Large Thermal Stresses Research Articles

3D Reconstruction and Thermal Stress Analysis of Re-Oxidized Ni-YSZ Anode Functional Layer of Solid Oxide Fuel Cells

Nickel (Ni) has been widely used in the anode of solid oxide fuel cells (SOFCs) because of its high electrocatalytic activity towards oxidation of H2, good electronic conductivity, and formation of pores when initial nickel oxide (NiO) is reduced to Ni. When SOFCs operate at high temperature (> 800 °C), Ni can be re-oxidized to NiO when there is an interruption of fuel supply, air inflow due to deterioration of seals, or excessive supply of H2O in fuels. Volume expansion of Ni can occur significantly during this re-oxidization, and it can induce severe stress which may cause cracks and changes in anode’s microstructure. In this study, re-oxidized Ni-YSZ anode functional layer (AFL) was 3D reconstructed by using image data obtained from focused ion beam-scanning electron microscopy (FIB-SEM), which is powerful technique to separate the elements and visualize the 3D structure. Then, microstructural properties such as volume fraction, phase boundary density, and connectivity were analyzed using image processing tool. 3D mesh model for finite element analysis (FEA) was generated and thermal stress was analyzed at various temperature. In thermal FEA, we calculated and found some NiO-YSZ interfaces exhibited large thermal stress with magnitude larger than 317 MPa, which is the ultimate tensile stress of Ni phase. This is because the lack of porosity in NiO-YSZ AFL inhibits the release of thermal stresses generated at high temperature. Thus, this high thermal stress could cause critical failure on the cell structure because re-oxidized Ni-YSZ anode still contains Ni phase (< 10 vol %). The details of the 3D reconstruction, image processing, and FEA results will be presented in the conference.

Design and simulation of thermal residual stresses of coatings on WC-Co cemented carbide cutting tool substrate

Large thermal residual stresses in coatings during the coating deposition process may easily lead to coating delamination of coated carbide tools in machining. In order to reduce the possibility of coating delamination during the tool failure process, a theoretical method was proposed and a numerical method was constructed for the coating design of WC-Co cemented carbide cutting tools. The thermal residual stresses of multi-layered coatings were analytically modeled based on equivalent parameters of coating properties, and the stress distribution of coatings are simulated by Finite element method (FEM). The theoretically calculated results and the FEM simulated results were verified and in good agreement with the experimental test results. The effects of coating thickness, tool substrate, coating type and interlayer were investigated by the proposed geometric and FEM model. Based on the evaluations of matchability of tool substrate and tool coatings, the basic principles of tool coating design were proposed. This provides theoretical basis for the selection and design of coatings of cutting tools in high-speed machining.

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.

Structural optimization for CFRP cryogenic tank based on energy release rate

In the present study, we conducted structural optimization for CFRP cryogenic propellant tank. Under the cryogenic temperature, large thermal stress between metallic material and CFRP might cause the debonding failure. Optimizations were performed by considering crack propagation. Energy release rate of debonding crack calculated by virtual crack closure method was used as objective function. Two analytical conditions were considered: when merely the temperature of tank was decreased and when the pressure was applied to the tank in the decreased temperature. Two types of optimizations were conducted for each condition. One is liner wall thickness optimization. Another optimization is curved wall shape optimization. On the basis of these results, we proposed a new design of the CFRP cryogenic tank. The new design that has notch and optimized dorm shape can reduce energy release rate near the cap and prevent crack propagation.

Evolution and interaction of surface cracks under thermal shock load

Thermal cracks would be easily formed on the surface of material under thermal shock. To investigate the thermal crack evolution behavior, the cyclic thermal shock test is applied and the finite element simulation is proposed to study the thermal stress field and the interactions between thermal cracks. Test results show that the thermal cracks nucleate on the specimen surface largely and finally form in stable thermal crack network. The length of the cracks could reach several millimeters, however, their depth is only about hundreds of microns. Simulation results show that large temperature and thermal stress gradient would occur in the cooling process, and the higher the cooling speed the higher the maximal thermal stress. Parallel and coplanar cracks both reflect weak interactions in crack shielding and enhancing effect, respectively. And the influence factors are also modeled in different crack distributions. All the simulation results agrees well with the experiment. DOI: http://dx.doi.org/10.5755/j01.mech.22.2.12671

Effects of surface roughness on thermo-mechanical fatigue life of a P91 power plant steel

P91 martensitic steel has now been widely used for power plant components such as steam pipe sections and headers. With the shift to renewable sources, traditional fossil power plants are increasingly expected to operate under so called “two shifting” conditions (high frequency start up/shut down cycles from a partial load condition) to match market demands. Such conditions increase the potential for large thermal stresses to be induced in thick walled components, making thermo-mechanical fatigue (TMF) and creep-fatigue interaction a life limiting concern. It is important to investigate the behaviour of P91 power plant steel under cyclic creep-fatigue interaction conditions in order to estimate the component remnant life under various possible operating strategies. Specimens used for TMF testing are commonly hollow (unlike solid specimens used in isothermal tests) to allow for higher cooling rates (with insignificant radial temperature variations) by injecting air. It is difficult to polish the internal surface to the same extent as the external surface of the specimen (with a roughness (Ra) of 0.8μm). Concerns have been expressed as to whether this type of uncontrolled surface roughness could significantly affect the fatigue life of the specimen since most fatigue cracks often initiate at the surface of the material. In this work, the roughness profile of the internal surface of the TMF sample is measured using Alicona optical profilometer. Resultant surface profiles are idealised and used to simulate distributions of stress and plastic strain under fatigue load using multi-axial visco-plasticity model. Concentration of stress and higher plastic stain accumulations are observed at the peak region of the roughness profile and crack initiations are expected to occur at those regions. Using accumulated plastic strain as a failure criterion for the fatigue, shorter fatigue lifetime is expected for specimen with rougher surface relative to the polished specimen. Optical and scanning electron microscopy (SEM) has been used to investigate the nature of the cracks initiating from the internal and external (polished) surfaces of a failed TMF test specimen.

Influence of Powder Bed Preheating on Microstructure and Mechanical Properties of H13 Tool Steel SLM Parts

Powder bed preheating is a promising development in selective laser melting (SLM), mainly applied to avoid large thermal stresses in the material. This study analyses the effect of in-process preheating on microstructure, mechanical properties and residual stresses during SLM of H13 tool steel. Sample parts are produced without any preheating and are compared to the corresponding parts made with preheating at 100°, 200°, 300°, and 400°C. Interestingly, internal stresses at the top surface of the parts evolve from compressive (-324MPa) without preheating to tensile stresses (371MPa) with preheating at 400°C. Nevertheless, application of powder bed preheating results in a more homogeneous microstructure with better mechanical properties compared to H13 SLM parts produced without preheating. The fine bainitic microstructure leads to hardness values of 650-700Hv and ultimate tensile strength of 1965MPa, which are comparable to or even better than those of conventionally made and heat treated H13 tool steel.

Thermal Stress Analysis of Powder Metallurgy Sintering Process Based on ANSYS

Powder metallurgy is a technology process that the powder is pressed and sintered to get the metal and composite products. The sintering temperature is one of the main technology parameters of pressing sintering. In significant measure, the sintering temperature determines the organization and geometry size of the material, which directly affects the performance of the material. This paper takes magnesium powder metallurgy sintering material as the research object, studying the transient thermal analysis with the ANSYS finite element software, systematic researching the sinter temperature field and thermal stress field of sintered body in the sintering process, and observing in the corresponding distribution of stress field and deformation under different sintering temperature distribution graph. Temperature field and thermal stress field are the main factors affecting the quality of the sintered body in sintering process. Uneven temperature distribution will cause larger thermal stress. When the thermal stress exceeds a certain limit, the sample will be deformed. With the temperature increases, the sintering temperature difference increases, the stress distribution in the constantly changes. The stress and deformation is not entirely visible for sintered body, the deformation and cracking will be side by side in the sintering process. Therefore, theory of temperature field and thermal stress distribution are studied and discussed in certain sintering processing, the sintering process optimization can be designed to provide quantitative basis of the theoretical analysis.

Development of Functionally Graded Tungsten/EUROFER Coating System for First Wall Application

Reduced activation Ferritic/Martensitic (RAFM) steels, e.g. EUROFER are to be used as structural material for the First Wall (FW) of future fusion power plants. The interaction between plasma and FW, especially physical sputtering will limit the FW lifetime under normal operation. Therefore tungsten coating is selected to protect the FW due to its very low sputtering yield and low activation. However, the mismatch in thermo-physical properties between tungsten and EUROFER can lead to large residual thermal stresses and even failure. To overcome the issue of erosion a protective tungsten coating with a functionally graded (FG) tungsten/EUROFER layer (FG tungsten/EUROFER coating system) on EUROFER substrate will be developed and optimized.Non-linear finite element simulations are performed to predict optimal parameters of the coating system. Thereby the potential of the FG-layer in reducing inelastic strains and improving lifetime is demonstrated, and the investigated thickness of FG-layer is suggested. Based on the simulation results samples are fabricated by vacuum plasma spraying (VPS) with three different thicknesses of FG-layer. The microstructural observations revealed that the coating system has fine gradation and variable thickness as designed, low porosity, as well as a sound interface. Berkovich and Vickers hardness identify basic properties of those layers.

Fracture behavior of soldered Al2O3 ceramic to A356 aluminum alloy and resistance of the joint to low temperature exposure

Active solder alloy 92Sn3Ag3Ti was used to join Al2O3 ceramic on cast A356 aluminum alloy substrate using an ultrasonic assisted soldering process. Due to the large discrepancy in thermal shrinkage coefficients between materials, large thermal stresses occur during cooling down to room temperature. Further cooling to −78°C and −196°C was used to inflict additional thermal stress in the samples. Shear strength, image analysis of fractured surfaces and microstructure analysis were performed to study the influence of cooling. Ceramic samples with different surface finish were used to investigate the influence of the contact between ceramic and aluminum alloy substrate on the fracture behavior. Two types of failure were observed, a detachment of the solder alloy from the ceramic surface and a cohesive failure through the solder layer. The results show that cooling the joined sample to −78°C does not significantly affect the bonding strength of the samples. However, samples that were rapidly cooled to −196°C showed reduced shear strengths even when the amount of cohesive failure area was taken into account. Many newly formed cracks in the Sn matrix were observed in the joint after the cooling which were identified as strength-determining defects.

Thermal breakage and fallout behaviors of non-tempered glass under the effect of water film

Delayed activation of the sprinkler or water film system may accelerate the crack and fallout process of the heated glass when encountered in a fire. A series of tests were conducted to investigate the thermal breakage and fallout behavior of a non-tempered glass façade with down-flowing water films. The most widely used non-tempered glass, low emissivity coated glass (Low-E), and float glass were studied in this work. An n-heptane pool fire was employed to heat up the glass panes and the water film was acted when the maximum glazing temperature reached the different designed values. The results illustrate that water film has little influence on the fracture behavior of float glass. However, the water film does accelerate the crack and fallout process of Low-E glass significantly when the glazing temperature is over 70°C. This is because large thermal stress is generated inside the glass due to the continuous cooling of the cold water film, which exceeds the ultimate tensile stress of glass. The water film also has a large impact on the crack propagation and fallout ratio of the glass panes.

Numerical assessment of functionally graded tungsten/EUROFER coating system for first wall applications

Reduced activation ferritic/martensitic (RAFM) steels, e.g. EUROFER, are to be used as structural material for the first wall (FW) of future fusion power plants. The interaction between the plasma and the FW, especially physical sputtering, will limit the FW lifetime under normal operation. Therefore, a tungsten coating should be selected to protect the FW due to its low sputtering yield, low activation, high melting point and high thermal conductivity. However, the mismatch of thermo-physical properties between W and EUROFER induces large residual thermal stresses and even failure of components. Functionally graded material (FGM) is considered as an appropriate solution to mitigate the high residual stresses.In this work, W coatings on EUROFER substrates with W/EUROFER FG-layer (the coating system) are investigated by means of finite element (FE) simulations considering elasto-perfectly plastic and elasto-viscoplastic material models. For determining optimal parameters of the coating system the vacuum plasma spraying (VPS) fabrication process and the operation phase of the fusion reactor are simulated. Based on the FE results creep assessment of the coating system is performed demonstrating the gain in lifetime to be expected when using a FG-layer and investigating its dependence on the thickness of the FG-layer.

Residual Stress in NiO-YSZ Composites and Its Relationship to Microstructure

From the view point of mechanical reliability, the thermal expansion coefficient of anodes is adjusted to add proper amount of YSZ into NiO to prevent large thermal stress between anodes and electrolytes and keep the stress conditions as compressive from R.T. to operation temperatures for protect the electrolyte from tensile destruction and buckling destruction. By contrast, Nickel oxide and YSZ, which thermal expansion coefficient is 14.1 and 10.8 [10-6 K-1], were contacted directly in anode. This suggests that large thermal strain and stress exist in anode at R.T. caused by the difference of large thermal shrinkage from sintering temperature e.g. 1400 oC to R.T. Moreover, these thermal strain and stress are probably strongly influenced by the difference of microstructure such as porosity, volumetric ratio, average diameter, uniformity because the introduction of them occurs at contact area between NiO and YSZ. However, stress conditions in anode, i.e. NiO-YSZ composites, were never studied in author’s knowledge. In this study, we evaluated residual stress and strain in NiO-YSZ composites numerically and experimentally. Numerical simulation was performed by Finite element method (FEM) using MARC. Residual stress and strain were evaluated by X-ray diffraction method. Both results suggested that stress conditions in NiO and YSZ are tensile and compressive, agreed with the expected states from the difference of thermal expansion coefficient. By contrast, average strain and stress were not so high. In the case of uniform NiO-YSZ (50:50 vol%), average stress is ca. 100 MPa and ca. -100 MPa (minus was compressive) for NiO and YSZ, respectively. These phenomena were explained by stress and strain relaxation because contact area between nickel oxide and YSZ was spatially distributed and introduced stress and strain reduced each other. However, stress and strain distributions also suggested that stress and strain widely distributed and its FWHM, i.e. dispersion, was over 200 MPa. In addition, distribution analysis based on the simulation results of various uniformity NiO-YSZ revealed that the increment of ununiformity caused the increment of stress dispersion. These results indicated that bending moment introduced and stress concentration in the microstructure of NiO-YSZ composites causes these widely dispersion.

Thermophysical Properties and Solidification Defects of Fe-22Mn-0.7C TWIP Steel

The thermophysical properties and solidification defects of Fe–Mn–C twinning-induced plasticity (TWIP) steel have been experimentally studied. The liquidus and solidus of Fe–22Mn–0.7C TWIP steels are measured to be 1380 ± 10 and 1270 ± 10°C, respectively. The thermal conductivity of Fe–22Mn–0.7C steel is the lowest compared with that of carbon steel Q235 and stainless steels 304 and J4. Combined with the highest thermal expansion coefficient of TWIP steel, internal cracking is apt to occur due to the extremely large thermal stress during cooling and heating. Severe shrinkage porosity and segregation have been observed in high alloy TWIP steel with large mushy zone. The macrosegregation of manganese along the central line is not as serious as that of carbon in TWIP steel, and the manganese/carbon microsegregation increases with the increase of not only manganese/carbon but also carbon/manganese. TWIP steel is liable to exhibit central shrinkage porosity due to the coarse dendrite and serious microsegregation during solidification. In order to improve the homogeneity and compactness of TWIP steel during traditional slab casting, combining application of control technology, such as decreasing steel casting temperature, descending water cooling intensity, and utilizing electromagnetic stirring and/or soft reduction, is strongly recommended.

Experimental investigation of thermal mixing phenomena in a tee pipe

Abstract T-pipe designs have been widely used in the industry. Among them, mixing of hot and cold water is a common application. In the mixing process, cold and hot fluids are respectively injected through main and branch pipes, and are mixed in the downstream area of T-tube. High temperature hot water flows through the main pipe for a long time; hence, the pipe wall is at high temperatures. The fluid injected into the branch pipe is a cooling fluid. After mixing, the wall of the main pipe is under high thermal fluctuations causing strong thermal stresses, which will eventually lead to pipe damage and water loss. Through flow rate adjustments of the branch and main pipes, when the branch/main velocity ratio was greater than 7.8, showing that cold water hit the bottom of the main pipe and created a reverse flow. This reverse flow created large thermal stresses on the wall. Hence, the branch/main velocity ratio and the hot-water-mixing phenomenon are the focus of this study.

A review on the research progress of push-out method in testing interfacial properties of SiC fiber-reinforced titanium matrix composites

Experimental analysis of single-fiber push-out for SiC fiber-reinforced titanium matrix composites (TMCs) is complicated by the incorporation of large thermal residual stresses, strong chemical bond of the fiber/matrix interface and matrix plastic deformation. This paper summarizes the development of push-out test and the characteristics of push-out test for TMCs such as crack initiating at the bottom face and theoretical analysis of the test. Moreover, it deeply analyzes the progresses of interfacial shear strength and fracture toughness, and work focus is pointed out in future.

FE ANALYSIS OF 6063 ALUMINIUM PROFILES WITH COMPLEX CROSS-SECTION DURING ONLINE QUENCHING PROCESSES

Thermal-elastic-plastic finite element (FE) theory and solution of thermal-structural coupled field were investigated on the basis of commercial software ANSYS. The FE model used to solve the transient air and water cooling processes of extrusion profiles was built through developed programs on the basis of ANSYS. The calculated temperature and displacement of a plate during water cooling are in good agreement with the measured value and the FE model is reliable. Consequently, the temperature, displacement and equivalent stress of a complex cross-section profile from a plant during online quenching process were investigated. Thickness and boundary conditions difference leads to the temperature difference across the section increases up to about 120C firstly and then decrease to less than 40C. For the complex cross-section profile, the non-uniform deformation is more obviously and the larger thermal stress is the main reason of plastic deformation, fracture generation and occurrence of higher residual stress. A method used external stretching force to improve the distortion was also proposed in quenching processes and the displacement and equivalent stress is reduced sig-nificantly through external stretching force. However, larger external stretching force perhaps results in more serious distortion and fracture.DOI: http://dx.doi.org/10.5755/j01.mech.21.2.11733

Special Section on Ratcheting.

The design methods for pressure vessels and piping are based on the avoidance of potential modes of failure, such as collapse, excessive deformation, and cracking. Collapse and excessive deformation from a single load application are addressed by methods such as buckling analysis, primary stress analysis, and plastic limit analysis. For instance, limit load design underlies the evaluation methods for design conditions in most existing pressure vessel design codes. When significant cyclic loading is applied, the evaluation of fatigue and ratcheting effects becomes necessary. Fatigue analysis is concerned with avoiding the initiation and propagation of cracks that could eventually cause a sudden fracture. Ratcheting is a failure mode typically associated with components that are subjected to pressure loading and simultaneously large cyclic thermal stresses. It is characterized by deformations or plastic strains that accumulate with increasing load cycles. Continued deformation can eventually render the component unserviceable and strain accumulation can accelerate fatigue cracking, which is not accounted for in the fatigue analysis. Ratcheting can occur in metals, but also in nonmetallic materials. In order to avoid ratcheting, the cyclic loads must be kept within a specific limit that depends on the level of simultaneously applied mechanical loading (shakedown limit). By maintaining the stresses below the shakedown limit, some incremental plastic deformation may occur during the initial loading cycles, but the deformation in the subsequent cycles will be stable cycling, either in the elastic range (elastic shakedown) or involving alternating plasticity (plastic shakedown). Typical simplified evaluation methods in the design codes are based on perfect (nonhardening) plasticity. Such methods and their extensions, such as direct methods of shakedown analysis, promise simple and efficient solutions and are still being developed. Due to the numerical methods that are now available, a full cyclic plastic simulation that could include work hardening is also becoming feasible. There are, however, some knowledge gaps in the application of hardening plasticity models to shakedown or ratchet analysis. For example, apparently equivalent descriptions of a plastic stress–strain curve by different plasticity models which give comparable results for static loading and even for steady cyclic response can have widely different ratcheting behavior. Depending on the plasticity model, the response may vary from guaranteed shakedown independent of loading to unexpected ratcheting under some stress states. Therefore, there is active research involving experimental studies of ratcheting and development of plasticity models to better describe the material response to various loading combinations. Simultaneously, efforts are underway to identify simple existing plasticity models that include hardening and are suitable for an engineering analysis of shakedown or ratcheting. This special section on ratcheting has a total of 12 papers which represent a good mixture of theoretical, numerical, and experimental research in this area.

팔라듐 합금 수소분리막의 내구성 향상

Pd alloy hydrogen membranes for hydrogen purification and separation need thermal stability at high temperature for commercial applications. Intermetallic diffusion between the Pd alloy film and the porous metal support gives rise to serious problems in long-term stability of Pd alloy membranes. Ceramic barriers are widely used to prevent the intermetallic diffusion from the porous metal support. However, these layers result in poor adhesion at the interface between film and barrier because of the fundamentally poor chemical affinity and a large thermal stress. In this study, we developed Pd alloy membranes having a dense microstructure and saturated composition on modified metal supports by advanced DC magnetron sputtering and heat treatment for enhanced thermal stability. Experimental results showed that Pd-Cu and Pd-Ag alloy membranes had considerably enhanced long-term stability owing to stable, dense alloy film microstructure and saturated composition, effective diffusion barrier, and good adhesive interface layer.

In situ synchrotron X-ray diffraction analysis of deformation behaviour in Ti-Ni-based thin films.

Deformation mechanisms of as-deposited and post-annealed Ti50.2Ni49.6, Ti50.3Ni46.2Cu3.5 and Ti48.5Ni40.8Cu7.5 thin films were investigated using the in situ synchrotron X-ray diffraction technique. Results showed that initial crystalline phases determined the deformation mechanisms of all the films during tensile loading. For the films dominated by monoclinic martensites (B19'), tensile stress induced the detwinning of 〈011〉 type-II twins and resulted in the preferred orientations of (002)B19' parallel to the loading direction (∥ LD) and (020)B19' perpendicular to the LD (⊥ LD). For the films dominated by austenite (B2), the austenite directly transformed into martensitic variants (B19') with preferred orientations of (002)B19' ∥ LD and (020)B19' ⊥ LD. For the Ti50.3Ni46.2Cu3.5 and Ti48.1Ni40.8Cu7.5 films, martensitic transformation temperatures decreased apparently after post-annealing because of the large thermal stress generated in the films due to the large differences in thermal expansion coefficients between the film and substrate.