Letteratura scientifica selezionata sul tema "Thermal shock behavior"

High temperature thermal shock causes the breakdown of Thermal Barrier Coating (TBC) systems. This paper focusing attention on the Zirconate TBC coating to study the thermo mechanical behavior such as wear and thermal shock test has been conducted inter metallic bond coat and Zirconate TBC to know the wear and thermal characteristics, and wear behavior has been studied on intermetallic bond coat using dry abrasion test and thermal characteristics studied on Zirconate TBC systems using thermal shock resistance test and finally the coatings characteristics before and after thermal cycling were evaluated.

The thermal fatigue behavior of alumina-magnesia based and alumina-chromia based purging plug materials are comparatively studied. By comparing thermal shock parameters, the changes of elastic modulus and hot modulus of rupture after thermal shock cycles, we come to a conclusion that microcracks emerge in the alumina-magnesia based material, which hinder the crack growth during thermal shock cycles. The fine-grained and network structure of alumina-magnesia based material are also helpful to improve thermal shock resistance. However, cracks are difficult to form in the alumina-chromia based material but it tends to fracture damage quickly once the cracks nucleation due to coarse-grained structure of alumina-chromia based material.

To develop materials suitable for aerospace applications, silicon nitride/boron nitride (Si3N4/BN) fibrous monolithic ceramics with varying BN contents were prepared. Employing analytical techniques such as XRD and SEM, coupled with mechanical testing equipment, the influence of BN concentration on the thermal shock resistance of Si3N4/BN fibrous monolithic ceramics was assessed. When the thermal shock differential is less than 800 °C, its residual flexural strength gradually decreases as the thermal shock differential increases. Conversely, when the differential exceeds 1000 °C, the residual flexural strength of the material increases. The residual strength of all samples reached its peak after undergoing a thermal shock assessment at a 1500 °C differential. When the BN mass fraction is 5 wt.%, the residual strength after a thermal shock at a temperature difference of 1500 °C is 387 ± 19 MPa, which is 124% higher than the original strength of the sample that did not undergo thermal shock (25 °C, 311 ± 18 MPa). The oxide layer formed on the thermal shock surface played a role in bridging defects introduced during material surface processing.

The mechanical properties and thermal shock behavior of Mg-PSZ/LaPO4 ceramics was investigated. The thermal shock resistance of the materials was evaluated by water quenching and a subsequent three-point bending test to determine the flexural strength degradation. Mg-PSZ/15LaPO4 composite showed a higher thermal shock resistance and behaved as a typical refractory. The calculation of thermal shock resistance parameters for the composites and the monolith had indicated possible explanations for the differences in thermal shock behavior.

Crack healing phenomena were observed in mullite and mullite + Yb2SiO5 environmental barrier coating (EBC) materials during thermal shock cycles. Air plasma spray coating was used to deposit the EBC materials onto a Si bondcoat on a SiCf/SiC composite substrate. This study reveals that unidirectional vertical cracks (mud cracks) formed after several thermal shock cycles; however, the cracks were stable for 5000 thermal shock cycles at a maximum temperature of 1350 °C. Moreover, the crack densities decreased with an increasing number of thermal shock cycles. After 3000 thermal shock cycles, cracks were healed via melting of a phase containing SiO2 phase, which partially filled the gaps of the cracks and resulted in the precipitation of crystalline Al2O3 in the mullite. Post-indentation tests after thermal shock cycling indicated that the mullite-based EBC maintained its initial mechanical behavior compared to Y2SiO5. The indentation load–displacement tests revealed that, among the materials investigated in the present study, the mullite + Yb2SiO5 EBC demonstrated the best durability during repetitive thermal shocks.

The purpose of the present study is to evaluate thermal shock properties of the ATJ graphite using laser irradiation techniques. Cracks of thermal shock specimens are initiated by maximum tensile stress field. Thermal shock resistance of the ATJ graphite is correlated with thermal parameter and mechanical property. To simulate the thermal stress conditions of rocket nozzle throat for the evaluation of the thermal shock resistance of ATJ graphite, the laser irradiation was applied at the central area of disk specimen. Thermal shock resistance was related to the geometry, the maximum stress, and the thermal and mechanical property. Also the analyses of transient temperature and thermal stress were performed by the finite element method with nonlinear code ABAQUS. Analyses were specially performed for several kinds of shape to determine the minimum power density which could be cracked the specimen. The shape of the thermal shock specimen which was cracked under the lower power density was obtained and the result will be proved to the test.

In service refractory materials are submitted to local temperature gradients that originate thermal stresses causing a thermal shock (TS) damage to the material. Practical tests for evaluating the thermal shock resistance (TSR) determine the variation or change of some characteristic property of the test sample like E (elastic module) or the strength before and after quenching. In this work, the microstructure and thermal shock behavior of Zircon based refractories are analyzed. Several compositions (eight), from pure Zircon to 70 % of Zircon were studied. The main structural and mechanical properties of the refractories were characterized, as modulus of rupture, elastic modulus, porosity, and microstructure. The dynamic elastic modulus E of the refractories was measured by the excitation technique. The TS behavior was evaluated by measuring the decrease in E and the modulus of rupture, before and after a quenching test. The influence of the presence of other phases was also analyzed. Refractories showed Zircon as the main crystalline phase. In some materials, m-ZrO2 appears coming from Zircon dissociation. The thermal shock behavior of refractory of high Zircon content is typical of the brittle ceramic materials. Materials showed a relation between elastic module and strength. Dynamic elastic modulus measurements have shown to be suitable for evaluation the TSR for these materials.

This paper reports a comparative study on characterization and thermal shock behavior of air plasma sprayed Al2O3-13wt.%TiO2 coatings using two kinds of raw materials, i.e. nanostructural and micro-structural (traditional) feedstock powders. The characterization, before and after thermal shock test, was carried out using micro-Vickers hardness tester, XRD and SEM. The thermal shock test was carried out using a water quenching method by employing cyclic heat treatment between ambient temperature and 650°C in air. The results showed that in spite of having denser structure, the nanostructural coating showed hardness a little lower than the traditional one at both conditions of before and after thermal shock tests. However, the nanostructural coating showed very good thermal shock behaviour.

The response of samples of unidirectional and cross-ply Nicalon fibre-reinforced calcium aluminosilicate (CAS) to a variety of thermal regimes has been examined using microscopy techniques and retained mechanical property measurements. The degree of matrix damage has been investigated by observation and measurement of cracking features and the results used in simple models in order to relate the occurrence of matrix cracking to stress distributions in the laminates. Thermal shock induced matrix crack damage was first seen to appear on the end faces of the unidirectional [0]16 laminate at a temperature differential of 400 °C and in the transverse plies, parallel to the longitudinal fibre direction, in the [0/90]3s cross-ply composite at a temperature differential of 350 °C. At more severe thermal shocks the next damage in both laminates was cracking in the matrix perpendicular to the fibre direction. The density of matrix cracking was seen to increase, initially, with increasing severity of thermal shock, but then to be less extensive at the highest temperature differentials (800 °C) used in this study. Crack density data for the unidirectional material at increasingly severe thermal shocks were compared with literature data for cracking under quasi-static loading using a simple thermal shock analysis incorporating a stress reduction factor. The effect of matrix cracking on retained mechanical properties has been examined by means of three-point flexure testing and values for Young's modulus, onset of non-linear behaviour and retained strength of the composites have been determined. Multiple thermal shock tests indicated that thermal treatment of previously cracked samples accelerated the rate of deterioration in the retained properties of the composite. It was proposed that the response of the samples to changes in the duration and severity of thermal treatments was consistent with interfacial modifications that have been reported to occur in this composite system at elevated temperatures. The suitability of applying a modified ACK model to predict critical temperature differentials for matrix cracking in the unidirectional laminate and longitudinal plies in the cross-ply composite has been tested. This approach combined applied thermal stresses, calculated using the simple thermal shock formula, with residual stresses, obtained from the model proposed by Powell et at. (1993). This method was found to be valid for the unidirectional material providing that some of the key parameters were determined independently. The use of a tunnelling crack model to predict thermal shock induced matrix cracking in the transverse plies of the cross-ply composite was less successful. This was partially attributed to the observed cracking patterns generated in the cross-ply material by flexure tests not conforming to those expected from stress calculations or reported from tensile tests.

Les matériaux réfractaires sont largement utilisés dans les applications à haute température mais ne sont pas toujours enclins à résister aux chocs thermiques sévères. Pour résoudre ce problème, une microstructure incorporant des microfissures préexistantes est une solution bien connue pour améliorer la résistance aux chocs thermiques. Néanmoins, une telle microstructure endommagée nécessite une meilleure compréhension pour optimiser son design sans compromettre l'intégrité du matériau. Dans un tel contexte, le Titanate d'Aluminium (Al₂TiO₅, AT) présentant une forte anisotropie de dilatation thermique, constitue un système modèle idéal pour créer un réseau de microfissures adapté afin d'améliorer la flexibilité et le comportement à la rupture. Cette thèse étudie les propriétés thermomécaniques des matériaux réfractaires développés à base d'AT, comprenant des céramiques polycristallines et des composites alumine/AT, en mettant l'accent sur les relations entre la microstructure et les propriétés macroscopiques. Dans le cas de ces deux matériaux, les microfissures préexistantes jouent un rôle clé sur le module de Young, le comportement de dilatation thermique, la réponse contrainte-déformation en traction, l'énergie de rupture et donc la résistance aux chocs thermiques. Un effet d’hystérésis significatif sur le module de Young et l’expansion thermique en fonction de la température témoigne des mécanismes de fermeture-réouverture de microfissures. Des essais de traction uniaxiale ont mis en évidence des lois de comportement non linéaires, impactant l'énergie de rupture et la résistance aux chocs thermiques. En particulier, des essais de traction incrémentale à 850 °C ont montré des comportements antagonistes à la montée ou à la descente en température du fait de l’histoire thermique. Les composites (alumine/AT) avec des 0 à 10 % d’inclusions présentent des microfissures diffuses dues à un différentiel d’expansion thermique. Ils présentent un module de Young réduit, des lois de comportement fortement non linéaires et une déformation à la rupture plus élevée à température ambiante. Les essais de choc thermique effectués par le dispositif innovant ATHORNA pour tous les matériaux à base d'AT étudiés ont confirmé leur résilience sous gradient thermique élevé. Ces résultats fournissent des informations précieuses pour le design de futurs matériaux réfractaires avancés présentant une résistance aux chocs thermiques améliorée
Refractory materials are widely used in high-temperature applications but are not always prone to resist severe thermal shock. To address this problem, microstructure incorporating pre-existing microcracks are already well known to improve thermal shock resistance. Nevertheless, such damaged microstructure needs a better understanding to optimize their design without compromising material integrity. In such context, Aluminum Titanate (Al₂TiO₅, AT) exhibiting a great thermal expansion anisotropy, constitutes an ideal model system for creating a tailored microcracks network in order to improve flexibility and fracture behavior. This thesis investigates the thermomechanical properties of developed AT-based refractory materials, including polycrystalline AT and alumina/AT composites, with emphasis on the relationship between microstructure and macroscopic properties. In both materials, pre-existing microcracks play a key role on Young's modulus, thermal expansion behavior, tensile stress-strain response, fracture energy, and thus thermal shock resistance. A significant hysteretic effect on Young's modulus and thermal expansion as a function of temperature indicates microcracks closure-reopening mechanisms. Uniaxial tensile tests revealed nonlinear stress-strain laws, impacting fracture energy and thermal shock resistance. In particular, incremental tensile tests at 850 °C showed contrasting behaviors during heating and cooling, attributed to thermal history. Composite materials with AT inclusions (0 - 10 wt.%) embedded in an alumina matrix exhibit diffuse microcracking due to thermal expansion mismatch. These composites exhibited reduced Young's modulus, highly nonlinear stress-strain laws, and higher strain to rupture at room temperature. Thermal shock tests performed by the innovative ATHORNA device for all studied AT-based materials confirmed their resilience under high thermal gradients. These findings provide valuable insights for the design of future advanced refractory materials with improved thermal shock resistance

[EN] The test for determining the resonance frequencies has traditionally been used to investigate the mechanical integrity of concrete cores, to assess the conformity of concrete constituents in different accelerated durability tests, and to ascertain constitutive properties such as the elastic modulus and the damping factor. This nondestructive technique has been quite appealed for evaluation of mechanical properties in all kinds of durability tests. The damage evolution is commonly assessed from the reduction of dynamic modulus which is produced as a result of any cracking process. However, the mechanical behavior of concrete is intrinsically nonlinear and hysteretic. As a result of a hysteretic stress-strain behavior, the elastic modulus is a function of the strain. In dynamic tests, the nonlinearity of the material is manifested by a decrease of the resonance frequencies, which is inversely proportional to the excitation amplitude. This phenomenon is commonly referred as fast dynamic effect. Once the dynamic excitation ceases, the material undergoes a relaxation process whereby the elastic modulus is restored to that at rest. This phenomenon is termed as slow dynamics. These phenomena (fast and slow dynamics) find their origin in the internal friction of the material. Therefore, in cement-based materials, the presence of microcracks and interfaces between its constituents plays an important role in the material nonlinearity. In the context of the assessment of concrete durability, the damage evolution is based on the increase of hysteresis, as a result of any cracking process. In this thesis three different nondestructive techniques are investigated, which use impacts for exciting the resonant frequencies. The first technique consists in determining the resonance frequencies over a range of impact forces. The technique is termed Nonlinear Impact Resonant Acoustic Spectroscopy (NIRAS). It consists in ascertaining the downward resonant frequency shift that the material undergoes upon increasing excitation amplitude. The second technique consists in investigating the nonlinear behavior by analyzing the signal corresponding to a single impact. This is, to determine the instantaneous frequency, amplitude and attenuation variations corresponding to a single impact event. This technique is termed as Nonlinear Resonant Acoustic Single Impact Spectroscopy (NSIRAS). Two techniques are proposed to extract the nonlinear behavior by analyzing the instantaneous frequency variations and attenuation over the signal ring down. The first technique consists in discretizing the frequency variation with time through a Short-Time Fourier Transform (STFT) based analysis. The second technique consists of a least-squares fit of the vibration signals to a model that considers the frequency and attenuation variations over time. The third technique used in this thesis can be used for on-site evaluation of structures. The technique is based on the Dynamic Acousto- Elastic Test (DAET). The variations of elastic modulus as derived through NIRAS and NSIRAS techniques provide an average behavior and do not allow derivation of the elastic modulus variations over one vibration cycle. Currently, DAET technique is the only one capable to investigate the entire range of nonlinear phenomena in the material. Moreover, unlike other DAET approaches, this study uses a continuous wave source as probe. The use of a continuous wave allows investigation of the relative variations of the elastic modulus, as produced by an impact. Moreover, the experimental configuration allows one-sided inspection.
[ES] El ensayo de determinación de las frecuencias de resonancia ha sido tradicionalmente empleado para determinar la integridad mecánica de testigos de hormigón, en la evaluación de la conformidad de mezclas de hormigón en diversos ensayos de durabilidad, y en la terminación de propiedades constitutivas como son el módulo elástico y el factor de amortiguamiento. Esta técnica no destructiva ha sido ampliamente apelada para la evaluación de las propiedades mecánicas en todo tipo de ensayos de durabilidad. La evolución del daño es comúnmente evaluada a partir de la reducción del módulo dinámico, producido como resultado de cualquier proceso de fisuración. Sin embargo, el comportamiento mecánico del hormigón es intrínsecamente no lineal y presenta histéresis. Como resultado de un comportamiento tensión-deformación con histéresis, el módulo elástico depende de la deformación. En ensayos dinámicos, la no linealidad del material se manifiesta por una disminución de las frecuencias de resonancia, la cual es inversamente proporcional a la amplitud de excitación. Este fenómeno es normalmente denominado como dinámica rápida. Una vez la excitación cesa, el material experimenta un proceso de relajación por el cual, el módulo elástico es restaurado a aquel en situación de reposo. Este fenómeno es denominado como dinámica lenta. Estos fenómenos ¿dinámicas rápida y lenta¿ encuentran su origen en la fricción interna del material. Por tanto, en materiales basados en cemento, la presencia de microfisuras y las interfaces entre sus constituyentes juegan un rol importante en la no linealidad mecánica del material. En el contexto de evaluación de la durabilidad del hormigón, la evolución del daño está basada en el incremento de histéresis, como resultado de cualquier proceso de fisuración. En esta tesis se investigan tres técnicas diferentes las cuales utilizan el impacto como medio de excitación de las frecuencias de resonancia. La primera técnica consiste en determinar las frecuencias de resonancia a diferentes energías de impacto. La técnica es denominada en inglés: Nonlinear Impact Resonant Acoustic Spectroscopy (NIRAS). Ésta consiste en relacionar el detrimento que el material experimenta en sus frecuencias de resonancia, con el aumento de la amplitud de la excitación. La segunda técnica consiste en investigar el comportamiento no lineal mediante el análisis de la señal correspondiente a un solo impacto. Ésta consiste en determinar las propiedades instantáneas de frecuencia, atenuación y amplitud. Esta técnica se denomina, en inglés, Nonlinear Single Impact Resonant Acoustic Spectroscopy (NSIRAS). Se proponen dos técnicas de extracción del comportamiento no lineal mediante el análisis de las variaciones instantáneas de frecuencia y atenuación. La primera técnica consiste en la discretización de la variación de la frecuencia con el tiempo, mediante un análisis basado en Short-Time Fourier Transform (STFT). La segunda técnica consiste en un ajuste por mínimos cuadrados de las señales de vibración a un modelo que considera las variaciones de frecuencia y atenuación con el tiempo. La tercera técnica empleada en esta tesis puede ser empleada para la evaluación de estructuras in situ. La técnica se trata de un ensayo acusto-elástico en régimen dinámico. En inglés Dynamic Acousto-Elastic Test (DAET). Las variaciones del módulo elástico obtenidas mediante los métodos NIRAS y NSIRAS proporcionan un comportamiento promedio y no permiten derivar las variaciones del módulo elástico en un solo ciclo de vibración. Actualmente, la técnica DAET es la única que permite investigar todo el rango de fenómenos no lineales en el material. Por otra parte, a diferencia de otras técnicas DAET, en este estudio se emplea como contraste una onda continua. El uso de una onda continua permite investigar las variaciones relativas del módulo elástico, para una señal transito
[CAT] L'assaig de determinació de les freqüències de ressonància ha sigut tradicionalment empleat per a determinar la integritat mecànica de testimonis de formigó, en l'avaluació de la conformitat de mescles de formigó en diversos assajos de durabilitat, i en la terminació de propietats constitutives com són el mòdul elàstic i el factor d'amortiment. Esta tècnica no destructiva ha sigut àmpliament apel·lada per a l'avaluació de les propietats mecàniques en tot tipus d'assajos de durabilitat. L'evolució del dany és comunament avaluada a partir de la reducció del mòdul dinàmic, produït com resultat de qualsevol procés de fisuración. No obstant això, el comportament mecànic del formigó és intrínsecament no lineal i presenta histèresi. Com resultat d'un comportament tensió-deformació amb histèresi, el mòdul elàstic depén de la deformació. En assajos dinàmics, la no linealitat del material es manifesta per una disminució de les freqüències de ressonància, la qual és inversament proporcional a l'amplitud d'excitació. Este fenomen és normalment denominat com a dinàmica ràpida. Una vegada l'excitació cessa, el material experimenta un procés de relaxació pel qual, el mòdul elàstic és restaurat a aquell en situació de repòs. Este fenomen és denominat com a dinàmica lenta. Estos fenòmens --dinámicas ràpida i lenta troben el seu origen en la fricció interna del material. Per tant, en materials basats en ciment, la presència de microfissures i les interfícies entre els seus constituents juguen un rol important en la no linealitat mecànica del material. En el context d'avaluació de la durabilitat del formigó, l'evolució del dany està basada en l'increment d'histèresi, com resultat de qualsevol procés de fisuración. En esta tesi s'investiguen tres tècniques diferents les quals utilitzen l'impacte com a mitjà d'excitació de les freqüències de ressonància. La primera tècnica consistix a determinar les freqüències de ressonància a diferents energies d'impacte. La tècnica és denominada en anglés: Nonlinear Impact Resonant Acoustic Spectroscopy (NIRAS). Esta consistix a relacionar el detriment que el material experimenta en les seues freqüències de ressonància, amb l'augment de l'amplitud de l'excitació. La segona tècnica consistix a investigar el comportament no lineal per mitjà de l'anàlisi del senyal corresponent a un sol impacte. Esta consistix a determinar les propietats instantànies de freqüència, atenuació i amplitud. Esta tècnica es denomina, en anglés, Nonlinear Single Impact Resonant Acoustic Spectroscopy (NSIRAS). Es proposen dos tècniques d'extracció del comportament no lineal per mitjà de l'anàlisi de les variacions instantànies de freqüència i atenuació. La primera tècnica consistix en la discretización de la variació de la freqüència amb el temps, per mitjà d'una anàlisi basat en Short-Time Fourier Transform (STFT). La segona tècnica consistix en un ajust per mínims quadrats dels senyals de vibració a un model que considera les variacions de freqüència i atenuació amb el temps. La tercera tècnica empleada en esta tesi pot ser empleada per a l'avaluació d'estructures in situ. La tècnica es tracta d'un assaig acusto-elástico en règim dinàmic. En anglés Dynamic Acousto-Elastic Test (DAET). Les variacions del mòdul elàstic obtingudes per mitjà dels mètodes NIRAS i NSIRAS proporcionen un comportament mitjà i no permeten derivar les variacions del mòdul elàstic en un sol cicle de vibració. Actualment, la tècnica DAET és l'única que permet investigar tot el rang de fenòmens no lineals en el material. D'altra banda, a diferència d'altres tècniques DAET, en este estudi s'empra com contrast una ona contínua. L'ús d'una ona contínua permet investigar les variacions relatives del mòdul elàstic, per a un senyal transitori. A més, permet la inspecció d'elements per mitjà de l'accés per una sola cara.
Eiras Fernández, JN. (2016). Studies on nonlinear mechanical wave behavior to characterize cement based materials and its durability [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/71439
TESIS
Premiado

Les matériaux carbones industriels sont constitues d'un agglomérat de grains et de liant. Les grains sont a base d'anthracite calcine ou de matériaux graphite; le liant est du brai de houille. Cuits a environ 1100#oc, ces produits sont ensuite employés comme revêtements dans les fours et hauts fourneaux et comme cathodes dans les cuves d'électrolyse de l'aluminium. Ils sont donc soumis à des sollicitations mécaniques et thermiques. L'objet de ce travail est l'étude des mécanismes d'endommagement sous contrainte mécanique et thermique, de ces matériaux. En particulier nous avons analyse l'effet de la microstructure sur la résistance à la propagation de fissures de divers produits. Le suivi de l'émission acoustique en cours d'essai de rupture a permis de préciser certaines influences des différentes phases de la microstructure, notamment en ce qui concerne la nature des grains et le taux d'anthracite. Ces analyses ont été complétées par des observations microscopiques des chemins de fissuration et des faciès de rupture. Nous avons pu ainsi dégager les caractéristiques schématiques de la microstructure de matériaux carbones présentant un faible et une forte résistance à la propagation de fissure. Par ailleurs, en vue de classer les matériaux en terme de résistance au choc thermique, nous avons mis au point un test base sur un impact laser tire au centre de disques. L'émission acoustique enregistrée pendant et après l'impact est associe à l'évolution et à la micro fissuration des produits soumis à des températures de cuisson. Il ne se produit pas réellement d'endommagement du au choc thermique de cette méthode n'est donc pas satisfaisante. Une comparaison des matériaux par les différents critères théoriques de résistance au choc thermique est effectuée et il semble au vu des applications concernées qu'on doive rechercher une bonne résistance à l'endommagement sous choc thermique
Industrial carbon-based materials are composed of grains and binder. The grains are based on calcined anthracite or coming from graphite materials; the binder is coal-tar pitch. The materials are heattreated at around 1100°C. They are used as linings in chemical and blast furnaces and as cathodes in aluminium smelters. Consequently they are submitted to severe mechanical and thermal conditions. This work is concerned with the study of damage mechanisms under mechanical and thermal stresses in theses materials. The effect of microstructure on crack growth resistance of a wide variety of materials bas been analysed. The complementary study of acoustic emission occurring during fracture tests bas shown some particular influence of microstructure, especially grain nature and anthracite content. These results have been confirmed with microscopic observations of crack pathes and crack surfaces. A model of the schematic microstructure of carbon materials having a weak and a strong crack growth resistance, proposed. Besides a thermal shock procedure bas been studied, in order to be able to classify carbon materials in terms of their thermal shock resistance. It is based on a laser bit at the centre of discs. Acoustic emission recorded during and after laser bit is related to the materials transformation and micro cracking, as the materials are submitted to temperatures higher th an their processing one. No real thermal shock damage occurs and consequently this test is not satisfactory. A comparison of the different materials with the various theoretical thermal shock parameters is made and it seems that, in view of the kind of applications considered, it is the thermal shock damage resistance that bas to be improved

Abstract This paper reports on the latest in a series of projects aiming at the qualification of new and proven materials in components under a severe service environment. In the initial stages of the project (HWT I & HWT II), a test loop at Unit 6 of the GKM Power Plant in Mannheim was used to study the behavior of components for advanced ultra-supercritical (A-USC) plants made from nickel alloys at 725 °C under both static and fluctuating conditions. Due to recent changes in the operation modes of existing coal-fired power plants, the test loop was modified to continue operating the existing nickel components in the static section while applying thermal cycles in a different temperature range. HR6W pipes and valves were added to the bypass of the static section, and all components in the cyclic section were replaced with P92, P93, and HR6W components. The test loop achieved approximately 9000 hours of operation and around 800 cycles with holding times of 4 and 6 hours. After dismantling the loop, nondestructive and destructive examinations of selected components were conducted. The accompanying testing program includes results from thermal fatigue, fatigue, thermal shock, and long-term creep tests, focusing on the behavior of base materials and welds, particularly for HR6W, P92, P93, and other nickel-based alloys. Additionally, test results on dissimilar welds between martensitic steel P92 and nickel alloys A617 and HR6W are presented. Numerical assessments using standardized and numerical lifetime estimation methods complement the investigations. This paper provides insights into the test loop design and operational challenges, material behavior, and lifetime, including advanced numerical simulations and operational experiences with valves, armatures, piping, and welds.

Abstract A narrow circumferential helium quench was used to thermally shock and fatigue internally heated alumina, reaction bonded, and sintered alpha silicon carbide tubes at 500°C and 1000°C. During these tests, transient temperature measurements required for thermal and stress-profile calculations were obtained through the use of micro-thermocouples positioned along the internal surface of the tubes. Acoustic emissions were also employed for in situ monitoring of crack initiation and propagation of the resident flaw populations during the single and repeated (up to 5) thermal shocks. Post-quench inspections and destructive burst tests were used to correlate the existence, extent, and statistical (Weibull) nature of the damage induced by the cycling. Results indicated progressive strength degradation in alumina tubes with repeated thermal cycles. In contrast, the thermally-cycled silicon carbide samples either showed no damage at all, or suffered minimal progressive strength degradation after the first cycle. In any case, the complex stress distributions computed from an FEA-based inverse heat transfer analysis were required to understand the observed damage (crack paths) and apparent fatigue behavior.

Abstract When testing the thermal cycling resistance of thermal barrier coatings, the surface temperature of the materials must be controlled so that test results can be used for coating life prediction. In this study, the temperature at the surface of plasma-sprayed TBCs was controlled during thermal shock testing using feedback from a double-color IR thermometer and high-rate cooling. The results are presented and discussed, highlighting the capability of the recently designed thermal shock test.

Abstract Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The coating comprised of YSZ – 0, 20, 50 and 80 wt % Al2O3; NiCrAlY bond coat; and AISI 304L stainless steels substrate. Thermal shock tests were performed over the specimens, at 1000 °C and 1200 °C for 5 min and then forced air quenching. The samples were directly pushed into a tube furnace at 1000 °C and 1200 °C. The results were prominent in forced air quenching tests at 1000 °C , where the lives of the TBCs were observed more than 66 cycles. It was noticed that with increase of Al2O3 content the thermal shock life of the specimens decreased. Further, the coating roughness decreased by Al2O3 addition. It was observed that with decrease in coating roughness, the thermal shock life decreased slightly.

During friction under shock conditions, interface is submitted to very strong heat flux. Thus, it may reach a temperature as high as melt temperature of one of the materials constituting the contact. As a consequence, the income and outcome of heat at the interface governs the friction and the contact behavior. This article exposes a model that resolves the non-linear heat equation in the vicinity of the interface. This way, it takes into account the variations of thermal properties of materials constituting the interface. First results indicate that such variations influence the tribological behavior of the contact.

Elastic and inelastic thermal shock analysis for simple shapes subject to convective heat transfer and the use of spreadsheets to automate this analysis is described. The completed spreadsheet application serves as a design tool to quickly calculate the elastic thermal stress distribution at peak loading or the residual stress distribution for inelastic regime. Spreadsheets were developed for the heating and cooling of a flat plate or wall, a beam, solid cylinder, and disc. The design tool results for elastic behavior are in good agreement with numerical methods over a wide range of Biot number. Inelastic analysis assumes elastic perfectly plastic behavior, and an approximate method is used to model it. Design tool results for inelastic analysis are also in good agreement with the literature for moderate values of Biot number with constant yield strength. The inclusion of temperature dependent yield stress and comparison with numerical methods is shown to give fair agreement.

Abstract High temperature oxidation behavior of MCrAlY coatings was studied at several temperatures in the range from 800 to 1100°C. In this study the MCrAlY coatings were obtained by plating using CrAlY as precursor powders in an electrolytic bath containing nickel and cobalt salt in solution. The size of the precursor CrAlY powders used was generally below 10 um. As-plated coatings consisted of a random distribution of CrAlY particles in the Ni-Co matrix. The heat-treatment of the as-plated coatings at elevated temperature resulted in the development of a gamma and beta structure. Both as-deposited and oxidized coatings were characterized by optical, scanning electron microscope and electron beam microprobe. During oxidation the coatings formed alumina scale with a negligible amount of transient nickel and chromium oxides. The spallation resistance of the oxide scale was investigated by thermal shock testing. The test consisted of a rapid cooling from 1000°C to 100°C with a two- minute dwell time at the maximum temperature. The thermal shock test was conducted in a) as–deposited and heat-treated condition and b) after preoxidation at 800°C and 1050°C, respectively. The coatings retained the alumina scale during thermal shock cycling.

Abstract In previous works, laser irradiation was associated to thermal spraying in the so-called MELTPRO process to improve the performance and the durability of TBCs. This study aims at presenting results concerning the thermal behavior of TBC's manufactured according to this process. Two types of severized thermal cycling were implemented: (i) “oxidizing” test: isothermal shocks were performed at different temperatures, ranging from 900°C to 1050°C down to 0°C; (ii) “quite non-oxidizing” test: thermal shocks were implemented from a temperature of 1100°C down to 50°C. Moreover, thermal annealing at 1100°C were performed to compare sintering phenomenon. TBC microstructure and its evolution during heat treating were characterized using image analysis, Knoop micro-indentation and XRD analysis. MELTPRO process was shown to increase twofold the lifetime of TBCs during isothermal shock tests. This is attributed to the fact that the columnar structure and the pore-crack architecture of remelted coatings improve the compliance property and decrease the permeability of TBCs. XRD analyses show that, in the Y-PSZ TBCs, the main phase is the metastable tetragonal (t’) phase both for as-sprayed and MELTPRO processed coatings. Moreover, remelted TBCs show a higher phase stability than as-sprayed TBCs during thermal shock tests: it seems that the remelted coatings have higher phase stability thanks to their pore architecture, which lead to a better compliance in relation to the thermo mechanical stresses, and so to a decrease in the stress variations undergone by the structure during the thermal cycles. During thermal annealing, it seems that MELTPRO processed coatings are less affected by sintering than as-sprayed coatings. Sintering phenomenon primarily concerns inter-lamellar cracks of non remelted areas. Besides, whatever the coating manufacturing process, the main remaining phase is the metastable tetragonal phase whatever the heat treating duration.