Thermal Stress Concentration Research Articles

Experimental and numerical investigation of laser hot wire welding

With respect to autogenous laser welding, laser welding assisted by a hot wire was capable of saving the consumption of laser power, tailoring the mechanical and physical properties of welds, and improving the gap-bridging capability. In this work, the influences of welding parameters were investigated to combine the positive aspects of laser beam and hot wire. Sound welds were obtained when the laser beam was focused inside the samples. The welding direction had a significant effect on the gap-bridging capability, while the offset distance between the laser focal spot and hot wire tip determined the stability of the feeding of the hot wire into the molten pool. The effect of hot wire on the temperature field and thermally induced residual stress of laser welding was investigated by using the finite element method. X-ray diffraction technique was used to measure the residual stress at the obtained weld beads, and a good agreement between the experimental and numerical results was achieved. Results showed that the transverse and longitudinal stresses prevailed in the laser hot wire welding process, and the thermal stress concentration occurred in the weld pool. When the hot wire voltage was lowered, the residual stresses, especially the transverse residual stress, were markedly reduced.

Modeling of Thermal and Mechanical Behavior of ZrB2-SiC Ceramics after High Temperature Oxidation

The effects of oxidation on heat transfer and mechanical behavior of ZrB2-SiC ceramics at high temperature are modeled using a micromechanics based finite element model. The model recognizes that when exposed to high temperature in air ZrB2-SiC oxidizes into ZrO2, SiO2, and SiC-depleted ZrB2 layer. A steady-state heat transfer analysis was conducted at first and that is followed by a thermal stress analysis. A “global-local modeling” technique is used combining finite element with infinite element for thermal stress analysis. A theoretical formulation is developed for calculating the thermal conductivity of liquid phase SiO2. All other temperature dependent thermal and mechanical properties were obtained from published literature. Thermal stress concentrations occur near the pore due to the geometric discontinuity and material properties mismatch between the ceramic matrix and the new products. The predicted results indicate the development of thermal stresses in the SiO2 and ZrO2 layers and high residual stresses in the SiC-depleted ZrB2 layer.

Finite-Element Analysis Study to Predict the Thermo-Mechanical Stability of Planar-Type NaS Batteries

The sodium sulfur (NaS) battery is one of the most promising candidates for grid scale energy storage systems (ESS) applications. Within the structure of the NaS cell, there are joint parts comprised of different types of materials such as metallic alloys or glasses to bond between insulating ring/metals and insulating ring/electrolytes, respectively. During the freeze and thaw processes, the ambient temperature changes may result in interfacial decohesion or bulk fractures of these joint parts, which may lead to a catastrophic failure of the cell. It is considered that such thermomechanical stability at the joints is one of the vital issues in developing commercialized NaS cells. In this presentation, therefore, we will first review the current thermal stability problems of NaS cells, and then the computational approach will be introduced to address the quantitative prediction of such thermal stress concentration in these metallic and glass joints area of NaS batteries. Specifically, the impacts of the structures, geometries, and sizes of cell components on the thermomechanical behavior of the planar-type NaS cell have been explored based on the finite-element analysis (FEA) computational technique. In contrast to the conventional cylindrical type cells, we will introduce a planar-type NaS design recently developed at RIST as an effort to increase the efficiency and performance. In developing the computational model, we have incorporated relevant stress-strain curves of each material assuming that they follow the J2 flow theory with isotropic hardening. Realistic geometries of the battery structure were digitally transferred to the FEA solver using sophisticated mesh-generation software. The computation includes all of the detailed cell assembly procedures with corresponding temperature changes. After construction of the 3 dimensional cell geometries, a comprehensive analysis including the tangential and normal stress concentration at the interfaces and the bulk section of the joint area upon heating and cooling has been performed. The results of the FEA computation show that the thermal stress concentration in the joint areas is strongly dependent on the geometry and the size of the cells. Especially, substantial increase of the stress accumulation was predicted in the cells with angular geometries. In this presentation, we will also show quantitative analyses on the probable fracture scenarios in the cell joints. It is expected that the outcomes from the developed prediction tool employing the FEA can provide a guidance to enhance the stability of the NaS cell with optimized materials and geometrical designs.

Thermal stress and relieving method of the ITER lower vertical stabilization coil

Structural analysis of the ITER lower vertical stabilization (VS) coil has concluded thermal stress concentration is a serious issue obstructing the coil design. To optimize the lower VS coil, its structural performance has been studied in detail to find methods of solving the stress concentration issues. Apparently, increasing filet radius and decreasing the notch depth are the means commonly used to deal with stress concentrations in the singular regions. However, the real effects are not so simple due to very limited optimization space of the coil geometry. Larger filet radius will significant mitigate the stress concentration but still not enough if the material keeps original. On the other hand, stress concentration at the interface between the conductor jacket and clamp also needs to be mitigated. Changing the clamp size or distance is an obvious and effective way to solve this issue. However, introducing sliding contact at the interface is more effective and simpler. It requires the jacket and clamp are not tightly welded together but connected by bolts or other analogous means with which local sliding is permitted. Additionally, the effect of gap between conductor, spine and clamp is also researched. All in all, the stress concentrations of ITER lower VS coil can be mitigated to meet the acceptance criteria based on the specific optimizations and combining them together.

An Educational Platform in Structural Mechanics

Thermal residual stresses often arise due to a manufacturing process, involving localised thermal induction, or to the existence of structural components with different thermal expansion coefficients. The existence of thermal residual stresses within a structural member is usually undesired, as it decreases the mechanical resistance of structures. Hence, it is desirable to obtain both a minimum level of residual stresses and smoother stresses transitions in the materials interfaces. Regarding the mitigation of thermal residual stress concentration, the use of materials which properties can vary along the component directions has great interest. This work addresses the use of dual-phase functionally graded materials, which microstructure varies gradually from a material to another according to a given gradation function. On the order hand, it is also addressed the use of a population based optimization algorithm in order to attain the referred minimum stress level. Summarizing, the current work presents an educational platform directed to structural mechanics students, which aims to give the tools to understand both the influence of design parameters in the thermal residual stress level and distribution along the material and the advantages of using a structural optimization technique in order to minimize the drawback thermal residual stresses effects.

Thermal transparency with the concept of neutral inclusion

The concept of the electromagnetic wave transparency is introduced into the thermal field. The conditions of the thermal transparency for a multilayered sphere with isotropic coatings, a coated spheroid with an isotropic coating, and a coated sphere with a radial anisotropic core or a radial anisotropic coat are deduced with the help of the idea of the neutral inclusion. The thermal transparency can be achieved by making the effective thermal conductivity of the composite inclusion equal to the thermal conductivity of the surrounding matrix. The validity of the theoretical analysis is checked by the corresponding simulated results, which indicate that the designed neutral inclusion can be transparent perfectly. A specific case of interest of the thermal transparency is its application to cancel the thermal stress concentration resulting from the existence of the inclusions in the particle (even the thermal-insulated particle) -reinforced composites.

Behaviors of full compositional W/Cu functionally gradient materials exposed to the edge plasma of HT-7 tokamak

To investigate the behaviors of graded structure materials under plasma loading, six-layered W/Cu functionally gradient materials (FGMs) with full compositional distributions (from 0% to 100%) were successfully fabricated by resistance sintering under ultra-high pressure (RSUHP) method and were exposed to the edge plasma of the HT-7 tokamak. After about 210 shots plasma loading with an estimated power density in the range of 2–7MW/m2, there is no detaching found at the graded interface corners where the thermal stress concentration is expected and the fatigue failures often occur initially, however, the brittle W layer develops the macroscopic cracks which can extend deeply into the interface and results in the interface failure or even the exfoliated W layer. To avoid these destructions for W/Cu FGMs during wall application, it is necessary to ameliorate the W layer by optimizing the initial powders and sintering parameters.

Investigation on safety aspects of forward light propagation during laser surgery.

During laser treatment of prostate, urological surgeons occasionally experience fiber-cap failure due to concentration of thermal stress on the fiber tip. Upon the cap breakage, laser light becomes forward-propagating and may adversely affect the bladder tissue such as perforation. The purpose of the current study was to identify any bladder perforation with forward-propagating laser light (λ = 532nm) at 80 and 120W with an assumption of fiber-cap failure. Perforation time was measured and compared in terms of fiber distance. The results showed that 80 and 120W perforated the tissue up to 2 and 2.5cm, respectively with perforation threshold of 17.2kW/cm(2), and the minimum perforation time was approximately 7s. No perforation occurred at the distance of 3cm for 1-min irradiation at both power levels, but severely carbonized lesions were generated around the irradiated tissue. Although equivalent ablation speed was found between the two power levels, 120W created up to 20% wider craters regardless of fiber distance. With consideration of dense collagen fibers in bladder structure and long surgical distance, direct incidence of laser perforation on bladder wall could be unlikely to happen upon fiber-cap failure during laser surgery.

Thermal Stress Concentration in Plates from Different Materials

DOI: 10.2514/1.C031661 Manufacturing of welded plates with different materials are widely employed in industrial application that is exposed to mechanical and temperature loads. In this study, thermal stress distribution in plates from different materials containing a defect, a rectangular hole with semicircular caps, near a joining line is numerically investigated under the homogeneous change of temperature. Investigations are carried out for the cases of butt and bevel-jointweldsfordifferentlocationsof thehole.Maximumofstress concentrationfactorisencountered whenthe longitudinal-axis of the hole is at an angle of 45 deg to the joining line in the case of butt-joint weld. Variation of the tangentialstresscomponentofthehole-tiphasajumpwhenitcrossesoverthejoiningline.Thestressintensityfactors in considered structures are determined using the obtained results of stress concentration factors for variations of radius of hole-caps. An expression is proposed to obtain the stress in the bevel-joint using values in the butt-joint plate. For a comparison, thermal stresses are also investigated in a model made from an optical sensitive material using the method of mechanical modeling of photothermoelasticity for a butt-joint plate. The comparison shows the agreement between numerical and experimental solutions.

Thermoelastic analysis of a partially insulated crack in a strip under thermal impact loading using the hyperbolic heat conduction theory

In this paper, the transient temperature and thermal stresses around a partially insulated crack in a thermoelastic strip under a temperature impact are obtained using the hyperbolic heat conduction theory. Fourier and Laplace transforms are applied and the thermal and mechanical problems are reduced to solving singular integral equations. Numerical results show that the hyperbolic heat conduction parameters, the thermal conductivity of crack faces, and the geometric size of the strip have significant influence on the dynamic temperature and stress field. The results based on hyperbolic heat conduction show much higher temperature and much more dynamic thermal stress concentrations in the very early stage of impact loading comparing to the Fourier heat conduction model. It is suggested that to design materials and structures against fracture under transient thermal loading, the hyperbolic model is more appropriate than the Fourier heat conduction model.

Study on Temperature Field Characteristic and Temperature Detection of Shuttle-Type High-Temperature, High-Pressure, Anti-Erosion Special Valve

Check valve is an important fluid control component of pressure pipeline system, shuttle-type high-temperature, high-pressure, anti-erosion special valve is a new type of check valve. The simulation with CFD software Fluent and analysis for its internal temperature field was done, and then make a comparison between Shuttle-type high-temperature, high-pressure, anti-erosion special valve and swing check valve, and it can be concluded that the former doesn't exist local thermal stress concentration. Ultrasonic speed in high-pressure water and steam rely on its temperature, according to this relation, it can be calculate the temperature of fluid in the valve by measuring the transit time of ultrasonic.

Effects of imperfect adhesion on thermal micro-residual stresses in polymer matrix composites

This paper investigates the influences of imperfect bonding between the fiber and matrix on thermal micro-residual stress fields in polymer matrix composites. For this purpose, a representative volume element consisting of a three-phase composite material subjected to a uniform temperature change is considered. Based on the energy method, a three-dimensional closed-form solution for micro-residual stresses is obtained. Besides, a finite element model is developed and the results are compared with the analytical solution. Both the energy method and finite element analysis show similar trend for thermal stress distribution along the fiber length, while due to the stress singularity, the interfacial shear stress from the finite element solution cannot satisfy the stress-free condition at the fiber end. The analysis shows that the magnitude of thermal stresses and their distribution mainly depend on the bonding efficiency parameter. An increase in thermal and elastic properties bonding efficiencies leads to a considerable decrease in composite axial and shear residual stresses, while the Poisson's ratio bonding efficiency does not affect the thermal stress fields. The interfacial radial residual stress distribution is approximately independent of the bonding conditions. Inefficient bonding may result in higher residual stresses in comparison with the perfect bonding condition. It means that in cases of low bonding efficiency conditions, the ability of composites to sustain and transmit load decreases drastically. Thermal stress concentration occurs at the vicinity of the fiber ends, although peak values depend on the bonding efficiency value.

Thermomechanical analysis of multi-bead pulsed laser powder deposition of a nickel-based superalloy

In laser powder deposition (LPD) repair of nickel-based turbine blades, hot cracking is the most common defect. The cracking has been found to be associated with thermal stress concentration and low-melting constituents on the grain boundaries. For directionally solidified and single-crystal blades, a positive correlation is established between cracking and the “stray grain” formation. Control of the deposit molten pool shape has been proposed as an effective method to limit the stray grains. However, in multi-bead, multi-layer LPD with a pulsed laser, due to a much more complex bead geometry, appearance of stray grains seems to be random. To obtain insight into cracking and stray grain formation during multi-bead LPD process on nickel-based superalloys, a 3-D transient finite element (FE) model has been developed and a thermomechanical analysis is performed. Time-dependent temperature and thermal strain fields have been predicted. The developed thermal model has been applied to predict and analyze the distribution of thermal stress concentration and the tendency of stray grain formation. The reliability and accuracy of the model are verified experimentally by the measured temperature field profile and the observation of microstructure.

C111 蒸気タービンケーシング簡易モデルによるリスクベース設計および保守管理手法(OS6 火力および診断技術)

Thermal transient stress analyses and pressure stress analyses were conducted using simplified turbine casing models with the variations of dimensional parameters. Thermal stress concentration factors were evaluated by the empirical formula and utilized for elastic-plastic strain analyses. Creep-fatigue damage was evaluated by the cumulative damage rule expressed in terms of cycle fraction and time fraction. The 2-dimensional probability function was used for calculating the failure probability under creep-fatigue damage, which was converted into time-based and cycle-based probability functions. The optimum dimensions were selected by equi-probability contours and the optimum maintenance intervals were determined using risk-cost functions. The proposed method was proved to be effective for the optimum design and maintenance decision making of high temperature components.

The Effects of the Cooling Gallery Position on the Piston Temperature Field and Thermal Stress

The effects of cooling gallery position on the piston temperature field and thermal stress are studied. A finite element model of 1015 engine piston is developed using Pro/E software and finite element analyses are achieved through ANSYS code. Numerical simulations are performed to find the temperature and thermal stress change rules with regard to the cooling gallery position. The results demonstrate that the axial direction position of cooling gallery has obvious effects on the piston temperature while the radial direction position of cooling gallery affects the piston thermal stress a lot. The highest position of cooling gallery should be as the same line as the top of the first ring groove. The distance between cooling gallery and piston radial edge should be more than a certain value to decrease the temperature gradient and thermal stress concentration.

NUMERICAL ANALYSIS ON THE MECHANICAL PROPERTIES OF ORGANIC THIN FILM TRANSISTOR

The organic thin film transistor (OTFT) on flexible substrate electroplated electrodes has many advantages as in the fabrication of low cost sensors, e-paper, smart cards, and flexible displays. In this study, we simulated the mechanical and electrical characteristics of the OTFT with various voltage conditions by using COMSOL. The model consisting of a channel, source and drain was employed to investigate the temperature distribution and thermal stress concentration. The channel length is 40 µm and the voltage ranged between -20V and -40V. The OTFT was fabricated using pentacene as a semiconducting layer and electroplated Ni as a gate electrode. Mechanical properties of the fabricated OTFT were characterized by thermal stress which was predicted with the result of stress distribution.

3D finite element modeling of the thermally induced residual stress in the hybrid laser/arc welding of lap joint

In this study, a three-dimensional (3D) finite element model is developed to investigate thermally induced stress field during hybrid laser–gas tungsten arc welding (GTAW) process. In the hybrid welding case, we focus on the GTAW process sharing common molten pool with laser beam and playing an augment role in the hybrid welding system. An experiment-based thermal analysis is performed to obtain the temperature history, which then is applied to the mechanical (stress) analysis. A modified material model is used to consider the influence of face-to-face contact between the top and bottom metal sheets in the thermo-mechanical analysis of welding lap joints. Results show that the normal stress components prevail in the weld zone during hybrid welding process, and maximum thermal stress exceeding the yield point of material exists at the heat affected zone (HAZ) near the weld pool. Increasing the welding speed causes the penetration and width of weld bead to decrease, and the thermal stress concentration at the welded joint is also reduced accordingly. After welding and cooling down, longitudinal tensile stress (SZ) and transverse compressive stress (SX) are retained in the formed weld, and the higher longitudinal compressive stress exists around the weld bead. In addition, a series of experiments are performed to validate the numerical results, and a qualitative agreement is achieved. Compared to the welded joint obtained by GTAW and laser welding alone, the residual stress concentration in the weld joint obtained by hybrid laser–GTAW is the minimal one.

Thermal Stress Distribution in CF/EP Composite in Low Earth Orbit

Thermal cycle in low earth orbit environment has a negative influence on thermal stress distribution in carbon fiber/epoxy (CF/EP) composite. In our experiment a 3D finite element analysis method was used to analyze thermal stress distribution and failure mechanism in CF/EP composite. Thermal stress distribution was characterized using Von Mises stress, composite potential failure zone was analyzed using parabolic failure criterion, and composite micro-cracking distribution was studied using birth and death technique. Results indicated that thermal stress at different parts of the composite are different: at composite free end zone, the maximum thermal stress 179.9 MPa located at fiber surface with the longest distance between two fibers; while in composite inner zone, the maximum thermal stress is 29.5 MPa located in EP matrix with the shortest distance between two fibers. Thermal stress concentration at composite free end zone induces micro-cracking between fiber and the matrix, besides as thermal cycle increase micro-cracking progress from composite free end zone to composite inner zone, which will degrade the mechanical performance.

Microstructure Evolution of Sn-2.5Ag-2.0Ni Solder Joint with Various Ni Platings on SiC<sub>p</sub>/Al Composites

A novel Sn-2.5Ag-2.0Ni alloy has been developed for soldering of SiCp/Al composites substrate with various types of Ni coatings. An investigation about electroplated Ni layer, electroless Ni-5 wt.% P, Ni-10wt.% P and Ni-B layers has been carried out. It is found that the solder joints possess a single intermetallic compound (IMC) Ni3Sn4, which coarsens with an increase in aging time. The formation of Ni2SnP has been observed to significantly affect the reliability of the solder joints. But the formation of Ni2SnP can be suppressed by lowering the P contents in as-deposited Ni coatings. It has been also found that the thermal stresses generated in solder joint increases with the decrease of P contents in Ni-P layer. Furthermore, the concentration of thermal stresses in the electroplated Ni solder joint is found to be higher than that in other three electroless Ni layers. Out of four as-deposited Ni coatings, the Ni-B layer exhibits good wettability with solder and low IMC growth rate during aging. Also, the shear strength of solder joint decreases with an increase in aging time and Ni-B solder joint demonstrates the highest shear strength after long term aging.

Thermal Stress of Blast Furnace Hearth Linings under Erosion State

The structure of cooling stave was simplified with equivalent convective heat transfer coefficient, and the thermal stress axisymmetric calculation model of blast furnace hearth linings under erosion state was established. The thermal stresses of familiar erosion states were analyzed. The thermal stress concentration of erosion part is an importance cause of erosion development. ‘Elephant-foot’ erosion seldom develops to ‘boiler-bottom’ erosion. ‘Boiler-bottom’ erosion is a ideal long life erosion state. When the erosion stabilizes, smaller cooling intensity is favorable to linings stabilization.