loading is studied. An asymptotic analysis in conjunction with displacement potentials is used to develop transient thermo-mechanical stress fields around the propagating crack-tip. Asymptotic temperature field equations are derived for exponentially varying thermal properties, and later, these equations are used to derive transient thermo-mechanical stress fields for a curving crack in FGMs. The effect of the transient parameters (loading rate, crack-tip acceleration, and temperature change) and temperature gradient on the maximum principal stress and circumferential stress associated with the propagating crack-tip is discussed. Finally, using the minimum strain energy density criterion, the effect of temperature gradient, crack-tip speeds, and T-stress on crack growth directions is determined and discussed.

Dynamic curving cracks in functionally graded materials under thermo-mechanical loading

Mixed-mode dynamic crack growth along an arbitrarily smoothly varying path in functionally graded materials (FGMs) under thermo-mechanical loading is studied. The property gradation in FGMs is considered by varying shear-modulus, mass density, thermal conductivity and coefficient of thermal expansion exponentially along the gradation direction. Asymptotic analysis in conjunction with displacement potentials is used to develop the stress fields around propagating cracks in FGMs. Asymptotic temperature fields are developed first for the exponential variation of thermal conductivity and later these temperature fields are used to derive thermo-mechanical stress fields for a curving crack in FGMs. Using these thermo-mechanical stress fields, various components of the stresses are developed and the effect of curvature parameters, temperature and gradation on these stresses are discussed. Finally, using the minimum strain energy density criterion, the effect of curvature parameters, crack-tip speeds, non-homogeneity values and temperature gradients on crack growth directions are determined and discussed.

Analysis of Thermo-Mechanical Stresses in Laminated Hollow Circular Cylinder with Finite Length

Thermo-mechanical stresses analysis of laminated hollow circular cylinder with finite length are carried out in this study based on the theory of laminated composites. The hollow circular cylinders with finite length are simply supported at four edges and are subjected to nonuniform thermal and mechanical loadings on the inner and outer surfaces. Analytical solutions of temperature distribution and thermo-mechanical stress fields are derived by using variable separation approach and series solving method. A four -layer laminated hollow circular cylinder with finite length is numerically analyzed by using the present method. All results are graphically presented and briefly discussed.

Mixed-mode dynamic crack propagation in graded materials under thermo-mechanical loading

Mixed-mode dynamic crack growth behavior in functionally graded materials (FGMs) under thermo-mechanical loading is studied. Asymptotic analysis in conjunction with displacement potentials has been used to develop thermo-mechanical stress fields for a mixed mode propagating crack in a FGM. The shear modulus, mass density, thermal conductivity and coefficient of thermal expansion of the FGM are assumed to vary exponentially along the gradation direction. First, asymptotic temperature fields are derived for an exponential variation of thermal conductivity and later these temperature fields are used in deriving stress fields. Using asymptotic thermo-mechanical stress fields the variation of maximum shear stress, circumferential stress and strain-energy density as a function of temperature around the crack tip are generated. Finally, utilizing the minimum strain-energy density criterion and the maximum circumferential stress criterion, the crack growth direction for various crack-tip speeds, non-homogeneity coefficients and temperature fields are determined.

Thermo-mechanical stress fields and strain energy associated with a mixed-mode propagating crack

Thermo-mechanical stress field equations are developed for a mixed-mode crack propagating at constant velocity in homogeneous and isotropic materials using an asymptotic approach along with displacement potentials. Asymptotic temperature field equations are first developed for steady state temperature conditions using insulated crack-face boundary conditions. These temperature field equations are later used to derive the first three terms of thermo-mechanical stress field equations for a steady state propagating mixed-mode crack. Using these thermo-mechanical stress fields, various components of the stresses are developed, and the effects of temperature on these stress components are discussed. Further, strain energy density and the circumferential stress criteria are employed to study the effect of temperature and the crack-tip velocity on crack growth direction.

Theoretical Investigation on Thermo-mechanical Stresses in Laminated Cylindrical Panels

Three-dimensional thermo-mechanical stresses in laminated cylindrical panels are theoretically investigated. The cylindrical panels are simply supported at four edges and are subjected to nonuniform thermal and mechanical loadings on the inner and outer surfaces. The theoretical model of this problem is established by using 3D thermo-elastic theory. Analytical solutions of 3D temperature distribution and thermo-mechanical stress fields are derived by using variable separation approach and series solving method. A three-layer laminated cylindrical panel is numerically analyzed by using the present method. All results are graphically presented and briefly discussed.

The influence of thermomechanical loading history on the stress level in WWER NPP reactor pressure vessels under thermal shock

We present the results of calculations of the kinetics of stress-strain state and stress intensity factors for surface and under-the-cladding circumferential cracks in modeling the emergency core cooldown conditions for the WWER-1000 reactor. The calculation procedure is based on a mixed finite-element method statement which provides stability of numerical solution and a high accuracy of results for both the displacements as well as stresses and strains. The authors analyze the influence of the density of the finite-element discretization of the crack-tip area for the surface and under-the-cladding circumferential cracks on the accuracy and convergence of computation of fracture-mechanics parameters in the modeling of thermal shock conditions. The results of calculation of kinetics of stress intensity factors allowing for the thermomechanical loading history and residual process-induced stress fields are reported. It is demonstrated that if the elastoplastic deformation history and residual process-induced stress fields are disregarded in the calculations of stress intensity factors for under-the-cladding cracks the reactor pressure vessel strength and lifetime may turn out to be overestimated.

Effect of thermomechanical loads on the propagation of crack near the interface brittle/ductile

The purpose of this paper is to understand the combined effect of thermal and mechanical loading on the initiation and behaviour of sub-interface crack in the ceramic. In this study a 2D finite element model has been used to simulated mixed mode crack propagation near the bimaterial interface. The assembly ceramometalic is subjected simultaneously to thermomechanical stress field. The extent of a plastic zone deformation in the vicinity of the crack-tip has a significant influence on the rate of its propagation. The crack growth at the joint specimen under four-point bending (4PB) loading and the influence of residual stresses was also evaluated by the maximum tensile stress criterion. The J-integral at the crack tip is generally expressed by the thermomechanical local stresses. The results obtained show the effect of the temperature gradient ΔT, the size of the crack and the applied stresses on the J-integral.

Thermoelastic Fields of a Functionally Graded Coated Inhomogeneity With Sliding/Perfect Interfaces

The determination of the thermo-mechanical stress field in and around a spherical/cylindrical inhomogeneity surrounded by a functionally graded (FG) coating, which in turn is embedded in an infinite medium, is of interest. The present work, in the frame work of Boussinesq/Papkovich-Neuber displacement potentials method, discovers the potential functions by which not only the relevant boundary value problems (BVPs) in the literature, but also the more complex problem of the coated inhomogeneities with FG coating and sliding interfaces can be treated in a unified manner. The thermo-elastic fields pertinent to the inhomogeneities with multiple homogeneous coatings and various combinations of perfect/sliding interfaces can be computed exactly. Moreover, when the coatings are inhomogeneous, as long as the spatial variation of the thermo-elastic properties of the transition layer is describable by a piecewise continuous function with a finite number of jumps, an accurate solution can be obtained. The influence of interface conditions, stiffness of the core, spatial distributions of thermal expansion coefficient and shear modulus of FG coating, and loading condition on the stress field will be examined.

Boundary element analysis for thermomechanical interface stress in the thin-layered media under sliding contact

The present work examines the influence of a surface coating on the temperature distribution and the thermomechanical stress field produced by friction in a sliding contact. A two-dimensional transient model of a layered medium moving with constant velocity relative to a heat source is presented. A solution technique based on the boundary element method employing the multiregion technique is utilized. Results are presented showing the influence of coating thickness, thermal properties, Peclet number, and mechanical properties. It shows that the mechanical properties and thickness of coating have a significant influence on the stress field. The effect of the ratio of shear modules is more important than the effects of the ratios of other mechanical properties.

A Study on Stress Distribution Using Boundary Element Analysis Due to Surface Coating in Sliding Contact

The present work examines the influence of surface coating on the temperature and the thermo-mechanical stress field produced by friction due to sliding contact. A two-dimensional transient model of a layered medium submitted to a moving heat flux is prsented. A solution technique based on the boundary element method employing the multiregion technique is utilized. Results are presented showing the influence of coating thickness, thermal properties, Peclet number, and mechanical properties. It has been shown that the mechanical properties and thickness of coating have a significant influence on the stress field, even for low temperature increase. The effects of the ratios of shear modulus become more important for low temperature increase than the effects of the ratios of other mechanical properties.

Methodology for service life increase of hot forging tools

Hot forging is widely used in the manufacturing of automotive components. The high production rate induces severe thermomechanical stresses in the tools. The lifetime of dies is commonly driven either by wear or thermal cracking. This paper describes the methodology that has been applied to gain understanding of the thermomechanical stress field in a cemented carbide punch used for the manufacture of airbag container type parts. The stresses are the result of a combination of purely mechanical stresses due to the forging process , and thermomechanical stresses induced by the thermal cycling of the punch surface during successive hot forging and waiting periods. Simulation results have been validated as a result of experimental investigations. The results show that in the critical area subjected to thermal cracking, the thermomechanical stress contribution is as high as 75% of the total stress field. As a consequence, it has been determined that an increased tool life could be expected by a global modification of the nominal hot-forging process parameters, i.e. by the modification of billet temperature and forging rate.

Effects of a time-dependent interfacial layer on the thermo-mechanical response of polymer matrix composites

In the present study, the micro-mechanical analysis of a fiber-matrix interphase is conducted. Finite-element analysis based on the generalized plane-strain condition is used to evaluate the thermo-mechanical deformation and stress fields in polymer matrix composites. Non-homogeneous visco-elastic constitutive relationships are considered for the matrix and the interphase. The analysis includes mechanical and time-dependent thermal loadings. When a unit cell is subjected to uniform transverse resultant forces, creep responses in the interphase and matrix are observed. For a uni-axial extensional load, most of the loading is carried by the fiber throughout the loading history, and stress relaxation in the matrix and interphase causes the fiber loading to increase slightly with time. Numerical evaluations also show that thermal expansion mismatches in the fiber, interphase and matrix result in the development of high residual stress at the interphase, which is responsible for debonding between the fiber and the matrix.