Thermal Stress Concentration Research Articles

Toward excellent performance of Al2O3-ZrO2 reticulated porous ceramics: New insights based on residual stress

In terms of improving properties of reticulated porous ceramics (RPCs) prepared by polymer sponge replica technique, vacuum infiltration process (VI process) is applied to fabricate Al2O3-ZrO2 RPCs with multi-layered struts. This paper focuses on the effect of additives on the rheological properties of Al2O3-ZrO2 slurry. Furthermore, the influence of the ratio of Al2O3/ZrO2 in slurries on the properties of Al2O3-ZrO2 RPCs are investigated, also the residual stress within the Al2O3-ZrO2 multi-layered struts is systematically explored. The results show that the Al2O3-ZrO2 slurry with 0.2 wt% FS20, 0.3 wt% PVA and 0.6 wt% CMC exhibits excellent rheological behavior. In addition, based on the results of finite element method and experiments, as the RPCs with a large amount of Al2O3 in outer layer the compressive residual stress (tangential direction) will exist in the outer layer and reduce the thermal stress concentration near the surface and the edge of struts, which are beneficial to improve the properties of RPCs.

Thermal-Hydraulic Analysis of the LBE Spallation Target Head in JAEA

To perform basic research and development to realize future accelerator-driven systems, a lead-bismuth eutectic (LBE) alloy spallation target will be installed within the framework of the Japan Proton Accelerator Research Complex (J-PARC) project, Japan Atomic Energy Agency. The target will be bombarded by high-power pulsed proton beams (250 kW, 400 MeV, 25 Hz, and 0.5 ms in pulse duration). The beam window (BW) of the spallation target is critical because it should survive under severe conditions that occur, i.e., high temperature, high irradiation, intense stress, and various kinds of damage. Therefore, the target vessel should be carefully designed to obtain an adequate safety margin. Our previous research indicates that there is a stagnant flow region in the LBE at the BW tip due to the symmetric configuration of the target, which causes high temperature and concentration of stress on the BW. On the basis of our previous work, three types of upgraded target head designs are studied in the current research to reduce/move the stagnant flow region from the BW tip and to increase the target safety margin. Thermal-hydraulic analyses and structural analyses for the target head designs are carried out numerically under a steady-state condition. Results illustrate that the designs can almost eliminate the stagnant flow region in the LBE. As a consequence, the concentration of thermal stress on the BW is released and greatly decreased. The safety margin of the target is improved through this study.

Evaluation of the increase in threading dislocation during the initial stage of physical vapor transport growth of 4H-SiC

The increase in threading dislocation during the initial stage of physical vapor transport growth of n-type 4H-SiC crystals was evaluated by cross-sectional X-ray topography. Crystals were grown under two different temperature conditions. A significant increase in threading dislocation was observed in crystals grown at a high, not low, temperature. The local strain distribution in the vicinity of the grown/seed crystal interface was evaluated using the electron backscatter diffraction technique. The local nitrogen concentration distribution was also evaluated by time-of-flight secondary ion mass spectrometry. We discuss the relationship between the increase in threading dislocation and the local strain due to thermal stress and nitrogen concentration.

Fabrication and Thermal Structural Characteristics of Ultra-high Temperature Ceramic Struts in Scramjets

ZrB2-SiC based ultra-high temperature ceramic (UHTC) struts were firstly proposed and fabricated with the potential application in the combustor of scramjets for fuel injection and flame-holding for their machinability and excellent oxidation/ablation resistance in the extreme harsh environment. The struts were machined with electrospark wire-electrode cutting techniques to form UHTC into the desired shape, and with laser drilling to drill tiny holes providing the channels for fuel injection. The integrated thermal-structural characteristic of the struts was evaluated in high-temperature combustion environment by the propane-oxygen free jet facility, subject to the heat flux of 1.5 MW/m2 lasting for 300 seconds, and the struts maintained integrity during and after the first experiment. The experiments were repeated for verifying the reusability of the struts. Fracture occurred during the second repeated experiment with the crack propagating through the hole. Finite element analysis (FEA) was carried out to study the thermal stress distribution in the UHTC strut. The simulation results show a high thermal stress concentration occurs at the hole which is the crack initiation position. The phenomenon is in good agreement with the experimental results. The study shows that the thermal stress concentration is a practical key issue in the applications of the reusable UHTC strut for fuel injection structure in scramjets.

Optimization study on structural analyses for the J-PARC mercury target vessel

The spallation neutron source at the Japan Proton Accelerator Research Complex (J-PARC) mercury target vessel is used for various materials science studies, work is underway to achieve stable operation at 1 MW. This is very important for enhancing the structural integrity and durability of the target vessel, which is being developed for 1 MW operation. In the present study, to reduce thermal stress and relax stress concentrations more effectively in the existing target vessel in J-PARC, an optimization approach called the Taguchi method (TM) is applied to thermo-mechanical analysis. The ribs and their relative parameters, as well as the thickness of the mercury vessel and shrouds, were selected as important design parameters for this investigation. According to the analytical results of 18 model types designed using the TM, the optimal design was determined. It is characterized by discrete ribs and a thicker vessel wall than the current design. The maximum thermal stresses in the mercury vessel and the outer shroud were reduced by 14% and 15%, respectively. Furthermore, it was indicated that variations in rib width, left/right rib intervals, and shroud thickness could influence the maximum thermal stress performance. It is therefore concluded that the TM was useful for optimizing the structure of the target vessel and to reduce the thermal stress in a small number of calculation cases.

In situ measurement on TSV-Cu deformation with hotplate system based on sheet resistance

The in situ measurement of TSVs deformation at different temperature is meaningful for learning more about the thermal deformation schemes of 3D TSVs in the microelectronic devices. An efficient and smart hotplate based on sheet resistance is designed for offering more heat, producing a uniform temperature distribution, relieving thermal stress and heat concentration issues, and reducing room space, which was optimized by the finite element method (FEM). The fabricated hotplate is efficient and smart (2.5 cm × 2.0 cm × 0.5 cm) enough to be located in the limited space during measuring. The thermal infrared imager was employed as the temperature sensor for monitoring the temperature distribution of TSVs sample. The 3D profilometry was adopted as the observer for TSVs profiles survey. The in situ 2D top surface profiles and 3D displacement profiles of TSVs sample at the different temperature were measured by 3D profilometer. The in situ average relative deformation and effective plastic deformation of the TSV sample were measured. With optical measurement method, 3D profilometry, the TSV sample can be tested repeatedly.

Failure Investigation of an AlSi9Cu3 Alloy Die-Cast Cavity Insert

The examination of a fractured cavity insert, cooperated die to finish geometry intricate aluminum alloy die-casting product, is described. The dominating failure mechanism in the investigated insert is the combination of thermal fatigue cracking and stress concentration. A visual fractographic examination was performed at the cavity insert and crack initiated at the periphery adjacent to a sharp corner. Detailed scanning electron microscopy observations and energy-dispersive X-ray suggest that the crack growth was facilitated by a number of elements: oxidation of the cracks’ surfaces, filling of cast alloy, and high stress concentration of the sharp corner. The thermal cracks were also produced product around the inside wall of the cooling channel. Similarly, these cracks were also subjected to oxidation and filling with scale deposit and oxide. Finally, the schematic of the cavity insert fracture evolution during die-casting is discussed.

Effect of an upstream bulge configuration on film cooling with and without mist injection

To meet the economic requirements of power output, the increased inlet temperature of modern gas turbines is above the melting point of the material. Therefore, high-efficient cooling technology is needed to protect the blades from the hot mainstream. In this study, film cooling was investigated in a simplified channel. A bulge located upstream of the film hole was numerically investigated by analysis of the film cooling effectiveness distribution downstream of the wall. The flow distribution in the plate channel is first presented. Comparing with a case without bulge, different cases with bulge heights of 0.1d, 0.3d and 0.5d were examined with blowing ratios of 0.5 and 1.0. Cases with 1% mist injection were also included in order to obtain better cooling performance. Results show that the bulge configuration located upstream the film hole makes the cooling film more uniform, and enhanceslateral cooling effectiveness. Unlike other cases, the configuration with a 0.3d-height bulge shows a good balance in improving the downstream and lateral cooling effectiveness. Compared with the case without mist at M = 0.5, the 0.3d-height bulge with 1% mist injection increases lateral average effectiveness by 559% at x/d = 55. In addition, a reduction of the thermal stress concentration can be obtained by increasing the height of the bulge configuration.

Thermo-mechanical characterization of laser textured LaMgAl11O19/YSZ functionally graded thermal barrier coating

The ablation resistance and adhesion strength of laser textured and as-sprayed functionally graded LaMgAl11O19/YSZ based thermal barrier coating (FG-TBC) was studied and compared with a duplex coating of LaMgAl11O19 and YSZ (DC-TBC). The thermal barrier coatings were sprayed over Hastelloy C263 using atmospheric plasma spray process (APS). The laser texturing was performed using a picosecond pulsed laser. The width-to-depth ratio and groove spacing were varied to analyse and correlate the significance of groove parameters in providing strain tolerance under extreme thermal loading conditions. The ablation behaviour of the coatings was investigated using a scramjet test rig. The shear force induced through the ablation load spalled the as-sprayed DC-TBC. The thermal stress mismatch instigated through the difference in coefficient of thermal expansion (CTE) between discrete YSZ and LaMgAl11O19 layers led to the failure of the as-sprayed DC-TBC. The as-sprayed FG-TBC exhibited an improved ablation resistance. The varying thermo-mechanical properties along the ceramic layer thickness in as-sprayed FG-TBC minimise the concentration of thermal stress across the coating and imparts better thermal stability. The improved mechanical interlocking of molten splats during re-solidification of five discrete layers of ceramic composites inhibits the concentration of stress around the weak links. Compared to the textured DC-TBCs, the textured FG-TBCs exhibited minimal level of distortion and better resistance to ablation. The textured surfaces restrict the propagation of crack across the top surface and also provide improved strain tolerance. The graded structure minimises the stress built up and improves the coating resistance compared to textured DC-TBCs. The adhesion strength of all test surfaces was analysed following the standard test procedure. The textured surfaces having higher surface roughness (in terms of surface area) exhibited improved adhesion strength. The groove parameters were observed to have significant influence in improving the ablation resistance and adhesion strength of the coatings.

Effect of five kinds of pores shape on thermal stress properties of thermal barrier coatings by finite element method

Thermal ablation is a very important technique to characterize the thermal properties of coating systems. On account of the concentration of thermal stress, thermal barrier coatings (TBCs) often break off from the substrate partly or completely during the thermal erosion. In this paper, the thermal erosion simulation of finite element geometric models based on the possible pore shapes were implemented, especially, the influence of pore shapes on the distribution of coating temperature, X component of stress, Y component of stress, XY-shear stress and von-Mises stress were focused on. The effects of the different porosity of square pore coatings on thermal insulation properties and thermal stresses were discussed in term of the simulation results. The simulation results indicate that different shape pores not only affect the thermal stress distribution above the contact area between the bond coating and top coating surface, but also affect the plastic deformation behavior of TBCs. The micromechanism was discussed in details in this study. The computed results verified that, the computational method can successfully predict thermal shear, crack initiation and normal failure mode of the studied TBCs. All the results are in good agreement with the corresponding experimental findings. The failure mechanism factors in this paper are of great importance to explain the failure micro-mechanism of TBCs.

The characteristics of Ta2O5 films deposited by radio frequency pure oxygen ion assisted deposition (RFOIAD) technology

The characteristics of Ta2O5 films deposited by radio frequency pure oxygen ion assisted deposition (RFOIAD) technology were compared to a gas mixture of oxygen and argon (O2/Ar) ion beam assisted deposition experimentally and theoretically, inclusive of optical, electronic, and laser-induced damage resistance. The results showed that the RFOIAD could make removal of the pinholes and oxygen vacancies in the film and increase of the band gap, which resulted in the enhancement of refractive index, breakdown voltage, and the diminution of absorptivity as well as leakage current. The laser induced damage testing suggested that the damage induced damage threshold of the film deposited by RFOIAD could be increased, but the cracks appeared easily due to the concentration of thermal stress. While for the Ta2O5 film deposited using a sputtering gas of the mixtures of oxygen and argon, the porous characteristics were helpful to relief the thermal stress concentration, and the formation of cracks can be avoided.

(Invited) All-Wet TSV Fabrication Using Electroless Plated Barrier and Cu Seed Layers with Pd Nanoparticle Catalyst

Fabrication of TSV is one of key technologies for 3D-ICs. Filling of Cu electrode materials in a small dimension of TSV is essential for realizing fine-pitch TSVs, which reduce the lisk of thermal stress concentration issues. Cost reduction is also required to be feasible TSVs for a variety of devices. All-wet fabrication of TSVs using electroless plating of barrier and seed is one of solutions to fullfill these requirements. For realizing conformal thin barrier metal layers of Co-alloy or Ni-alloy electroless barriers in a high aspect TSV, Pd nanoparticles as catalyst are very effective because their adsorption is uniform throughout top to bottom of TSV. Furthermore they shows improvements of adhesion between dielectric and metal [1,2]. Adhesion issue is a serious obstacle for electroless plated films in general, however, we found displacement plating of Cu on electroless plated barrier layer has an excellent adhesion property. Finally, we succeeded in formation of a high aspect ratio TSV with superfilled Cu on electroless barrier/seed layers. Reliability aspects relating interdiffusion of Cu against the barrier layers will be discussed further. [1] F.Inoue, T.Shimizu, T. Yokoyama, H. Miyake, T.Ito, H. Seki, Y.Shinozaki, T.Yamamoto,and S.Shingubara, Electrochimica Acta, 82, 372-377 (2012). [2] F. Inoue, T. Shimizu, H. Miyake, R. Arima, T. Ito, H. Seki, Y. Shinozaki, T. Yamamoto,S.shingubara, Microelectronic Engineering, 106, 164-167 (2013).

3D Dynamic Simulation Analysis of Thermal-Mechanical Coupling during 7075 Aluminum Alloy Micro-droplet Deposition Manufacture

Abstract In aluminum alloy droplet deposition manufacture process, thermal warping, layering and hot cracking of formed 3D components are the most common defects, which have been found to be associated with the large temperature gradient and thermal stress concentration. To obtain insight into the common defects formation mechanism in metal micro-droplet deposition manufacture, a 3D transient finite element (FE) simulation model has been developed by the APDL(ANSYS) code and element birth-death technique. The distribution and variation of time-dependent temperature and thermal stress fields were predicted and the thermo-mechanical behaviors were analyzed in fabricating aluminum alloy 3D components. And then, a series of deposition experiments were conducted using 7075Al alloy droplets under the setting process parameters (same to the initial and boundary conditions). The results show that the experimental results basically agree with the simulation results of thermo-mechanical behaviors. The reliability and correctness of the simulation model were verified experimentally by the measured temperature field and the observation of thermal deformation and hot cracking of formed 3D component. The work provides a useful theoretical and experimental guide for optimizing metal droplets deposition manufacture.

Degradation characteristics and Ni3Sn4 IMC growth by a thermal shock test in SAC305 solder joints of MLCCs applied in automotive electronics

In this study, the degradation characteristics in MLCCs SAC305 lead-free solder joints were verified by thermal shock testing (from −40°C to 125°C). The thermal stress distribution in the solder joints was simulated utilizing ANSYS 13.0 and the thermal stress concentration was verified between the Ni plating layer and Cu electrode. After the test, the degradation of the mechanical characteristics was compared by conducting shear tests. The shear force decreased by 21.1% after 1200 cycles in C1608. In the case of C1005, the shear force decreased by 21.3% after 1000 cycles. The Ni3Sn4 IMCs were confirmed at the starting point at the shear fracture surface through ingredient analysis after the thermal shock test. Also, by observation of the cross-sections before and after the test, cracks were verified after thermal shock at the 1400th cycle. The cracks formed between the Ni plating layer and the solder joint. After the 1800th cycle, the Ni3Sn4 IMC thickness in C1608 grew to 8 μm. In C1005, the IMC thickness increased to 9 μm. Therefore, the Ni3Sn4 IMC growth affected the degradation and reliability reduction characteristics in the MLCC SAC305 lead-free solder joints. In the future, research aimed at preventing Ni3Sn4 IMCs should be carried out.

열충격 시험을 통한 MLCCs SAC305 무연 솔더 접합부의 IMCs 성장과 접합특성 저하에 관한 연구

The bonding characteristics of MLCCs (multi layer ceramic capacitor, C1608) lead-free solder (SAC305) joints were evaluated through thermal shock test (-40℃~125℃, total 1,800 cycle). After the test, IMCs( intermetallic compounds) growth and cracks were verified, also shear strengths were measured for degradation of solder joints. In addition, The thermal stress distributions at solder joints were analyzed to compare the solder joints changes before and after according to thermal shock test by FEA (finite elements analysis). We considered the effects of IMCs growth at solder joints. As results, the bonding characteristics degradation was occurred according to initial crack, crack propagations and thermal stress concentration at solder-IMCs interface, when the IMCs grown to solder inside.

Characteristics of heat transfer and chemical reaction of methane-steam reforming in a porous catalytic medium

We did a numerical examination of the heat transfer and chemical reaction characteristics in methane-steam reforming, which is widely used in the petrochemical industry. In fact, the prediction of temperature variation along the reformer tube is essential for methanesteam reforming, as it also causes material failures, such as thermal stress concentration. Thus, the influence of the Reynolds number and the porosity variation of catalysts inside the reformer tube on methane-steam reforming was examined. The commercial code of Fluent (V. 13.0) was used for the current simulation. An axisymmetric reformer tube with porous catalytic medium was modeled and two kinds of porosity, which ranged from 0.35 to 0.50, were adopted. The temperature of the fuel gas and the external heat source were 780.15 K and 1291.55 K, respectively. The standard k-e model and the eddy-dissipation-concept model were employed. In addition, conjugated heat transfer was considered for estimation of the heat transfer from the external heat sources to the reformer tube. The Discrete ordinate (DO) model for radiation effects was also used. It was found that the radial temperature distribution of the high Reynolds number is lower than that of the low Reynolds number. The axial temperature distribution varied because of the heat transfer from the external heat source and the dominant endothermic chemical reactions. The mole fraction of products increased as the Reynolds number decreased. Both radial and axial temperature increased inside the reformer tube as the porosity was denser. However, the effect of porosity variation on the methane-steam reforming could not be distinctively observed in this study.

Multistress effects on goldfish (Carassius auratus) behavior and metabolism.

Crossed effects between climate change and chemical pollutions were identified on community structure and ecosystem functioning. Temperature rising affects the toxic properties of pollutants and the sensitiveness of organisms to chemicals stress. Inversely, chemical exposure may decrease the capacity of organisms to respond to environmental changes. The aim of our study was to assess the individual and crossed effects of temperature rising and pesticide contamination on fish. Goldfish, Carassius auratus, were exposed during 96 h at two temperatures (22 and 32 °C) to a mixture of common pesticides (S-metolachlor, isoproturon, linuron, atrazine-desethyl, aclonifen, pendimethalin, and tebuconazol) at two environmentally relevant concentrations (total concentrations MIX1 = 8.4 μg L(-1) and MIX2 = 42 μg L(-1)). We investigated the sediment reworking behavior, which has a major ecological functional role. We also focused on three physiological traits from the cellular up to the whole individual level showing metabolic status of fish (protein concentration in liver and muscle, hepatosomatic index, and Fulton's condition factor). Individual thermal stress and low concentrations of pesticides decreased the sediment reworking activity of fish and entrained metabolic compensation with global depletion in energy stores. We found that combined chemical and thermal stresses impaired the capacity of fish to set up an efficient adaptive response. Our results strongly suggest that temperature will make fish more sensitive to water contamination by pesticides, raising concerns about wild fish conservation submitted to global changes.

Optimized functionally graded La2Zr2O7/8YSZ thermal barrier coatings fabricated by suspension plasma spraying

In this paper, an optimized functionally graded coating (OFGC) was successfully fabricated by suspension plasma spraying (SPS) with feedstocks of the suspension of nanoparticles. La2Zr2O7/8YSZ OFGC with gradual compositional variation along the through-thickness direction is proposed to mitigate spallation and crack formation owing to the high residual stresses caused by frequent thermal cycling for TBCs. The single ceramic layer coatings (SCLC) of LZ and double ceramic layer coatings (DCLC) of LZ/8YSZ were fabricated by SPS as comparison. The phase composition and microstructure of the SCLC, OFGC and DCLC were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). Moreover, the thermal cycling tests were carried out to evaluate their thermal shock behavior. Changes in weight and morphology of specimens were analyzed during thermal cycling tests. The results showed that OFGC has extended lifetime compared with SCLC and DCLC. The failure of DCLC with clear interface between different ceramic layers occurred via delamination mode, as a result of crack initiation and propagation generated by thermal mismatch between LZ and 8YSZ. While the failure of OFGC occurred in thermally grown oxide (TGO) layers, indicating that the gradual compositional variation avoided thermal stress concentration in the top ceramic layers.

Thermal Analysis of the Molten Lead KettleFailure at the Galvanizing Plant and Developmentof Novel Design Using CFD Techniques

The present work discusses problem of failure of kettle holding molten lead in a galvanizing plant. A CFD code Fluent was used to solve the problem using computational approach. The geometry is built in the GAMBIT environment and 3D solvers for turbulence, combustion and radiation heat transfer was applied. The study revealed the detection of the hot spot exactly at the same location of the kettle that was observed at the plant. This confirms the validity of the CFD Model. Based on this model the suggestions are made to overcome kettle bulging. For this two approaches are suggested. They are, differential heating of the kettle and the reduction in the width of the burner. The effect of both these alternatives is studied using a CFD model and the effectiveness of the results are discussed and compared using dimensionless surface parameter representing the temperature distribution and flue gas temperature. Based on the results obtained an improvised geometry for the kettle heating is proposed. The proposed novel design has the following features:  50% reduction in the fuel consumption and lower start-up time.  Separation of the zone of mechanical and thermal stresses and lower concentration of thermal stresses.  Increase in the heat transfer area and a reduction in the heat losses through the exhaust.  Reduction in the amount of lead used for the operation.The present work discusses problem of failure of kettle holding molten lead in a galvanizing plant. A CFD code Fluent was used to solve the problem using computational approach. The geometry is built in the GAMBIT environment and 3D solvers for turbulence, combustion and radiation heat transfer was applied. The study revealed the detection of the hot spot exactly at the same location of the kettle that was observed at the plant. This confirms the validity of the CFD Model. Based on this model the suggestions are made to overcome kettle bulging. For this two approaches are suggested. They are, differential heating of the kettle and the reduction in the width of the burner. The effect of both these alternatives is studied using a CFD model and the effectiveness of the results are discussed and compared using dimensionless surface parameter representing the temperature distribution and flue gas temperature. Based on the results obtained an improvised geometry for the kettle heating is proposed. The proposed novel design has the following features:  50% reduction in the fuel consumption and lower start-up time.  Separation of the zone of mechanical and thermal stresses and lower concentration of thermal stresses.  Increase in the heat transfer area and a reduction in the heat losses through the exhaust.  Reduction in the amount of lead used for the operation.

Numerical Modeling and Experimental Verification of Residual Stress in Autogenous Laser Welding of High-Strength Steel

A three-dimensional finite element (FE) model was developed to numerically calculate the temperature field and residual-stress field in the autogenous laser welding process. The grid independence of the FE model was verified to eliminate the variation of the heat flux between adjacent elements. A cut-off temperature method with combination of the tensile testing was used to consider the effect of high-temperature material properties on the numerical simulation. The effect of the latent heat of fusion and evaporation was also taken into consideration. High compressive initial stress was presented in the selected high-strength steel plates. A subroutine was written to consider the initial stress in the FE mode. Predicted residual stress agreed well with experimental data obtained by an X-ray diffraction technique. Results showed that the transverse and longitudinal residual stresses prevailed in the autogenous laser welding process, and the thermal stress concentration occurred in the molten pool and its adjacent regions. The effect of the welding speed on the distribution of residual stress was also studied. The values of residual stress decreased with an increase in the welding speed.