Computational investigation of porosity effects on fracture behavior of thermal barrier coatings

The influence of microstructural pore defects on fracture behaviour of Thermal Barrier Coatings (TBC) is analysed using finite element analysis involving cohesive elements. A concurrent multiscale approach is utilised whereby the microstructural features of the TBC are explicitly resolved within a unit cell embedded in a larger domain. Within the unit cell, a random distribution of pores is modelled along with three different layers in a TBC system, namely, the Top Coat (TC), the Bond Coat (BC) and the Thermally Grown Oxide (TGO). The TC/TGO and the TGO/BC interfaces are assumed to be sinusoidal of specified amplitude and frequency extracted from experimental observations reported in the literature. To simulate fracture in the TBC, cohesive elements are inserted throughout the inter-element boundaries in order to enable arbitrary crack initiation and propagation. A bilinear traction-separation relation with specified fracture properties for each layer is used to model the constitutive behaviour of the cohesive elements. Parametric studies are conducted for various pore geometrical features, porosity, fracture properties of Top Coat layer and Thermally Grown Oxide layer thicknesses. The results are quantified in terms of crack initiation and evolution. It is found that the presence of pores has a beneficial effect on the fracture behavior up to a certain value of porosity after which the pores become detrimental to the overall performance. Insights derived from the numerical results can help in understanding the failure behavior of practical TBC systems and further aid in engineering the TBC microstructure for a desired fracture behavior

Simulation numérique du comportement thermomécanique de systèmes multicouches (application au cas du système barrière thermique)

Les barrières thermiques obtenues par procédé de projection plasmasont largement utilisées pour prolonger la durée de vie des composants de la turbine. En raison de la faible conductivité thermique de la couche céramique, la température du substrat peut diminuer de quelques centaines de degrés. Un nouveau modèle par éléments finis a été développé pour évaluer les contraintes induites par le cyclage thermique et le développement des fissures dans un système barrière thermique obtenue par procédé de projection plasma. Un calcul thermomécanique a été réalisé en utilisant une distribution de température non homogène et prend en compte les effets des contraintes résiduelles issues du procédé d élaboration, les propriétés thermiques et mécaniques, la morphologie de l interface, l'oxydation et la déformation par fluage sur les contraintes locales qui sont responsables de la propagation des micro-fissures au cours du refroidissement, en particulier, près de l'interface métal/céramique. La propagation de fissures dans le système est simulée en fonction de la morphologie d'interface et de l'épaisseur de la couche d oxyde, grâce à l'outil de contact "Debond" présent dans le code par éléments finis ABAQUS. Les résultats montrent que le système ayant une interface sinusoïdale uniforme supporte des contraintes plus élevés que celles supportées par un système à interface non-uniforme. En outre, afin de prolonger la durée de vie et d'améliorer la fiabilité des systèmes barrières thermiques, nous devons contrôler la croissance de la couche d'oxyde.Air plasma sprayed thermal barrier coatings protection is widely used to prolong the lifetime of turbine components. Due to the low thermal conductivity of the top-coat layer, the substrate temperature can decrease by some hundred degrees. A finite element model is developed to evaluate the stresses induced by the thermal cycling and crack development in a typical plasma sprayed thermal barrier coatings system. A new thermo-mechanical model has been designed to function using a non-homogenous temperature distribution andtakes into account the effects of the residual stress generated due to coating process, the thermal and mechanical properties, the morphology of the top-coat/bond-coat interface, oxidation and creep deformation on the local stresses that are responsible of the micro-crack propagation during cooling, especially near the metal/ceramic interface. Crack propagation at the system is simulated as function of interface morphology and oxidation thickness, thanks tothe contact tool Debond present in the ABAQUS finite element code. The results show that the system having homogenous and uniform interface supports well the stress in comparison to the one having a non-uniform interface morphology of the interface. In addition, in order to extend the lifetime and to improve the reliability of TBC systems, we have to control the growth of the oxide layer.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

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چکیده هدف این پژوهش ارزیابی نقش آموزشهای مهارتی اعم از فنی و غیرفنی در فرآینـد تجـاری سـازی اختراعات و نوآوریهای کارآموزان آموزش فنی و حرفهای است. استراتژی تحقیق ، پیمایشی اسـت و جامعه هدف آن کـارآموزان مهـارت آموختـه مختـرع و نـوآوران سـازمان آمـوزش فنـی و حرفـه ای کشوراست. برای جمع آوری نظرات از پرسشنامه محقق ساخته با پایایی 8/0) آلفای کرونباخ) و بـرای بررسی نتایج تحقیق از آزمون دو جملهای، آزمون مقایسه میانگین رتبه، آزمون تی اسـتیودنت، آزمـون رتبه بندی فریدمن استفاده شده است. نتایج نشان داد که آموزش مهارتهای فنی و غیر فنی در فرایند تجاریسازی از اهمیت بالایی برخوردارند

Numerical Simulation of Temperature Distribution and Thermal-Stress Field in a Turbine Blade with Multilayer-Structure TBCs by a Fluid-Solid Coupling Method

To study the temperature distribution and thermal-stress field in different service stages, a two-dimensional model of a turbine blade with thermal barrier coatings is developed, in which the conjugate heat transfer analysis and the decoupled thermal-stress calculation method are adopted. Based on the simulation results, it is found that a non-uniform distribution of temperature appears in different positions of the blade surface, which has directly impacted on stress field. The maximum temperature with a value of 1030 °C occurs at the leading edge. During the steady stage, the maximum stress of thermally grown oxide (TGO) appears in the middle of the suction side, reaching 3.75 GPa. At the end stage of cooling, the maximum compressive stress of TGO with a value of -3.5 GPa occurs at the leading edge. Thus, it can be predicted that during the steady stage the dangerous regions may locate at the suction side, while the leading edge may be more prone to failure on cooling