Deep levels in homoepitaxial boron-doped diamond films studied by capacitance transient spectroscopies

International audienceDeep level transient spectroscopies (DLTS) applied to Schottky junctions made on homoepitaxial boron-doped diamond films show the existence of two traps. A deep acceptor, negatively charged and strongly attractive for holes, 1.57 eV above the valence band edge displays the characteristic features of a complex defect due to interacting centers and impurities, also displaying some evolutions after thermal cycles, possibly due to hydrogen effusion or diffusion. It is tentatively ascribed to association of a boron atom, a vacancy and several hydrogen atoms. A deep donor, 1.13 eV above the valence band edge, able to compensate the boron acceptors, is attributed to a defect correlated with dislocations. It could be due to the positively charged carbon vacancy. These conclusions are drawn from the Fourier transform-DLTS results coupled with isothermal time domain algorithms allowing the discrimination of multiple emission rates with high resolution

An assessment of thermo-mechanically induced fatigue damage of a steam turbine shaft

The increasing demands on the flexibility of steam turbines due to the use of renewable energy sources substantially alters the fatigue strength requirements of components of these facilities. The work summarized hereafter was initiated by the effort to develop the methodology of prediction of thermo-mechanical fatigue of steam turbine rotors. A significant effort was put into local thermo-mechanical stress-strain response modelling in the shaft material. An FE model of the structure assuming 2D axisymmetry idealisation was developed and verified. In-house codes based on a variety of approaches to assess critical location and fatigue damage, including the Manson-McKnight and the Nagode methods, were created. The experimental programme aimed to investigate the material fatigue behaviour under the thermo-mechanical conditions was initiated in order to provide data for calibrating and verifying the fatigue prediction procedures. A preliminary study on thermo-mechanical fatigue behaviour was conducted and the results are summarized in the paper

Numerical simulations of fatigue crack growth in a steam turbine rotor blade groove

With increasing share of renewable energy sources in the electricity production strict demands are placed on thermal power plants that have to cover the power shortages more frequently. Increasing number of steam turbine (ST) start-ups and shutdowns, as well as requirements on higher ramping of operating conditions, has detrimental effect on the overall lifetime of ST components. In the ST design process, this situation has to be dealt by applying advanced prediction methodologies handling the thermo-mechanical fatigue mechanism, for instance. On the other hand, in the case of currently operating STs, regular inspection and maintenance schedule as well as technologies for turbine operation control have to be reconsidered or newly developed. To cope with these challenges, the international consortium of energetic turbine producers and research institutes initiated the TURBO-REFLEX project funded by EU's H2020 program. One of the principal aims of the project is development of a damage tolerance approach that may be suitable for scheduling the ST rotor maintenance, for instance. Decisive factors in this effort are ST rotor operating conditions, material fracture properties and geometry that constitute the crack initiation site and crack growth rate and direction. This forms a complex task that has to be handled numerically by using a Finite Element (FE)-based code accompanied by in-house scripts for detecting the most probable way of crack propagation. In this contribution, the adopted fracture-mechanics approach applied to low-pressure section of ST rotor and results that have been achieved are presented

Differences in the response to in-phase and out-of-phase multiaxial high-cycle fatigue loading

This paper discusses the phase shift effect occurring between two and more load channels of multiaxially loaded specimens. The discussion concludes that there is an extreme shortage of good experimental data that would prove the existence and the trend of the phase shift effect in the high-cycle fatigue region. It is no wonder that there are so many fatigue strength estimation criteria that use quite different computational concepts, because the response to the phase shift effect in the experimental base is often hidden in a conglomeration of other interacting effects. The paper presents results of a sensitivity study that compares the fatigue strength estimation results for various such criteria for the same stress amplitudes, but for different phase shifts between the push-pull and torsion load channels. These results show that, with the exception of criteria, that assume a zero phase shift effect, the phase shift affects the results of each studied fatigue strength estimation criterion in a different way. If well-organized experiments were available, experiments corresponding to the described comparison between in-phase and out-of-phase loading would show the right trends, and the optimum criterion could be selected. A proposal for such an experimental setup is provided in the paper

Thermo-Mechanical Fatigue of a CrMo Steel Applicable to Steam Turbine Shafts

The paper is an overview of the work conducted in order to investigate thermo-mechanical fatigue (TMF) performance of a CrMo steel applicable to steam turbine shaft design. Uniaxial TMF tests of tubular specimens under various temperature ranges and phase shifts were carried out. Damage Operator Approach (DOA) proposed by Nagode et al. (2009) was calibrated by low-cycle fatigue experiments and its quality of prediction was verified by the TMF tests. The obtained results show that the DOA predicts the tests with higher range of temperatures (100-600 °C) much better than those subjected to 450-600 °C. The method can distinguish well the effect of different mechanical-to-temperature phase shift

THERMO-MECHANICAL FATIGUE ANALYSIS OF A STEAM TURBINE SHAFT

Increasing demands on the flexibility of steam turbines due to the use of renewable energy sources substantially alters the fatigue strength requirements of components of these devices. Rapid start-ups as well as the increased number of the load cycles applied to the turbines must be handled by design methodologies. The goal of the work presented in this paper was to provide a computational framework applicable to the thermo-mechanical fatigue (TMF) prediction of steam turbine shafts. The so-called Damage Operator Approach by Nagode et al. has been implemented to the software codes and applied to fatigue analysis of the thermo-mechanical material response computed numerically by the finite element analysis. Experimental program conducted in order to identify the material thermo-mechanical behavior and to verify numerical simulations is introduced in the paper. Some results of TMF prediction of a sample steam turbine shaft are shown