Elevated-Temperature Life Assessment

This article provides some new developments in elevated-temperature and life assessments. It is aimed at providing an overview of the damage mechanisms of concern, with a focus on creep, and the methodologies for design and in-service assessment of components operating at elevated temperatures. The article describes the stages of the creep curve, discusses processes involved in the extrapolation of creep data, and summarizes notable creep constitutive models and continuum damage mechanics models. It demonstrates the effects of stress relaxation and redistribution on the remaining life and discusses the Monkman-Grant relationship and multiaxiality. The article further provides information on high-temperature metallurgical changes and high-temperature hydrogen attack and the steps involved in the remaining-life prediction of high-temperature components. It presents case studies on heater tube creep testing and remaining-life assessment, and pressure vessel time-dependent stress analysis showing the effect of stress relaxation at hot spots.

A Comprehensive Comparison Between Different Multiaxial Cycle Counting Procedure

High-temperature nuclear design codes, such as Section III, Division 5 of the American ASME Boiler and Pressure Vessel Code and the French RCC-MRx, require evaluating fatigue damage for qualifying high-temperature components. Both codes provide clear guidance for counting cycles under uniaxial loading conditions, but neither provides a cycle counting procedure for multiaxial loading conditions. The ASTM E1049 also does not address multiaxial cycle counting. However, several widely utilized multiaxial cycle counting procedures are available in the open literature, but there is no agreement on the most appropriate method for high-temperature applications. Applying the different cycle counting methods to the same loading history generally produces different results. Comparisons between cycle counting procedures are available for low-temperature high-cycle fatigue but not for high-temperature low-cycle dwell-fatigue applications. This work presents an extensive comparison between different multiaxial cycle counting procedures potentially suitable for high-temperature low-cycle dwell-fatigue applications. Furthermore, how to conservatively assemble design transients to construct a loading history is also an open question. This work also investigates the uncertainty related to the loading sequence. The results guide the selection of the most appropriate cycle counting procedure, strain range metric, and cycle distribution for ASME Section III, Division 5 applications.

Towards digital design of gas turbines

This paper shows the current research to move towards the full digital design of a gas turbine. In the last years new manufacturing technologies, such as additive manufacturing, become more common for gas turbine applications, allowing greater flexibility in the design space. There is a need to fully exploit this flexibility and to design and validate in a digital environment new solutions. This work shows how optimization methods, mainly based on topology optimization strategies, requires more accurate estimator for critical applications, such as high temperature components of high pressure stages. For this reason a comparison of recent Gene Expression Programming and Neural Networks in topology optimization are shown. In particular it is shown how a RANS estimator in fluid topology optimization is capable of obtaining predictions compatible to high fidelity DES.

A solution to the challenges of particle erosion and thermal cycle for PS-PVD 7YSZ thermal barrier coatings

Advanced aero-engine is a key technique that is used all over the world, where many high-temperature components such as turbine blades and combustor, are made of Ni/Co/Fe based superalloys. However, they need high-temperature protection to avoid fast performance degradation. Generally, the superalloy high-temperature components are protected by thermal barrier coatings (TBCs) obtained via an atmospheric plasma spray (APS) and an electron beam-physical vapor deposition (EB-PVD). Here, a novel 3rd generation TBCs process using plasma spray-physical vapor deposition (PS-PVD) is presented, showing a more promising use than the traditional APS and EB-PVD. The PS-PVD feature uses evaporating ceramic powder, which results in the deposition of a feather-like columnar coating. This special microstructure showed good strain tolerance and non-line-of-sight (NLOS) deposition, giving great potential for application. In a working aero-engine, the high-temperature components face a serious environment, where foreign particle erosion is a great challenge and is the first barrier to the application of PS-PVD TBCs. As a solution, an Al-modification approach was proposed in this investigation. The results demonstrate that this approach can improve particle erosion resistance. Also, the thermal cycle performance had an apparent optimization.

High Temperature Creep Behaviour of Cast Nickel-Based Superalloys INC 713 LC, B1914 and MAR-M247

Cast nickel-based superalloys INC713 LC, B1914 and MAR-M247 are widely used for high temperature components in the aerospace, automotive and power industries due to their good castability, high level of strength properties at high temperature and hot corrosion resistance. The present study is focused on the mutual comparison of the creep properties of the above-mentioned superalloys, their creep and fracture behaviour and the identification of creep deformation mechanism(s). Standard constant load uniaxial creep tests were carried out up to the rupture at applied stress ranging from 150 to 700 MPa and temperatures of 800–1000 °C. The experimentally determined values of the stress exponent of the minimum creep rate, n, were rationalized by considering the existence of the threshold stress, σ0. The corrected values of the stress exponent correspond to the power-law creep regime and suggest dislocation climb and glide as dominating creep deformation mechanisms. Fractographic observations clearly indicate that the creep fracture is a brittle mostly mixed transgranular and intergranular mode, resulting in relatively low values of fracture strain. Determined main creep parameters show that the superalloy MAR-M247 exhibits the best creep properties, followed by B1914 and then the superalloy INC713 LC. However, that each of the investigated superalloys can be successfully used for high temperature components fulfils the required service loading conditions.

Brazing Coupling Performance of Piezoelectric Waveguide Transducers for the Monitoring of High Temperature Components

Piezoelectric waveguide transducers possess great potential for the online monitoring of high temperature critical components, in order to improve their operational safety. Due to the use of a waveguide bar, the sensory device is not susceptible to high temperature environments, which enables the long-term service of the piezoelectric transducers. However, the coupling between the waveguide bar and the high-temperature component has been proven to be the most important part of the monitoring system. In order to effectively transmit waves through the junction of the waveguide bar and the monitoring target, it is necessary to research a reliable coupling method to connect the waveguide transducers with the host structure. In the present research, the feasibility of brazing coupling for wave propagation through the junction was investigated through experiments. Piezoelectric waveguide transducers were welded using various kinds of brazing filler metals. The experimental results indicate that the coupling effects of the brazing welding depend on the filler metals. At the same time, some filler metals for the effective coupling of the transducer and the target monitoring component were identified. The brazing coupling method was verified that it can non-dispersively and effectively propagate waves into the host structure with much better reliability than the conventional dry coupling approach. Moreover, the high-temperature experimental results show that the brazing-coupled waveguide bar system can work reliably and stably in high temperatures at 300 °C for a long time. This work strives to pave a solid foundation for the application of piezoelectric waveguide transducers for the structural health monitoring of high temperature critical components.