Thermomechanical Stability Research Articles

Nonlinear thermo-mechanical stability of shear deformable FGM sandwich shallow spherical shells with tangential edge constraints

This paper presents an analytical approach to investigate the nonlinear axisymmetric response of moderately thick FGM sandwich shallow spherical shells resting on elastic foundations, exposed to thermal environments and subjected to uniform external pressure. Material properties are assumed to be temperature independent, and effective properties of FGM layer are graded in the thickness direction according to a simple power law distribution in terms of the volume fractions of the constituents. Formulations are based on first-order shear deformation shell theory taking geometrical nonlinearity, initial geometrical imperfection, Pasternak type elastic foundations and various degree of tangential constraint of boundary edge into consideration. Approximate solutions are assumed to satisfy clamped boundary condition and Galerkin method is applied to derive closed-form expressions of critical buckling loads and nonlinear load-deflection relation. Effects of geometrical parameters, thickness of face sheets, foundation stiffness, imperfection, thermal environments and degree of tangential edge constraints on the nonlinear stability of FGM sandwich shallow spherical shells are analyzed and discussed.

Methacrylate based cross-linkers for improved thermomechanical properties and retention of radiation detection response in plastic scintillators

Pulse shape discrimination (PSD) is an important method that can efficiently sort and separate neutron and gamma radiation signals. PSD is currently achieved in plastic scintillators by over-doping poly(vinyl toluene) (PVT) matrices with fluorescent molecules. Meaningful separation of the signals requires addition of >20 wt% 2,5-diphenyloxazole (PPO) fluor in PVT. At these concentrations PPO acts as a plasticizer, negatively affecting the physical properties of the final plastic such as hardness, machinability, and thermomechanical stability. This work addresses these issues by implementing a cost-effective solution using cross-linking chemistry via commercially available bisphenol A dimethacrylate (BPA-DM), and a synthesized fluorinated analogue. Both improve the physical properties of over-doped PPO based plastic scintillators without degrading the measured light yield or PSD and Figure of Merit (FoM). In addition, the fluorinated analogue appears to enhance the hydrophobicity of the surface of the plastic scintillators, which may improve the scintillators’ resistance to water diffusion and subsequent radiation response degradation. The new formulations improve the feasibility of widely deploying long lifetime PSD capable plastic scintillators in large area coverage assemblies.

Investigations of the thermo-mechanical stability of hybrid layers for tribological applications: Nitrided layer/CrCN coating system

The paper contains a description of investigations related to an analysis of thermo-mechanical couplings in nitrided and non-nitrided steel substrate/CrCN coating systems with the use of dilatometry with temperature modulation. In this method, measured quantities are changes in the linear elongation of substrates in layered systems that occur as a result of given thermal loads. This enable i.a. to assess thermally activated changes in the states of residual stresses in the system as a result of a number of physical and chemical processes in the coating and in the substrate. Independently, results of these investigations were confronted with the results of scratch test enabling the assessment of coating adhesion and fracture toughness. On the grounds of the investigations carried out, it was found that CrCN coatings deposited on the nitrided substrates demonstrate a stronger thermo-mechanical coupling with the substrate in comparison with coatings deposited on the non-nitrided substrate and are characterized by a higher thermal stability. The results obtained explicitly confirm that systems with nitrided substrates are characterized by a higher mechanical load-bearing capacity and more favorable state of the adhesion of the coating to the substrate is observed as compared to systems with non-nitrided substrates.

YSZ/Al2O3 multilayer thick films deposited by spin coating using ceramic suspensions on Al2O3 polycrystalline substrate

Yttria Stabilized Zirconia (YSZ) high chemical and thermomechanical stability under Solid Oxide Fuel Cells (SOFCs) standard operational conditions and its high electrolytic domain indicate that an interesting alternative for lowering SOFCs operating temperature would be the development of high conductive YSZ-based electrolytes compared to conventional electrolytes through microstructure engineering. In this work, YSZ/Al2O3 micrometric multilayer films obtained by spin coating presented electrical conductivity 3.5 times higher than traditional YSZ ceramics. The multilayer film was formed by three layers of YSZ with 3.7 µm of thickness each were intercalated by two 7.4 µm thick layers of Al2O3. Therefore, the high electrical conductivity observed in nanometric multilayer films obtained by advanced techniques was achieved, even with lower magnitude, with ceramic suspensions and in the micrometric scale. Computer simulation shows higher stresses on YSZ than on Al2O3 layers, correlating the heterofilms higher conductivity with the stress imposed on the YSZ layer during sintering.

The changes of crosslink density of polyurethanes synthesised with using recycled component. Chemical structure and mechanical properties investigations

Abstract This paper aims at the utilisation of glycerolysate (Gly) obtained in polyurethane recycling process by means of crude glycerine, which has in its structure hydroxyl end groups that allow for further processing. Polyurethanes (PUs) were synthesised using prepolymer method with the mixture of neat polyol and glycerolysate, in different ratios, with 4,4-diphenylmethane diisocyanate (MDI). The prepolymer was subsequently extended using 1,4-butanediol (BD). The incorporation of glycerolysate caused the crosslinking of obtained polyurethane materials. The crosslink density was calculated from swelling measurements by means of Flory and Rehner equation. The effect of recycled component content on the structure, thermal and mechanical properties were investigated by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA) and mechanical tests, respectively. Morphology was also analysed by atomic force microscopy (AFM). As the content of recycled component increased, the swelling ratio decreased and simultaneously crosslink density increased, resulting in the increase of glass transition and storage modulus at room temperature. Thermomechanical stability, tensile strength, elastic modulus and hardness of PUs also increased with the incorporation of glycerolysate. Although elongation at break decreased, the recoverable deformation increased with the content of glycerolysate. Morphology reveals the transition from a microphase-separated microstructure to an interconnected hard segments domain structure with the incorporation of glycerolysate. These materials can be applied as protective coatings.

Ultrafine-grained W–25 wt-%Cu composite with superior high-temperature characteristics

ABSTRACTIn the present work, several ultrafine-grained W–Cu composites with bi-continuous microstructure have been prepared through combined processes of high-energy ball-milling, liquid-phase sintering and infiltration. The microstructure, relative density, hardness and electrical conductivity of the W–Cu composites with copper content ranging from 20 to 35 wt-% were investigated to determine the optimal composition. Then, the high-temperature characteristics such as thermo-mechanical stability, high-temperature compressive performance and high-temperature wear behaviour of the optimal W–25 wt-%Cu composite were systematically assessed in comparison with those of the micron-sized W–25 wt-%Cu composite. The results showed that the ultrafine-grained W–Cu composite exhibited distinctly superior thermo-mechanical stability, as well as high-temperature compressive and wear-resistant properties, indicating its great potential in enduring repeated contact-loading at high temperature.

Analysis and experimental investigation of ceramic powder coating on aluminium piston

Energy conservation and efficiency have always been the quest of engineers concerned with internal combustion engines. The diesel engine generally offers better fuel economy than its counterpart petrol engine. Even the diesel engine rejects about two thirds of the heat energy of the fuel, one-third to the coolant, and one third to the exhaust, leaving only about one-third as useful power output. Theoretically if the heat rejected could be reduced, then the thermal efficiency would be improved, at least up to the limit set by the second law of thermodynamics. Low Heat Rejection engines aim to do this by reducing the heat lost to the coolant. Thermal Barrier Coatings (TBCs) in diesel engines lead to advantages including higher power density, fuel efficiency, and multifuel capacity due to higher combustion chamber temperature. Using TBC can increase engine power by 8%, decrease the specific fuel consumption by 15-20% and increase the exhaust gas temperature by 200K. Although several systems have been used as TBC for different purposes, yttria stabilized zirconia with 7-8 wt.% yttria has received the most attention. Several factors playing important role in TBC life include thermal conductivity, thermo chemical stability at the service temperature, high thermo mechanical stability to the maximum service temperature and thermal expansion coefficient (TEC). This work mainly concentrates on the behaviour of three TBC powders under the same diesel engine conditions. This work finds out the best powder among yttria, alumina and zirconia to be used as a piston coating material i.e., the one resulting in lowest heat flux and low side skirt and bottom temperature has been chosen for the coating purpose. This work then analyses the coated sample for its surface properties such as hardness, roughness, corrosion resistance and microstructural study. This work aims at making it easier for the manufacturers choose the coating material for engine coating purposes and surface properties for operating them in their service period.

Thermo-mechanical Characteristics and Improvement of Antenna Structures in Supersonic Flows

In a supersonic airflow, smart skin antenna structure is analyzed to investigate the thermo-mechanical characteristics and stability boundaries. The skin is modeled as a multi-layer sandwich structure with a dielectric polymer. Especially, the layer is embedded on the middle surface of the face sheets similar to the honeycomb core to perform the role of antenna or radar structures. First-order shear deformation theory of plate and von Karman strain-displacement relations are adopted in this work. Also, aerodynamic pressure is based on the first-order piston theory for the aerodynamic loads. Newton-Raphson iterative method is applied to solve the nonlinear equations of motion. Special thermo-mechanical characteristics and improvement of the structural model are discussed.

A method for the systematic assessment of lubricant performance during superplastic sheet forming

AbstractFrictional forces at the die‐sheet interface during superplastic forming play an important role in controlling material flow. As such, lubricious coatings are routinely used to facilitate a low friction flow of sheet metal into the die cavity. They may also act as parting agents and/or oxidation barriers. The selection of coatings for the superplastic forming of Ti‐6Al‐4 V is limited by the requirement of thermomechanical stability at the high forming temperature, therefore, solid film coatings such as boron nitride are a common choice. However, little work has been published on coating performance during superplastic forming, particularly under true process conditions. This paper presents a method for the systematic assessment of superplastic forming coating performance by using a multi‐pocket die, which allows many coating types and combinations to be tested simultaneously during a single forming cycle. Sixteen coating combinations were evaluated in total and the resultant sheet thickness distributions compared, serving as an indicator of friction.

Deformation Mechanism Map of Cu/Nb Nanoscale Metallic Multilayers as a Function of Temperature and Layer Thickness

The mechanical properties and deformation mechanisms of Cu/Nb nanoscale metallic multilayers (NMMs) manufactured by accumulative roll bonding (ARB) are studied at 25C and 400C. Cu/Nb NMMs with individual layer thicknesses between 7 and 63 nm were tested by in-situ micropillar compression inside a scanning electron microscope Yield strength, strain-rate sensitivities and activation volumes were obtained from the pillar compression tests. The deformed micropillars were examined under scanning and transmission electron microscopy in order to examine the deformation mechanisms active for different layer thicknesses and temperatures. The analysis suggests that room temperature deformation was determined by dislocation glide at larger layer thicknesses and interface-related mechanisms at the thinner layer thicknesses. The high temperature compression tests, in contrast, revealed superior thermo-mechanical stability and strength retention for the NMMs with larger layer thicknesses with deformation controlled by dislocation glide. A remarkable transition in deformation mechanism occurred as the layer thickness decreased, to a deformation response controlled by diffusion processes along the interfaces, which resulted in temperature-induced softening. A deformation mechanism map, in terms of layer thickness and temperature, is proposed from the results obtained in this investigation.

Experimental and numerical analyses of the thermo-mechanical stability of an embankment with shady and sunny slopes in a permafrost region

The hydro-thermo-mechanical state is important for embankments in permafrost regions, especially for the embankments with shady and sunny slopes. It is difficult to determine the state because the complex hydro-thermo-mechanical interaction usually occurs within the embankments during freezing-thawing processes. To explore the thermo-mechanical stability of embankments in permafrost regions, an in-situ experiment of highway with asphalt pavement was performed firstly. Based on the observed results, a mathematical model was developed to describe the hydro-thermo-mechanical process. Subsequently, the model was validated by the observed data. Finally, the long-term stability of the embankment was analyzed and predicted with a climate warming rate of 0.052°C/a. The results show that, (1) the asphalt pavement heat absorption effect and shady-sunny slope effect reduce the stability of the embankment; (2) the deformation of the embankment demonstrates the periodic freezing-thawing fluctuation and the long-term creep settlement characteristics; (3) the settlement is mainly caused by the freezing-thawing behavior of the active layer, the moving down of the permafrost table, and the warming of permafrost; (4) the shady-sunny slope effect induces the uneven settlement, and the settlement difference increases with operation time. The study will be helpful for the design and maintenance of embankments in permafrost regions.

Evaluation of polypropylene hybrid composites containing glass fiber and basalt powder

Abstract Polypropylene composites filled with inorganic fillers are widely used due to their good mechanical and thermal properties. The modification efficiency of filler incorporated into thermoplastic polymer strongly depends on its shape and dimension. Therefore, the development of novel hybrid composites modified with particle and fibrous shaped fillers expands the range of thermoplastic composite applications. This work investigates the influence of glass fiber and basalt powder and their shape on the mechanical properties of polypropylene-based composites. Mechanical properties of hybrid composites were evaluated using static tensile test, impact resistance, and hardness measurements. The thermomechanical stability of the materials was evaluated via dynamic mechanical thermal analysis. Results indicated that the incorporation of inorganic fillers significantly improved the composite sample stiffness at a wide range of temperatures. The research was complemented with structure investigations realized using scanning electron microscopy. Moreover, the incorporation of basalt powder, which is well known for its low friction coefficient, improved the processing properties, as proven by the melt flow index test.

Thermomechanical cyclic stability of porous NiTi shape memory alloy

It is known that shape memory and phase transition response of NiTi shape memory alloy (SMA) is dependent on mechanical or thermal background, and the presence of phases such as B2(NiTi), B19'(NiTi), Ni4Ti3, Ni3Ti2 and NiTi2. The volume fraction of metastable Ni4Ti3 phase in NiTi matrix can be easily changed by heat treatments. Both Ni4Ti3 phases and Ni ingredient affect the phase transition kinetics and the shape memory response. This work deals with the effect of thermomechanical cycling on the shape memory response of a porous Ni50.8Ti49.2 SMA, which is homogenized at 1050°C and aged at 800°C, afterwards slow cooled. Full recoverable shape memory response and cyclic stability were achieved during thermomechanical cycling under a compressive stress of 5MPa.

Thermo-mechanical stability analysis of cooling embankment with crushed-rock interlayer on a sloping ground in permafrost regions

Some embankments of highways are inevitably built on slopes in permafrost regions. It is usually difficult to ensure the stability of the embankments on sloping ground because of the asymmetric geotemperature and stress distribution. The crushed-rock layer is often layed in the slope areas to control the geotemperature. Cooling performance is focused to evaluate the thermal stability of crushed-rock embankment, but the mechanics are poorly concerned. This study developed a thermo-mechanical model to evaluate the influence of the cooling effect of crushed-rock layer on the mechanical state of embankment on sloping ground. Two embankments are taken as examples according to the engineering practice in permafrost regions on the Qinghai-Tibet Plateau, i.e., the unprotected embankment and the crushed-rock interlayer embankment. To analyze the stability of sloping embankments, the geotemperature, principal strain, deformation and safety factor in four typical seasons are simulated during operations. Numerical results indicate that the crushed-rock interlayer can effectively cool the sloping embankment and its foundation. Meanwhile, the cooling effect of the porous layer can reduce the uneven settlements to improve the safety reverse of the embankment. This study can also provide scientific basis and reference for the design of similar engineering structures in permafrost regions.

Thermal conductivity of self-ion irradiated nanocrystalline zirconium thin films

Thermomechanical stability and high thermal conductivity are important for nuclear cladding material performance and reliability, which degrade over time under irradiation. The literature suggests nanocrystalline materials as radiation tolerant, but little or no evidence is present from thermal transport perspective. In this study, we irradiated 10nm grain size zirconium thin films with 800keV Zr+ beam from a 6 MV HVE Tandem accelerator to achieve various doses of 3×1010 to 3.26×1014ions/cm2, corresponding to displacement per atom (dpa) of 2.1×10−4 to 2.28. Transmission electron microscopy showed significant grain growth, texture evolution and oxidation in addition to the creation of displacement defects due to the irradiation. The specimens were co-fabricated with micro-heaters to establish thermal gradients that were mapped using infrared thermometry. An energy balance approach was used to estimate the thermal conductivity of the specimens, as function of irradiation dosage. Up to 32% reduction of thermal conductivity was measured for the sample exposed to a dose of 2.1 dpa (3×1014ions/cm2).

Effect of Nb2O5 doping on improving the thermo-mechanical stability of sealing interfaces for solid oxide fuel cells

Nb2O5 is added to a borosilicate sealing system to improve the thermo-mechanical stability of the sealing interface between the glass and Fe-Cr metallic interconnect (Crofer 22APU) in solid oxide fuel cells (SOFCs). The thermo-mechanical stability of the glass/metal interface is evaluated experimentally as well as by using a finite element analysis (FEA) method. The sealing glass doped with 4 mol.% Nb2O5 shows the best thermo-mechanical stability, and the sealing couple of Crofer 22APU/glass/GDC (Gd0.2Ce0.8O1.9) remains intact after 50 thermal cycles. In addition, all sealing couples show good joining after being held at 750 °C for 1000 h. Moreover, the possible mechanism on the thermo-mechanical stability of sealing interface is investigated in terms of stress-based and energy-based perspectives.

Magnetically controlled thermoelastic martensite transformations and properties of a fine-grained Ni54Mn21Ga25 alloy

Comparative studies of physical characteristics (the electrical resistivity, the magnetic susceptibility, the magnetization, the bending deformation, and the degree of shape recovery during subsequent heating) of the Ni54Mn21Ga25 ferromagnetic alloy as-cast and rapidly quenched from melt have been performed in the temperature range 2–400 K. The results are compared to the results of studying the structural–phase transformations by transmission and scanning electron microscopy and X-ray diffraction. It is found that the rapid quenching influences the microstructure, the magnetic state, the critical temperatures, and the specific features of thermoelastic martensite transformations in the alloy. It is found that the resource of the alloy plasticity and thermomechanical bending cyclic stability demonstrates a record-breaking increase in the intercritical temperature range and during subsequent heating.

Three-Dimensional Modelling of a Microtubular SOFC: A Multiphysics Approach

There is an increasing interest in the use of Microtubular Solid Oxide Fuel Cells (µ-SOFC) for a broad spectrum of applications with power demands ranging from a few watts to several hundred watts. µ-SOFC’s possess inherently favourable characteristics over alternate configurations such as high thermo-mechanical stability, high volumetric power density and rapid start-up times. The use of computational modelling at the design level is essential for minimising cost and maximising productivity, giving critical insight into the complex phenomena occurring during SOFC operation and their interrelationships. To date, models have been limited by oversimplified geometries, often failing to account for cathode air supply complexities, gas distribution within pores and radiative heating effects[1-3]. In this study, a three-dimensional Computational Fluid Dynamics (CFD) model of an anode, electrolyte, cathode, current collectors, fuel inlet/outlet and furnace temperature effects is considered. The model has been validated with a mirroring experimental setup, varying fuel compositions, operating temperature and interconnect configuration. COMSOL Multiphysics is used to simulate the physics, describing the distribution of temperature, current density, electrical potential, pressure and gas concentrations throughout the cell. Results show good correlation with experimental data and the model is reliable for prediction of fuel cell performance as a function of operating conditions within these set parameters.

LSC Infiltrated LSCF Oxygen Electrode for High Temperature Steam Electrolysis

From last few years, there has been an increased interest in the development of high performing solid oxide electrolysis cells (SOECs) due to production of highly pure hydrogen. The basic concept of SOECs technology has been developed based on SOFCs and their materials research. The most common electrolyte and hydrogen/steam electrode are yttria stabilized zirconia (YSZ) and Ni-YSZ cermet respectively, as in SOFC, already exhibited good electrochemical performance and stability. In terms of oxygen electrode materials La1-x Sr x MnO3 (LSM), La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) and their composite with GDC or YSZ have been reported. Nevertheless, the electrochemical and thermo-mechanical stability of these materials during SOEC operation still requires further investigation. Recently several researchers have focused their work on the degradation mechanism during SOEC operation. In short term (during first few hours) of ageing, the formation of Sr- and/or Co-rich particles on the top surface of dense LSCF samples has been reported by the authors while only surface Sr-enrichment is usually observed in aged porous oxygen electrode [1, 2]. When it is kept for long duration at operating condition then particularly the delamination of oxygen electrode at the electrode/electrolyte interface is the most common mode of failure [3, 4]. In general, the oxygen evolution reaction at the oxygen electrode leads to a locally high partial pressure of oxygen (pO2) in the electrode and at the electrode/electrolyte interface in SOEC mode, further leads to increase in electrode overpotential [3]. So, the pO2 and overpotential (η) at the electrode/electrolyte interface are the governing factors and should be treated carefully considering the specific interface structure. One way to avoid these problems could be to increase the ionic conductivity at the electrode/electrolyte interface. Our current research work is focused on the oxygen electrode to avoid such kind of delamination by increasing the activity of oxygen electrode using infiltration of very active material like La0.58Sr0.4CoO3-δ (LSC), for the efficient hydrogen production. The electrochemical performance of LSC infiltrated LSCF oxygen electrode is investigated for steam electrolysis and compared with blank LSCF electrode. Furthermore it is also tried to investigate at which overpotential the delamination of oxygen electrode starts. In this respect, the symmetrical half-cell as well as full cell containing LSCF oxygen electrode with and without LSC infiltration are characterized using electrochemical impedance spectroscopy in the temperature range 750-900 °C. The symmetrical half-cells “8YSZ//CGO//LSCF” are characterized in a three electrode set up using a reference electrode with the aim to obtain detailed information on the different electrochemical processes and degradation mechanisms. The polarization resistance (R p) values obtained for symmetrical half-cells with and without infiltration are 24 mΩ.cm2 and 32 mΩ.cm2 respectively, at 800 °C. An improvement in the R pvalue is observed for LSC infiltrated sample. The degradation experiment performed up to two weeks with an increase of potential 50 mV in each two days. The postmortem analysis performed to investigate the behavior of electrode and electrode/electrolyte interface using SEM-EDX. Interestingly, no delamination is observed at least after two weeks with LSC infiltrated symmetrical half-cell, contrary to the blank LSCF symmetrical half-cell. The behavior of Ni-YSZ supported single cells “Ni-YSZ//8YSZ//CGO//LSCF" is also investigated from 750-900 °C temperature range under both SOFC and SOEC modes. A maximum current density value of 1.48 A.cm-2 is obtained at 800 °C under an applied electrolysis voltage of 1.5 V with 50 % H2O. The investigation of LSC infiltrated LSCF single cell is under progress. The comparison of electrochemical properties of symmetrical half-cells and single cells with and without infiltration will be presented and discussed in detail. Refrances [1] L. C. Baqué, A. L. Soldati, E. Teixeira-Neto, H. E. Troiani, A. Schreiber, A. C. Serquis, Journal of Power Sources, 337 (2017) 166-172. [2] J. Druce, I. Tatsumi, J. Kilner, Solid State Ionics, 262 (2014) 893-896. [3] Anil V. Virkar, International Journal of Hydrogen Energy, 35 (2010) 9527-9543. [4] J. R. Mawdsley, J. D. Carter, A. J. Kropf, B. Yildiz, V. A. Maroni, International Journal of Hydrogen Energy, 34 (2009) 4198-4207.

The effect of dynamic aging on the cyclic stability of Cu73Al16Mn11 shape memory alloy

This study focuses on the effect of dynamic aging on the thermo-mechanical cyclic stability of Cu73Al16Mn11 (at%) shape memory alloy (SMA) during constant stress (isobaric) and stress-free heating-cooling experiments. Isobaric heating-cooling experiments under various tensile stress magnitudes revealed almost no residual strain up to 60MPa. Subsequently, the cyclic stability of the samples was further investigated via thermal cycles under 60MPa with increasing upper cycle temperature (UCT) values. The increase in UCT led to the ordering of the parent phase and formation of the bainite and α phases. In addition to the effect of UCT on the formation of bainite, stress had an accelerating effect on the bainitic transformation. The martensitic transformation temperatures increased due to ordering when UCTs were set to 120°C and 145°C and then started to decrease due to the formation of the bainite and α phases when the UCTs were set to 175°C and 205°C. Consequently, it was determined that the cyclic stability of Cu-Al-Mn SMAs for high temperature applications strongly depends on the UCT.