Papers by Keyword: Thermal Cycling

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Authors: He Fei Li, Zhao Hui Zhou, Hesnawi A, Kuo Jiang, Sheng Kai Gong
Abstract: Thermal barrier coatings with one-layered/ two-layered NiAl bond coat were produced by electron beam physical vapor deposition (EB-PVD). Compared to the TBC with one-layered bond coat, the TBC with two-layered bond coat improved the thermal cycling resistance significantly. The failure mechanism of the two-layer NiAl bond coat TBC was investigated in this paper.
Authors: R.R. Keller, N. Barbosa, R.H. Geiss, D.T. Read
Abstract: A novel approach for measuring thermal fatigue lifetime and ultimate strength of patterned thin films on substrates is presented. The method is based on controlled application of cyclic joule heating by means of low-frequency, high-density alternating current. Such conditions preclude electromigration, but cause cyclic strains due to mismatch in coefficients of thermal expansion between film and substrate. Strain and stress are determined from measurement of temperature. Fatigue properties are a natural fit to testing by alternating current. Stress-lifetime (S-N) data were obtained from patterned aluminum lines, where stress amplitude was varied by changing current density, and lifetimes were defined by open circuit failure. Electron microscopy and electron backscatter diffraction observations of damage induced by a.c. testing suggested that deformation took place by dislocation mechanisms. We also observed rapid growth of grains – the mean diameter increased by more than 70 % after a cycling time of less than six minutes – which we attribute to strain-induced boundary migration. Ultimate strength was determined by extrapolating a modified Basquin relation for high cycle data to a single load reversal. A strength estimate of 250 ± 40 MPa was determined based on a.c. thermal fatigue data. In principle, an electrical approach allows for testing of patterned films of any dimension, provided electrical access is available. Furthermore, structures buried beneath other layers of materials can be tested.
Authors: Fateeha Nisar Siddiqui, Nada Saleh, Ayesha Rahat, Asif Israr, Atiq Ur Rehman
Abstract: Carbon Fiber Reinforced Composites are presently used in satellites structure for better performance during extreme thermal cycling space environment. These materials display unexpected failure because the satellite periodically goes into and out of the earth shadow region on orbit, leading to a change in its surface temperature. As the coefficient of thermal expansion of carbon fibers is an order of magnitude lower than that of the polymer matrix, repeated thermal stresses are generated in the composites under the alternative temperature field, resulting in damage to the materials and a decrease in mechanical properties. The main objective of this study is to develop an analytical model to predict the damage produce in the composites subjected to extreme thermal loading. These thermal loading also causes the material to release strain energy. The results are presented in terms of strain produced during thermal cycling and also in the process of delamination.
Authors: Cheng Jin, Chun Yuan Shi
Abstract: Effects of thermal cycling on the tensile strength of aluminum alloy welded joints are studied experimentally in this paper. The damage mechanisms are also analyzed based on the microstructure observations. Results reveal that certain thermal cycling can cause strength decrease especially at the heat affected zone of the aluminum alloy welded joint. The cyclic temperature and the external load are the key factors which influence the strength of the welded joint specimens, while the cyclic period has a minor effect in thermal cycling conditions. Microstructure analysis also shows that voids nucleation and evolution governs the damage process under thermal cycling condition.
Authors: Zhi Li, Cheng Ping Zhang, Mei Rong Zhao, Hai Jun Jin
Abstract: Using elastoplastic finite element method study the interface stress distribution of Sn3.5Ag0.75Cu lead-free solder at different temperatures and different strain rate. Numerical analysis results show that: when strain-rate is identical, as the temperature rises, the interface stress increased rapidly of Sn3.5Ag0.75Cu lead-free solder and the substrate binding sites, from 11 °C when 2.6MPa rose to 90 °C in 49.7MPa, so the temperature is very large have effects of the interface stress of the lead-free solder and the substrate binding sites; when the temperature is constant, as the strain rate increases, Sn3.5Ag0.75Cu lead-free solder and the substrate binding sites of the interface stress showed a slight increase, from 0.005% / S when the 49.47MPa rose to 0.005% / S when the 50.08MPa, so strain rate on lead-free solder and the substrate binding sites of the interface stress effect is very small, indicating Sn3 .5Ag0.75Cu lead-free solder has strong rate-independent nature.
Authors: Nicolas G. Wright, C. Mark Johnson, Alton B. Horsfall, Cyril Buttay, Konstantin Vassilevski, W.S. Loh, R. Skuriat, P. Agyakwa
Abstract: The adoption of SiC devices as a viable technology depends crucially on maximising the potential advantages of the material. This is best achieved by the adoption of co-design techniques in which the optimisation of the SiC device is performed in parallel to that of the package and the overall application. This paper considers suitable techniques for this co-design and describes new approaches to the development of SiC technology for practical applications.
Authors: Aurélie Vande Put, Djar Oquab, Daniel Monceau
Abstract: During service, TBC can suffer degradation by CMAS, FOD, erosion or spallation. Whereas the first three are due to foreign particles, the last one is related to thermal cycling. When subjected to high temperature exposures followed by rapid coolings under oxidizing conditions, a TBC system undergoes morphological changes and stress development. This will initiate cracks which propagate and finally lead to failure by spallation. Consequently, the aim of the present study is to understand better the mechanisms responsible for such spallation events. Two kinds of TBC systems with different bond coatings (NiCoCrAlYTa or Pt-modified nickel aluminide bond coatings) are thermally cycled. Subsequently, SEM investigations on TBC systems after spallation concentrate on failure path, defect, morphological and microstructural changes to propose way for improving TBC system lifetime.
Authors: A. Tony Fry, Jim P. Banks, John Nunn, Louise.J. Brown
Abstract: Ceramic Thermal Barrier Coatings (TBCs) have been developed for advanced gas turbine engine components to improve the engine efficiency and reliability. The integrity and reliability of these coatings is of paramount importance. Accurate prediction of service lifetimes for these components relies upon many factors, and is not straightforward as knowledge of the service conditions and accurate input data for modelling are required. The main cause of failure of coatings is through debonding which develops as a consequence of thermally induced strains between the metallic bondcoat and the alumina TGO layers due to the differences in the thermal expansion coefficients of the individual layers. Thermal transients due to the power cycles of turbines will then cause these fractures to grow between the TGO and the bondcoat. When these fractures reach a critical size they can grow rapidly and cause the TBC to spall off. Thermal cycling of TBCs is used therefore to evaluate and rank TBC performance. Within the laboratory this is often conducted under isothermal conditions. Whilst this test method has performed adequately in the past it does not fully simulate service conditions. Work has been underway therefore to develop a more complex test method, which better simulates the service conditions experienced by the TBC. The approach here employs a gas torch to heat the operating face of the TBC whilst cooling the rear of the substrate with compressed air, thereby imparting a heat flux on the specimen. The specimen is then cycled by removing the gas torch and cooling with compressed air on the front and rear faces. Tests have been conducted on a TBC system consisting of an IN738 substrate with a CN334 bondcoat and EBPVD TBC. Thermal cycling tests have been performed under both isothermal and heat flux conditions. During the course of the tests the samples were examined non-destructively using a thermal camera to identify early indications of spallation. This paper reports on the performance of the flame rig equipment and the results from the exposures on the TBC system.
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