Papers by Keyword: Thermal Fatigue

Abstract: During operation of transport and maneuverable gas-turbine units, there are crack formation in turbine disc rims what exerted by thermomechanical cycling loads. For in-depth study of these problems we have to use theories of plasticity and creep which form the basis for determining the complex stress-strain state in the stress concentration zone for disc rims, and a modern failure criterion which can predict lifetime under conditions of simultaneous plastic and creep strain accumulation. There is a finite-element method (FEM) that allows us to evaluate the stress-strain state in a stress concentration zone for a non-elastic material behavior. With plasticity and creep theories, it is possible to determine local strain quiet reliable by FEM.

Abstract: This paper presents results of experimental research and numerical calculations for thermal fatigue of G-X12CrMoVNbN9-1 martensitic cast steel. The tests were performed using hydraulic testing machine equipped with the heating chamber. The experimental research included static tests under mechanical load conditions at four temperature levels (20°C, 400°C, 550°C, 600°C) aimed at determining material characteristics needed for the numerical model. Main cyclic tests were carried out under thermal load conditions. The parameter, which was maintained at unchanged level during tests was the specimen strain ε = const = 0. The specimen temperature was changed. The numerical calculations for the same temperature change program were performed with the use of ABAQUS software. Experimentally and numerically determined stress value versus test time were compared.

Abstract: Microwave sintering has emerged in recent years as a new, fast, cheap and green technology for sintering a variety of materials. The main advantages of microwave heating can be summarized as follow: reduced processing times, energy costs and environmental benefits. Nevertheless, understanding how this specific heating drives to obtain ceramic materials with a combination of unique, structural and functional properties is the big challenge. The present work shows the different and improved properties achieved with β-eucryptite nanocomposite ceramic materials by microwave heating compared with the conventional method. Microcracking evolution in addition to the microstructure of the sintered materials along the several thermal cycles has been studied. Mechanical properties changes observed can be related to this process. Thus, the microwave technique is a promising tool for sintering new materials by controlling the composition of the phases, chemical reactivity and nanostructure, using up to 70% less energy in the whole sintering process than conventional heating. This technique becomes part of the new and innovative technologies "eco-green".

Abstract: Thermal barrier coating. Thermal fatigue. Exposure time. Thermal fatigue test is one of the most widely used method to evaluate the durability of thermal barrier coating (TBC). However, thermal fatigue test can be concluded in totally different results according to the test variations. Especially, Exposure time of thermal fatigue test can affect the delamination life cycle of TBC. In this study, using the same test equipment which Kim et al. used, thermal fatigue tests were performed with different holding time at high temperature, and the test results by Kim et al. and those by this study were compared. In addition, delamination map was come to perfection from the test results to define more accurate thermal fatigue life.

Abstract: The electron beam-physical vapor deposition (EB-PVD) coatings were selected to analyze the fatigue life. Firstly the failure mechanisms of oxidation and thermal fatigue of TBCs were considered together. The weight of thermally grown oxide (TGO) were measured during oxidation experiments and modeled. Thermal fatigue experiments were induced by thermal mismatched stress and thermal fatigue with holding time. The failure mechanism was obtained by the finite element analyses. The life prediction model was modified by the shear and normal strain range close to the interface of the top coat and TGO. Based on the experiment data, the life prediction model of thermal fatigue of TBCs was set up for EB-PVD coatings. Finally, using the life prediction model, the life of TBCs was predicted under the designed load case. The life prediction model was verified and all experiment data fall into the factor of 3 scatter band.

Abstract: Die casting is a cost-efficient method of forming geometrically complex near net-shaped products with close tolerances and excellent surface finishes.Due to the thermal shock characteristics, a number of test methods have been proposed to understand thermal fatigue behaviours of the die material as real die casting dies experience.This study reveals the differences of these methods by numerical modelling the thermal behaviour of the test specimen. Simplified die casting process is firstlyproposed to facilitate numerical computation. Test specimens are then optimized to be more representative of a real die. From the comparison results, we can conclude that least divergence can be obtained between immersion tests and induction heating based tests while furnace heating based tests would result in low thermal shock effects on the test sample.

Abstract: Thermal fatigue property of a directionally solidified nickel-base superalloy with different notch radius was studied. The results show that cycle numbers of crack initiation increase and resistance of thermal fatigue decreases with the rise of notch radius at the upper temperature of 1050°C. Thermal fatigue crack initiates at carbide or oxidized cavity. The join of oxidized cavity makes crack propagate. The propagation rate lowers with the increasing notch radius. Crack propagation direction is along <110> direction in {111} plane. The stress concentration drops and thermal fatigue property improves with the increase of notched radius.

Abstract: Components and systems of military aircraft are regularly subjected to severe operating conditions, which lead to the development of a wide range of failure modes. The Defence Science and Technology Organisations (DSTO) Forensic Engineering and Accident Investigation group investigates such failures for the Australian Defence Force (ADF). Correct diagnosis of these failures has provided the ADF with immediate advice that has contributed to increased aircraft safety, improved operational availability, and significant cost savings. This paper presents a number of case studies of recent fatigue failures which have occurred in Australian Defence aircraft. The case studies include examples of failures which occurred via differing fatigue initiating and driving mechanisms. Details of the forensic investigations relating to each case study are provided and the ensuing remedial actions discussed.

Abstract: The thermal fatigue behaviors of traditional Al-Si-Mg casting alloy and optimized Al-Si-Mg casting alloys at different thermal fatigue temperatures were investigated. Fatigue cracking appeared on the surface of traditional Al-Si-Mg alloy after 450 thermal cycles at 300 °C thermal fatigue temperature. However, the fatigue cracking was not found on the surface of optimized Al-Si-Mg alloy at the limited thermal fatigue cycles (less than 4450 times). Moreover, the optimized Al-Si-Mg alloy only occurred to elastic deformation and could not emerge in irreversible deformation. The grain refinement in the optimized Al-Si-Mg alloy could make the thermal fatigue cracking appeared much later and propagated much slowly. These results showed that the thermal fatigue resistance of optimized Al-Si-Mg alloy was superior to that of traditional Al-Si-Mg alloy.

Abstract: A hard-coating on hot work tool steel can be used to obtain higher corrosion resistance, as well as better wear resistance. This study investigates the thermal fatigue performance of AISI H13 hot work tool steel with and without hard chromium plating. Treated specimens were characterized using microstructural analysis, X-ray diffraction analysis and microhardness measurement. The thermal fatigue test is based on cyclic induction heating and water cooling. The specimen was heated to the maximum surface temperature of 670°C followed with water injection to bring the specimen down to a minimum temperature of 25°C. The thermal fatigue testing in this study was conducted using 500 cycles. A vacuum heat treated specimen with a hardness of 47 HRC was used as the reference material. The hard chromium plated layer with a thickness 35 μm had a hardness of 930 HV_0.1. The damage factor, defined as crack depth × crack width, of quenched and tempered H13 specimens and hard chromium plated specimens were 800 and 1760, respectively. The damage factor evaluation verified the vacuum heat treated specimen thermal fatigue resistance is superior to that of the hard chromium plated specimen.