Thermo-mechanical Stress Distribution Research Articles

Sequential thermomechanical stress and cracking analysis of photovoltaic modules with full and half-cut cells

The transition from conventional full-cell patterns to half-cell modules in the photovoltaic (PV) industry promises enhanced stability and efficiency. This study investigates the thermomechanical behaviour and stress distribution within half-cell and full-cell PV modules during the manufacturing and operational phases. Using finite element modeling, the research simulates sequentially the manufacturing and operating processes such as laser cutting, soldering, lamination, mechanical loading and thermal cycling. The Extended Finite Element Method (XFEM) predicts crack initiation and propagation in the crack-sensitive regions in PV modules during their entire life. Key findings highlight stress concentrations during manufacturing, influenced by parameters like laser power, scribing length and Silicon wafer size. Mechanical loading (ML) and thermal cycling (TC) induce additional stresses, impacting module reliability. The results are useful for the optimization of material properties and the development of advanced design strategies contributing to the durability and efficiency of PV systems.

Analysis of micro-defect-induced cracking in the IPyC layer of TRISO particle with the XFEM

PurposeWe use the extended finite element method (XFEM) to model the whole process of initiation and propagation of cracks in the inner dense pyrolytic carbon (IPyC) layer of tri-structural isotropic (TRISO) particle induced by the microdefect in an irradiation-induced thermomechanical coupling environment and study the effect of microdefect sizes on the propagation path.Design/methodology/approachThe irradiation-induced thermal–mechanical coupling analysis is first conducted for the representative volume element (RVE) of the TRISO particle by using the conventional finite element method (CFEM) so that the stress distribution is obtained. The stress results are then restored for the enriched elements, and the simulation of crack initiation and propagation is eventually carried out by using the XFEM.Findings1. As a crack initiates in the IPyC layer, it will terminate at the free edge of the RVE TRISO particle in the end. 2. The size of the microdefect has a significant impact on the propagation path.Originality/valueThe ceramic dispersion microencapsulated (CDM) fuel is a good accident-resistant fuel whose safe operation is crucial to the safety and reliability of the whole nuclear reactor. It is of great scientific significance and practical value to study the irradiation-induced thermomechanical coupling stress distribution and cracking behavior in the IPyC layer of TRISO particles for the CDM fuel. Crack initiation and propagation analysis is challengeable for this complex multi-layer structure. This can help understand the failure mechanism of TRISO particles and evaluate the operation safety of the reactor.

Thermo-Mechanical Stress Distributions in a Ceramic Matrix Composites Turbine Vane Coated with Environmental Barrier Coatings

It is of great significance to obtain an accurate stress assessment when replacing traditional metal components with ceramic matrix composites (CMCs) in turbine engines. The current study aims to investigate the stress characteristics of CMCs turbine vanes with multilayer-structured environmental barrier coatings (EBCs) using numerical simulation techniques. A three-dimensional finite element model of CMCs turbine vanes coated with EBCs was formulated. The distribution of thermal residual stresses generated during the manufacturing process of EBCs and the distribution of stresses under different loading conditions were calculated and compared. The results show that the hoop stress (σ11) and spanwise stress (σ22) in the turbine vanes are significantly higher than the through-thickness stress (σ33) under coupled loads. The maximum hoop stress (σ11) is approximately 346 MPa. The thermal residual stress induced during the EBCs manufacturing process reaches a maximum of approximately 360 MPa. The loading conditions significantly influence the stress distribution of EBCs, and the stress distribution of EBCs exhibits certain regularities at different heights under varying loading conditions. These results enable us to gain a deeper understanding of the failure mechanism of CMCs/EBCs turbine vanes and can improve the optimization capabilities for these components.

A Fully Coupled Model of Multi-Chip Press-Pack IGBT for Thermo-Mechanical Stress Distribution Prediction

In this article, a fully coupled multiphysical model of multichip press-pack insulated gate bipolar transistor (PPI) considering surface roughness is proposed. This model realizes the coupling of the electrical-, thermal- and mechanical domains through contact resistance. The simultaneous prediction of temperature and stress under various surface roughness is achieved. First, the fully coupled model and the contact resistance model are established. Second, both thermomechanical stress under the clamping phase and heating phase are analyzed. The influence of the surface roughness on the thermo-mechanical stress distribution is also investigated based on the proposed model. Results show that the surface roughness does not affect the thermomechanical stress distribution under the heating phase, while the amplitudes increase with surface roughness. Third, the correctness of the model is verified by the temperature measurement and deterioration observation results. The observed fretting scratches on the additional metallization area after the power cycling test validates the correctness of the deformation prediction. Finally, the strain-life fatigue model of PPI is established, it turns out that the surface roughness has an impact on the fatigue life of the chip metallization area.

Long-Term Reliability Evaluation of Power Modules With Low Amplitude Thermomechanical Stresses and Initial Defects

In renewable energy and grid applications, solder-attached power modules are subject to fatigue stress cycles of low amplitudes, but the devices will be in service for decades. This article aims to understand the slow aging process by focusing on the initial defects and their growth under the stress cycles of concern. A finite-element analysis (FEA) is performed to obtain the thermomechanical stress distribution around the defects, which is combined with the solder material’s property to give a lifetime model. Effort is made to validate the model by power cycling plus microscale computed tomography (CT) scanning. It is found that a void in the solder layer may first transform into a crack on the material boundary, which then grows progressively more rapidly until device failure. A numerical example is shown for evaluating the lifetime of an IGBT power module in a “soft-open-point” (SOP) converter designed for an 11-kV power network.

Comprehensive Study and Realizing an Enhanced Efficiency of the Thermoelectric Generator Along with Its Thermomechanical Properties

This paper proposes a real-time simulation model to simplify the research and outcome of a portable thermoelectric generator (TEG) system in real conditions. Consequently, the model is divided into three parts: conventional, segmented, and hybrid TEG systems. The conventional TEG system consisted of Bi2Te3 as the p-type and the n-type materials, whereas the segmented and the hybrid TEG systems consisted of a different combination of materials, including PbTe and Sb2Te3. The optimization of the TEG system length was carried out to achieve the highest power output, which was found to be 2 mm. In addition, thermomechanical stress distribution analysis of the module was conducted to determine the maximum load the TEG system could withstand before undergoing fracture, depending upon the yield strength of the material. The stress was analyzed in all three TEG systems, and the results were evaluated. Results were observed from the optimized length at 2 mm. The conventional, segmented, and hybrid TEG systems showed maximum power output of 147.122 mW, 171.934 mW, and 550 mW, respectively, with a temperature difference at 50 K.

Thermo-mechanical stress analyses of solid oxide fuel cell anode based on three-dimensional microstructure reconstruction

During high-temperature operation of a solid oxide fuel cell, the stresses caused by mismatch of thermal expansion coefficients between different materials and external mechanical loads may cause the rising of damage risk of nickel-yttria-stabilized zirconia anode. It is quite difficult to quantify the mechanical characteristics of a composite anode without investigating on the stress distribution in its real microstructure. However, the high operating temperature and extremely complex microstructure in micro-scale determine the high difficulty in in-situ measurement of thermo-mechanical stress distribution. In this work, the microstructures of six different anode samples, fabricated by using identical nickel oxide-yttria-stabilized zirconia powder mixture, are reconstructed in three-dimension based on the dual-beam focused-ion-beam-scanning-electron-microscopy. The three-dimensional thermo-mechanical stress distributions of different microstructures are conducted at operating temperature based on the finite element method. The effects of both thermal expansion coefficients mismatch between nickel and yttria-stabilized zirconia and external mechanical loads are analyzed. The mechanical failure probabilities of yttria-stabilized zirconia phase in different reconstructions are estimated based on the obtained stress distributions to investigate the influence of microstructure characterizations on nickel-yttria-stabilized zirconia anode strength.

Thermo-Mechanical Stress Simulation of Unconstrained Region of Straight X20 Steam Pipe

Abstract The thermo-mechanical stress, strain and temperature distribution across the thickness of X20 steam pipe at region less susceptible to thermo-mechanical failure was simulated using finite element analysis software, Abaqus. The mesh convergence studies conducted showed that 10 mm mesh size was suitable for the simulation. The temperature distribution profile across the thickness of Pyrogel showed that pyrogel is an excellent insulation jacket for steam pipes. The maximum stress value obtained from the simulation shows that the pipe is operating below the yield strength of X20 steel at the region under study. Hence, the pipe’s failure at this region due to thermo-mechanical stress or strain only is practically impossible over a long period of time. A deviation of 0.5% was found to exist between the analytical and simulated stress value obtained. This indicates a strong correlation between the simulated and analytical stress results.

Physical Limitations of Phosphor layer thickness and concentration for White LEDs

Increasing phosphor layer thickness and concentration can enhance the lumen flux of white LED (W-LED). In this work, we found that increasing the phosphor layer thickness and concentration can increase its temperature, and there is also a maximum thickness and concentration beyond which their increase will not lead to lumen increase, but only temperature increase. Higher thickness and higher concentration also results in warm light instead of White light. The maximum thickness and concentration are found to be limited by the scattering of light rays with higher % decrease of blue light rays than the yellow light rays. The results obtained in this work can also be used to compute the temperature and thermo-mechanical stress distribution of an encapsulated LED, demonstrating its usefulness to the design of encapsulated LED packages. Simulation software like ANSYS and TracePro are used extensively to verify the root cause mechanisms.

Multiphysics Characterization of a Novel SiC Power Module

This paper proposes a novel power module concept specially designed for highly reliable silicon carbide (SiC) power devices for medium- and high-power applications. The concept consists of two clamped structures: 1) a press-pack power stage accommodating SiC power switch dies and 2) perpendicularly clamped press-pack heatsinks, in which the heatsinks are in contact with electrically insulated case plates of the power stage. The concept enables bondless package with symmetric double-sided cooling of the dies and allows for an order of magnitude higher clamping force on the heatsinks than what can be applied on the dies. The concept has been evaluated in a first demonstrator (half-bridge configuration with 10 paralleled SiC dies in each position). The experimental methodologies, setups, and procedures have been presented. The commutation loop inductance is approximately 9 nH at 78 kHz. The junction-to-case thermal resistance is approximately 0.028 K/W. Furthermore, a simplified 3-D finite-element thermomechanical model representing the center unit of the demonstrator has been established for the purpose of future optimization. The accuracy of the simulated temperatures is within 4% compared to the measurements. Finally, a 3-D thermomechanical stress distribution map has been obtained for the simplified model of the demonstrator.

A numerical model for soldering process in silicon solar cells

Solar or Photovoltaic (PV) cells are utilized to convert solar energy into electricity through the photovoltaic effect. Although, Silicon solar cell is one of the most prevalent type of solar cells; manufacturing of this type of solar cells especially soldering copper electrodes to the silicon wafer is very challenging. If the soldering parameters are not selected properly, small cracks will be created in silicon layer, and finally, fracture of the solar cell will happen. The goal of the present study is to develop, for the first time, a finite element model to simulate the soldering process in the fabrication of silicon solar cell in which the soldering system is passing over the solar cell to connect the copper electrode to the silicon wafer. The temperature and thermomechanical stress distributions induced in different layers of the cell are extracted. The effects of several parameters such as soldering power, soldering speed, and the thickness of silicon wafer on the temperature and stress distributions are probed. The results show that the silicon layer is the most critical layer in a silicon solar cell during the soldering process which is susceptible to high thermomechanical stresses. It is shown that increasing the soldering system power and decreasing the soldering system speed lead to increase of thermomechanical stresses induced in the silicon wafer. In addition, using thicker silicon wafer can decrease the thermomechanical stresses generated in this layer.

Mechanical Deformation of Sintered Porous Ag Die Attach at High Temperature and Its Size Effect for Wide-Bandgap Power Device Design

The mechanical properties of sintered Ag paste with microporous structure have been investigated by tensile and shear tests, focusing on the temperature-dependent plastic deformation at various temperatures from 25°C to 300°C, corresponding to the target operating temperature range of emerging wide-bandgap semiconductor devices. Specimens were prepared by sintering hybrid Ag paste consisting of microflake and submicron spherical Ag particles, simulating a typical bonding process for power semiconductor die attach. Mechanical tests revealed that the unique microstructure caused a brittle-to-ductile transition at temperature of around 160°C, remarkably lower than that of bulk Ag. The obtained Young’s modulus and shear modulus values indicate obvious softening with increasing temperature, together with a remarkable decrease in Poisson’s ratio. These plastic behaviors at elevated temperature can be explained based on Coble creep in the microporous network structure. Fracture surfaces after tensile and shear tests indicated unique features on scanning electron microscopy, reflecting the variation in the ductile behavior with the test temperature. Furthermore, these temperature-dependent mechanical parameters were employed in three-dimensional finite-element analysis of the thermomechanical stress distribution in wide-bandgap semiconductor module structures including Ag paste die attach of different sizes. Detailed thermal stress analysis enabled precise evaluation of the packaging design for wide-bandgap semiconductor modules for use in high-temperature applications.

Dynamic thermoelectric model of a light-emitting structure with a current spreading layer

The non-stationary thermoelectric model of the axisymmetric heterostructure of a light-emitting device is considered taking into account positive feedback mechanisms and the effect of the current-spreading-layer resistance. Taking into account the current localization effect, the nonuniform distribution of the heterojunction current density over the heterostructure area is determined. The non-stationary thermal conductivity equation with temperature-dependent current density flowing into the heterojunction is solved by the numerical–analytical iterative method. Based on the developed model, the current density, temperature, and thermomechanical stress distributions for the heterojunction plane are determined.

Effect of Design Parameters on Thermomechanical Stress in Silicon of Through-Silicon Via

We examined the effect of the design parameters of a through-silicon via (TSV) on the thermomechanical stress distribution at the bottom of the TSV using finite element analysis. Static analyses were carried out at 350 °C to simulate the maximum thermomechanical stress during postplating annealing. The thermomechanical stress is concentrated in the lower region of a TSV, and the maximum stress in silicon occurs at the bottom of the TSV. The TSV diameter and dielectric liner thickness were two important determinants of the maximum stress in the silicon. The maximum stress decreased with decreasing TSV diameter, whereas the effect of aspect ratio was negligible. A thick dielectric liner is advantageous for lowering the maximum stress in silicon. The minimum dielectric thickness resulting in a maximum stress less than the yield stress of silicon was 520, 230, and 110 nm for via diameters of 20, 10, and 5 μm, respectively. The maximum stress also decreased with the thickness of the copper overburden.

Exact and Approximate Solutions of Thermoelastic Stresses in Functionally Graded Cylinders

Thermoelasticity behavior of functionally graded thick hollow cylinders with power law and exponentially variations of material properties versus radius is studied analytically. Temperature, displacement and thermomechanical stress distributions are obtained and discussed. Approximate homogeneous multilayer semi-analytical method is used successfully with adequate accuracy and finite layers. Different combinations of ceramics and metals are checked out for thermoelastic stresses due to high temperature and internal pressure. We conclude that the approximate method is simple and has appropriate results. The effects of high temperature on the stresses are more important than the high internal pressure.

Thermo-mechanical analysis of an internal cooling system with various configurations of a combustion liner after shell

The after-shell section, which is part of the gas turbine combustion liner, is exposed to the hottest combustion gas. Various cooling schemes have been applied to protect against severe thermal load. However, there is a significant discrepancy in the thermal expansion with large temperature differences, resulting in thermo-mechanical crack formation. In this study, to reduce combustion liner damage, thermo-mechanical analysis was conducted on three after-shell section configurations: inline-discrete divider wall, staggered divider wall, and swirler wall arrays. These array components are well-known heat-transfer enhancement structures in the duct. In the numerical analyses, the heat transfer characteristics, temperature and thermo-mechanical stress distribution were evaluated using finite volume method and finite element method commercial codes. As a result, we demonstrated that the temperature and the thermo-mechanical stress distribution were readily dependent on the structural array for cooling effectiveness and structural support in each modified cooling system. Compared with the reference model, the swirler wall array was most effective in diminishing the thermo-mechanical stress concentration, especially on the inner ring that is vulnerable to crack formation.

Energy Release Rate Estimation for Through Silicon Vias in 3-D IC Integration

The technology of 3-D IC integration is expected to satisfy the demand for high-performance, better reliability, miniaturization, and lower priced portable electronic products. Since through silicon via (TSV) is at the heart of 3-D IC integration architectures, the reliability issues with TSV interconnects are an area of extreme concern. Due to the large thermal expansion mismatch among the copper (Cu), silicon die, and silicon dioxide (SiO2) dielectric layer, the induced thermal stresses and strains can occur and become the driving forces that cause failures in TSV interconnects. Hence, thermomechanical stress analyses and failure mode investigations for TSVs are in urgent need. Among the typical failures, delamination is the mostly common failure type, which is caused when lower energy release rate (ERR) or higher critical stresses at interfaces are present. In this paper, the finite element analysis (FEA) for a symmetrical single inline Cu-filled TSV with redistribution layer is illustrated and has been used to realize the thermomechanical stress distribution for TSVs in 3-D IC integration. Moreover, four kinds of interfacial cracks that were embedded in the interface of SiO2 passivation and Cu seed layer (Cu pad and TSV wall delamination cases) and the critical stress areas observed from FEA are introduced to estimate the interfacial ERR using modified virtual crack closure technique. The parametric study has also been adopted to capture the most important mechanical factors of the TSVs to comprehend the corresponding ERR. The significance of discussed parameters such as crack length, TSV diameter, TSV pitch, TSV depth, SiO2 thickness, and Cu seed layer thickness are also examined. It is believed that these results would be helpful to avoid delamination of TSV interconnects in 3-D IC integration.

Thermomechanical Stress Distributions in a Gas Turbine Blade Under the Effect of Cooling Flow Variations

Thermomechanical stresses in gas turbine blades are investigated. Attention is focused on effects caused by varying the cooling airflow that runs through the blade interior, keeping constant a mainstream condition around the blade surface. Stress concentration was predicted numerically under engine real operating conditions. Temperature distributions in the metal blade surface produced by convective boundary conditions were linked with heat conduction within the blade using a conjugate solution. Results of stress concentration in the blade material for reduced cooling flow rate, blocked cooling ducts, and rotation rate were obtained. It is shown that temperature and stress distributions are a strong function of position in blade interior material and surface. Thermomechanical stress concentration was observed in the leading edge, with the endwall region affected by large stress concentration. Stress magnitude increments were found for combined cyclic thermal heating and sustained mechanical loads on specific planes of airfoil span for reduced cooling flow. Also, large stress gradients between leading and trailing regions of the blade were observed. The study reveals that blocking channels increase stresses in the central region of blade transversal cross section.

Using FEM to determine the thermo-mechanical stress in tube to tube–sheet joint for the SCC failure analysis

The stress corrosion cracking in a weld of the tube to tube–sheet region of heat exchangers is a common problem. Thermo-mechanical stress in tube to tube–sheet joints including welding effect should be determined in this situation for failure analysis. In this paper, the Finite Element Method (FEM) is used to predict the thermo-mechanical stresses including welding residual stress in a tube to tube–sheet weld. Both the thermo-mechanical stress distribution with and without the welding residual stress have also been investigated by numerical simulation. The welding, operating temperature, and operating pressure have effect on total stresses. Especially, the welding residual stresses play an important role in total stress state in tube to tube–sheet joints. Geometric discontinuities of the vicinity of gap cause the welding joint to experience a local stress concentration. A high tensile stress in the tube to tube–sheet region has been demonstrated by FEM, which is the stress aspect for the SCC phenomenon of austenitic stainless steel in chloride environment.

Stress Analysis of Ultra-Supercritical and Electrical Three-Way Valve Body Intensity in a High Pressure Heater Using FEM

Temperature and thermo-mechanical coupling stress distributions of the electrical three-way valve body in a high pressure heater are investigated by using finite element method analysis. Maximum stress at the outer wall region of the valve body has exceed the allowable stress of the material is obtained. The weak place is improved by increasing the wall thickness and the stress meet the requirement of the allowable stress of the material after FEM analysis. And the thermo-mechanical coupling stress are classified and evaluated in the most dangerous section. The evaluation results show that the various types of stress all meet intensity requirements, which provide a guideline for the design and optimization of the high temperature and high pressure valve in the future.