Thermal Stress Field Research Articles

Practical Green’s function for the thermal stress field induced by a heat source in plane thermoelasticity

The classical Green’s functions used in the literature for a heat source in a homogeneous elastic medium cannot lead to finite remote thermal stresses in the medium, so that they may not work well in practical thermal stress analyses. In this paper, we develop a practical Green’s function for a heat source disposed eccentrically into an elastic disk/cylinder subject to plane deformation. The edge of the disk/cylinder is assumed to be thermally permeable and traction-free. The full thermal stress field induced by the heat source in the disk/cylinder is determined exactly and explicitly via the Cauchy integral techniques. In particular, a very simple formula is obtained to describe the hoop thermal stress on the edge of the disk/cylinder, which may be conveniently useful for analyzing the thermal stresses in microelectronic components.

Thermal shock resistance behavior of auxetic ceramic honeycombs with a central crack or an edge crack

In this paper, the thermal shock resistance of an auxetic ceramic honeycomb plate is studied based on the fracture mechanics concept for the cases of a central crack or an edge crack. The transient temperature field and transient thermal stress field are obtained for both auxetic and non-auxetic structures. The relationship between the thermal stress intensity factor (TSIF) and the internal cell angle, crack length and time is determined and the critical temperature for the initiation of crack propagation is predicted. Results show that compared with the non-auxetic ceramic honeycombs which are at an internal cell angle of 30°, the critical temperature of the auxetic ceramic honeycombs whose cell are orientated at an angle of −30° increases by 78.5% and the TSIF at the crack tip decreases by 40%, respectively. Hence, the auxetic structures have better thermal shock resistance. This study indicates that auxetic ceramic honeycombs have significant potential applications in harsh temperature environments.

Three-dimensional crack analyses under thermal stress field by XFEM using only the Heaviside step function

A three-dimensional crack analysis method based on XFEM using only the Heaviside step function was applied to heat transfer and subsequent thermal stress analyses of cracked structures. The proposed method employs tip elements to discretize the weak form governing equations using the approximation function with discontinuity. Two kinds of XFEM analysis software, which perform steady-state or transient heat transfer analysis and thermal stress analysis to evaluate J-integrals and stress intensity factors, were developed. As the XFEM model for the stress analysis is consistent with the heat transfer analysis model, nodal temperature data can be directly imported for stress analysis as thermal load data. The developed codes were verified by solving some benchmark crack problems, including both mechanical and thermal loading cases, and comparing the results with those of conventional methods and a reference. It was shown that the developed codes provide appropriate results for practical thermal stress analysis of cracked structures considering temperature-dependent material properties.

Investigating the Thermal Stress of Millisecond Pulsed Laser Irradiation on Charge-Coupled Devices

In this study, a two-dimensional model describing thermal stress on a charge-coupled device (CCD) induced by ms laser pulses was examined. Considering the nonlinearity of the CCD’s material parameters and the melting phase transition process of aluminum electrode materials was considered by using equivalent specific heat capacity method, the physical process where a laser pulse irradiating a CCD pixel array was simulated using COMSOL Multiphysics software. The temperature field and thermal stress field were calculated and analyzed. In order to clarify the mechanism producing damage on the CCD detector, Raman spectra from silicon were measured with a micro-Raman spectrometer to determine stress change in the CCD chip. The procedure presented herein illustrates a method for evaluating strain in a CCD after laser irradiation.

Effect of Nano-Clay Filler on the Thermal Breakdown Mechanism and Lifespan of Polypropylene Film Under AC Fields

The wide application of nanocomposites in the insulation system has greatly contributed to the performance improvement of power equipment. However, nano fillers are not omnipotent for improving the properties of composite dielectrics. In some situations, nano-modified materials are in fact a compromise of improving some performance features while sacrificing others. In this work, the breakdown characteristics and time-to-failure of polypropylene film with nano-clay fillers have been evaluated under combined thermal stress and AC electric fields. Experiments on plain polypropylene (PP) samples have also been carried out under the same test conditions as control. Test results indicated that the time-to-failure of the samples with nano-clay filler was shorter than those without nano filler, which is different from the previous experience. SEM and EDS analyses were conducted to study how the failure mechanism had taken place in both plain polypropylene and the nano-clay filled polypropylene. The failure phenomenon in these materials can be explained by molecular thermodynamics. The main reason for the premature thermal breakdown of PP nanocomposite is essentially due to the weak coupling between nano-clay filler and polymer matrix. Finally, suggestions are proposed for nano modification methods and lifespan prediction models of composite dielectrics.

Microstructure modification of AISI1045 steel induced by high-current pulsed pseudospark electron beam

Pseudospark electron beam is a kind of high-current pulsed electron beam. It is characterized by high current (hundreds of amperes to thousands amperes), short pulse duration (hundreds of nanoseconds to thousands nanoseconds), high frequency (hundreds Hz to thousands Hz) and self-focused beam diameter (typically 0.5-5 mm), which are more favorable for metal surface treatment compared with the conventional thermal-emission electron beam diode. The pseudospark electron beam is easy to achieve high rate of beam energy rise, plasma quenching and to obtain fast and stable repetitive operation. Some unique properties of material processing applications have been induced by the pseudospark electron beam treatment. In this paper, the mechanism and microstructure modification of AISI1045 steel by pseudospark electron beam surface treatment have been investigated. Surface microstructure and chemical composition of AISI 1045 steel pre and post pseudospark electron beam treatment are investigated by using scanning electron microscope (SEM), electron back-scattered diffraction (EBSD) and transmission electron microscope (TEM). Results show that the average depth of the treated layer is several microns. The melt interface is not straight, indicating a non-uniform melting process. Besides, material’s grain size is significantly reduced after pseudospark treatment, especially in sublayer. The average grain size in the sublayer is smaller than in the top-layer. The relatively finer grain size in the sublayer is attributed to the temperature-induced dynamic thermal stress fields and the highest temperature gradient in the subsurface layer instead of on the top layer. Furthermore, the amorphous phase is observed on the surface of melted layer.

Thermal stress analysis of printed circuit heat exchanger based on thermal-structural coupling method

The supercritical carbon dioxide (S-CO2) Brayton cycle has the advantages of high efficiency, compactness and simplicity. It is expected to become the main technical route for the future advanced ship nuclear power platform. The printed circuit heat exchanger (PCHE) with ultra-high pressure, ultra-compact and ultra-high efficiency typical characteristics is the ideal configuration of the S-CO2 power generation system recuperator. The PCHE numerical analysis model is constructed and the thermal stress field of the PCHE core structure is analyzed by thermal-structural coupling method. The results show that the maximum thermal stress of the PCHE core structure is 117 MPa, which is less than the yield limit of the material and meets the requirement of strength safety. The thermal stress value of most regions in PCHE is less than 78.2MPa. Therefore, the current design scheme has a large safety margin. The PCHE compactness can be further improved by structural parameter optimization.

Analytical solution to thermal stresses of high speed PM rotor considering thermal load and heat convection

Aiming at the high speed permanent magnet (PM) rotor with the heat source, this work investigates the analytic solution to the transient temperature field and thermal stress field of the rotor, considering the influence of the forced air cooling of rotor surface on the stress field. Firstly, dimensionless formulation of the transient heat conduction equation including interior heat source is derived, where the axially non-uniform heat convection coefficient and the temperature of main flow region are equivalent to their mean values. Secondly, the Fourier integral transform method is used to solve the dimensionless heat conduction equation. Then, the obtained temperature field is loaded into the analytical solution of strength, in which three types of stress sources such as interference fit, centrifugal force and temperature gradient are included. Finally, examples are carried out to verify the analytical solutions and relative results are discussed.

Numerical Modeling of Transient Temperature and Stress in WC–10Co4Cr Coating During High-Speed Grinding

Based on the classical theory of heat conduction and thermoplastic mechanics, the three-dimensional heat transfer and thermal stress models of high-velocity oxygen fuel WC–10Co4Cr coating under the action of a moving heat source during grinding were developed. Considering temperature-dependent material properties, the transient temperature field and the thermal stress field generated in workpiece were simulated by a finite element method (FEM). As both the thermal conductivity and the specific heat capacity of WC–10Co4Cr are larger than 300 M, temperature rise mainly occurred in the coating. The rise in grinding temperature was discontinuous along the depth of the workpiece, and the temperature gradient at the coating–substrate interface was found to be the largest. Due to the large difference in thermal expansion coefficient between coating and substrate material, considerable thermal stress was generated at the bonding surface. Grinding temperature and thermal stresses at the coating–substrate interface started to increase with the decrease in coating thickness. Grinding surface temperatures were measured experimentally for different coating thicknesses, and simulation results were compared with the obtained experimental values. It was found that FEM results were well consistent with experimental observations.

Study of Crack Interaction Effects Under Thermal Loading by Digital Photoelasticity and Finite Elements

The effect of an interacting internal crack on the edge crack in a transient thermal stress field is evaluated using digital photothermoelastic experiments and finite element (FE) analysis. Initially, a transient thermal stress field is simulated using FE analysis for which the thermal boundary conditions are set based on the experimental isochromatic fringes. In this simulated thermal stress field, single edge crack with and without an internal short crack interacting at different configurations are modeled and the non-dimensional Stress Intensity Factor (SIF) values are evaluated at different time intervals. It is observed that the asymmetric and collinear configurations tend to increase the SIF of the edge crack whereas a parallel crack tends to decrease the SIF of the edge crack. For experimental evaluation, specimens with a single edge crack and asymmetric configuration of interacting cracks are considered and the non-dimensional SIF values are evaluated under a thermal stress field generated by edge heating and cooling. The results are compared with those obtained using the FE analysis.

Comprehensive Numerical Simulation on Thermally Grown Oxide and Internal Stress Evolutions in Thermal Barrier Coatings

TBCs (Thermal Barrier Coatings) is deposited on gas turbine blades to protect the substrate from a combustion gas flow. One of the serious problems occurred in gas turbine is TBC delamination which is caused by startup, steady and stop operation in service. TBC delamination results from subjecting to both cyclic thermal stress and evolution of internal stress due to thermally grown oxide (TGO). In this study, the finite element code which can simulate thermal and internal stress fields generated in TBC was developed. The developed code involves the follows: inelastic constitutive equation for ceramic coating, bilinear-type constitutive equation for bond coating and Chaboche-type inelastic constitutive equation for the substrate, and mass transfer equation in consideration of oxygen diffusion and chemical reaction with aluminum. Thermal cycling simulation was conducted using the developed code. It was confirmed that maximum stress and its location in the ceramic coating/bond coating interface were matched with the associated experimental results.

Mathematic model for describing the stress-tension behavior of an autoclave with integrated heating element

This paper describes a mathematical model and the method for determining the thermal stress fields in a cylindrical wall element of a pressure vessel with a built-in heating element and a layer of heat insulating material. The method presented is based on the calculation of contact pressures.

Interplay of temperature, thermal-stresses and strains in laser-assisted modification of collagenous tissues: Speckle-contrast and OCT-based studies.

Moderate heating of collagenous tissues such as cartilage and cornea by infrared laser irradiation can produce biologically nondestructive structural rearrangements and relaxation of internal stresses resulting in the tissue reshaping. The reshaping results and eventual changes in optical and biological properties of the tissue strongly depend on the laser-irradiation regime. Here, a speckle-contrast technique based on monochromatic illumination of the tissue in combination with strain mapping by means of optical coherence elastography (OCE) is applied to reveal the interplay between the temperature and thermal stress fields producing tissue modifications. The speckle-based technique ensured en face visualization of cross correlation and contrast of speckle images, with evolving proportions between contributions of temperature increase and thermal-stresses determined by temperature gradients. The speckle-technique findings are corroborated by quantitative OCE-based depth-resolved imaging of irradiation-induced strain-evolution. The revealed relationships can be used for real-time control of the reshaping procedures (e.g., for laser shaping of cartilaginous implants in otolaryngology and maxillofacial surgery) and optimization of the laser-irradiation regimes to ensure the desired reshaping using lower and biologically safer temperatures. The figure of waterfall OCE-image demonstrates how the strain-rate maximum arising in the heating-beam center gradually splits and drifts towards the zones of maximal thermal stresses located at the temperature-profile slopes.

Investigations on the thermal behavior and associated thermal stresses for short pulse laser heating

The fractional type Cattaneo heat conduction equation and the associated thermal stress equation are established for studying the thermal and related stress behavior of the short pulse laser heating. With the Laplace transform method, the analytical solution of the temperature distribution and thermal stress field are derived. Compared to the classical Fourier heat transfer model and the standard C-V one, the results of the present model show the thermal diffusion feature as well as the thermal wave behaviour, which is more realistic. Furthermore, the influences of the fractional order parameter on the thermal and related stress behavior are discussed. The thermal velocity, thermal variation rate and the peak value of the temperature and the thermal stress depend on the fractional order parameter. The fractional type Cattaneo heat model and associated thermal stress exhibit the non-local nature.

Thermal shock resistance of porous ceramic foams with temperature-dependent material properties

The impact of temperature dependence of material properties on thermal shock resistance of porous ceramic foams is studied in this paper. Two cases of thermal shock are carried out: sudden heating and sudden cooling. Finite difference method and weight function method are employed to get the thermal stress field at crack tip. The effects of time dependence and temperature dependence of material properties on thermal shock behavior are analyzed. The thermal shock resistance is acquired based on two different criteria: fracture mechanics criterion and stress criterion. By comparison analysis, results show that taking temperature dependence of the material properties into account is crucial in the assessment of thermal shock resistance of ceramic foams. Cold shock fracture experiments of Al2O3 foams with different relative densities are also made, and the obtained results are in coincidence with theoretical results very well.

Shifted Laser Surface Texturing (sLST) in Burst Regime

High laser-scanning speed and high precision are two opposite parameters for effective laser surface texturing (LST). Application of a sequence of laser pulses (called burst) helps to increase the processing effectivennes and speed, but precision control of laser pulses arriving becomes a difficult task for micro-texturing. In this work, one possible solution for this dilemma is presented: a scan-ning strategy called shifted laser surface texturing (sLST) in burst regime. This burst sLST repre-sents an alternative method, where the inertia of galvanoscan mirrors becomes a useful factor at higher speeds. Physical principles of laser burst interaction with a material surface and resulting subsurface thermal-stress fields are discussed. Heat accumulation was calculated from a semi-planar model of temperature distribution from laser spots in the line of the burst. Residual subsurface temperature and pressure is called positive heat accumulation in the case of minimal output roughness of laser-scanned surfaces. Experimental application of the burst sLST was performed with a pico-second laser with a galvanoscan system. Results were evaluated by newly developed shape analysis of objects detected on contrast images of laser-processed stainless steel surfaces painted with high-emissivity paint. Deviation in sLST precision was determined from larger and smaller diameters of detected microobjects on the surface with LabIR coating. The roughness of depth structure in microobjects was controlled by a contact surface profiler and compared with the goal profile and positive heat accumulation distribution. The sLST method in burst regime enables a significant increase of processing speed while maintaining good precision of the produced texture.

Thermal-stress distribution and damage characteristics of three-junction GaAs solar cell irradiated by continuous laser beam

According to the theory of Fourier heat transfer and thermal stress field, a model on thermal damage of three-junction GaAs solar cell is established. The temperature and thermal-stress distribution inside the solar cell, which irradiated by continuous laser, are calculated. Results show that the temperature and thermal-stress distribution are corresponded to the intensity distribution of the incident laser beam. The maximum values of the compressive and tensile stress increase with the increasing of the power density and waist radius of the incident laser. When the waist radius is 0.5 cm, the melting damage is the main damage pattern as the power density is below about 40 W/cm2. However, when the laser power density is higher the stress damage will change to the main damage form. As the waist radius increases, the change in damage processes will also occur at lower laser power density.

Experimental Study and Numerical Simulation of the Intermittent Feed High-Speed Grinding of TC4 Titanium Alloy

This paper proposes intermittent feed high-speed grinding, which shows considerable advantages in terms of reducing grinding temperature, relieving grinding wheel blockage and improving workpiece surface integrity. In this grinding, the continuous feed mode of the workpiece is changed into the normal feed + fast retreat reciprocating feed mode by a fast linear feed worktable. By reasonably setting the normal feed distance of single grinding, the action time of the grinding wheel and workpiece is reduced, so that the grinding heat transfer process does not reach a stable state, reducing the grinding temperature during single grinding. Besides this, the surface temperature is cooled to nearly room temperature and the grinding wheel is flushed by the timely retreating of the grinding wheel to allow the grinding fluid to enter the grinding zone fully, which greatly reduces the phenomenon of heat accumulation and grinding wheel loading. An intermittent feed high-speed grinding experiment on Ti-6Al-4V (TC4) titanium alloy was systematically carried out, and the influence of the grinding parameters on grinding force and grinding temperature was deeply analyzed. The instantaneous grinding temperature field and thermal stress field of TC4 titanium alloy in intermittent feed high-speed grinding were constructed with the finite element method. The surface morphology of the grinding wheel and TC4 titanium alloy specimens after intermittent feed grinding were analyzed and were compared with those after traditional continuous grinding. It was found that the curves of the grinding force and temperature varied with time in the process of machining, consisting of many “pulse” peaks. Under the same grinding parameters, the magnitude of the grinding force is the same as that of continuous grinding. In a certain range, the grinding temperature is greatly affected by the single feed distance and the interval time. The numerical simulation results are in good agreement with the experimental results, and the error is controlled within 12%. Compared with traditional high-speed grinding, under the same process parameters, the grinding temperature is greatly reduced, the surface integrity of the workpiece is better, and the clogging of the grinding wheel is greatly reduced.

Closed‐form solutions for a circular inhomogeneity in nonlinearly coupled thermoelectric materials

AbstractThermoelectricity is a class of material possessing the ability of interconverting heat and electricity. For the sake of high conversion efficiency, inclusions/fibers are usually introduced into thermoelectric materials. However, the discontinuity of material property and geometry brought by inclusions/fibers might result in severe stress concentration and thus greatly affect the mechanical properties of thermoelectric material in its engineering service. This paper introduces a nonlinear coupled thermal–electrical–mechanical formulation for the plane problem of a circular inhomogeneity embedded in a thermoelectric medium under a combined uniform electric current density and energy flux. A special attention is directed towards the induced thermal stress field which has often neglected in the literature. Using methods based on Cauchy integrals, the complex potentials of the electric field, temperature field and associated stress field are derived explicitly in both the inhomogeneity and the surrounding matrix. The analysis shows that the electrically and thermally induced stress has a linear relationship with the energy flux applied at infinity, but has a nonlinear relationship with the remote electric current density. Numerical simulations are also performed to examine how the inhomogeneity influences the electrically induced thermal stress concentration on the interface. The presented results can be directly used for reliability consideration in design and optimization of thermoelectric composites with circular fibers/inclusions.

Simulation and Modelling in Cracks Prevention of Early Age Concrete

Cracks often arise in mass concrete structures, due to the thermal stress and low tensile strength of early age concrete. To prevent the undesired thermal stress induced crack, controlling the temperature of concrete has been considered as an effective approach. In this paper, a temperature controlling measure evaluation system (TCMES) is proposed, which includes distributed fiber optic temperature monitoring, prediction of temperature and stress fields, and concrete crack risk evaluation. We first experimentally monitor the temperature evolution of the concrete using the distributed fiber optic temperature sensing. Then thermal parameters of in-situ concrete are retrieved by performing back-analysis. Subsequently, the concrete temperature field and thermal stress field can be predicted from the retrieved parameters and the experimental parameters of concrete. Under the concrete crack risk evaluation principles, temperature controlling measures for different stages are proposed. Our analysis indicates that the proposed system is an effective approach to prevent cracks of early age concrete.