Key Engineering Materials Vol. 827

Abstract: The joining points between martensite packets (laths) and their microstructure in low-carbon martensitic steel were TEM studied. In order to determine the real microstructure of the packet, martensite examinations were conducted before low-cycle fatigue (LCF) tests, considering the structure of the packets and types of their joining. The changes in microstructure occurred in the above places after austenite-martensite transformation were also analyzed. It was shown that after jointing some packets initiate arch-like contours in the laths, exhibiting a presence of local stresses. Several types of joints are considered, including the penetration of laths of one packet into that of neighboring one. It was revealed that the microstructure changes are exhibited in joining points without any external deformation, and result in the localized plastic deformation at LCF. It is assumed that microcrack initiation and commencement of fatigue failure of the material should be expected to happen just in these areas. All the above is explained from point of view of the peculiarities of martensitic transformation.

Abstract: TEM study of junctions between martensite packets (laths) and their microstructure in low-carbon martensitic steel were studied. It was revealed that formation of slip bands commences at junctions between laths. Heavy changes in microstructure occur at junctions of packets and in the near-boundary laths because of plastic deformation of martensite. A number of typical junctions between the laths after LCF are considered. Deformation process within the individual packet occurs inhomogeneously, some of the laths deform more heavily than the others. The coarser slip bands group within large laths and run along their whole lengths. It was shown that the above microstructure changes strongly affect the cyclic fracture of the steel. Some concrete sites of microcrack initiation are indicated.

Abstract: The theoretical model of quasi-static crack growth in the elastic-plastic material under load variation in a wide range. Small-scale yielding is principal assumption and main restriction of proposed theory. The model of crack growth provides for continues and interrelated both the crack propagation and plastic deformation development. The nonlinear first-order differential equation describes the quasi-static process of crack growth. In dimensionless form this equation invariant in respect to geometrical configuration and material. The critical size of the plastic zone is proposed as the characteristics of material resistance which is directly connected with the fracture toughness, but more convenient in practical applications of invariant equation. The demonstration of solution is performed for the double cantilever beam that widely used as the standard (DCB) sample for measurement of the mode-I interlaminar fracture toughness. he short analysis of some properties of solution of the invariant equation and its application is done.

Abstract: The UK Advanced Gas Cooled nuclear reactor fleet adopted CO₂ gas as the heat transfer medium. Over the plant service life carbon diffuses into the stainless steel components as part of the overall oxidation process. This carbon enrichment promotes carbide precipitation and changes overall microstructure, thereby altering temperature deformation and fracture behaviour. Due to difficulties of replicating the high temperature/high pressure CO₂ service environment, many tests are conducted under simulated CO₂ conditions. We compare the role of a range of surrogate atmospheres on steel test specimens to one which failed in service to establish the influence of testing atmosphere on creep deformation and fracture characteristics.

Abstract: The paper deals with an analysis of maximal operation amplitudes of piezoelectric energy harvesting systems generating electrical energy from ambient vibrations. Energy harvesting systems could be very interesting alternative for autonomous powering of ultra-low power electronics, sensors and wireless communication. A design of piezoelectric vibration energy harvester is based on the cantilever beam design with active piezoelectric layers. The output power is proportional to an amplitude of relative oscillation of this resonance mechanism. This paper presents an analysis based on the simulation model of multidisciplinary piezoelectric energy harvesting device, enabling an optimization of its key parameters ensuring a maximal efficiency of the system. Such analysis is also essential for development of new energy harvesting systems formed of new smart materials and structures which could be integrated in future development processes.

Abstract: The investigation of the stressed state of flat element with the geometrical stress concentrators was performed. The sample was made of piezo optic material. The oval-shape cutout and linear cuts were issued as the stress concentrators. On the symmetrical axles was cut out an oval. Linear cuts were made one by one under the different angles. Each time after appearing of the new cut, we fixed the stress fields in the model. Issue was performed using the axial compression at the elastic linear stage of loading. We used the method of photoelasticity. We estimated the quality and quantity of the influence of the size and the angle of cuts on the element stressed state, and received the stress fields during the different loadings. Also we estimated the stress coefficients near the tops of the cuts.

Abstract: Turbine components (blades, guides and casing) of gas turbine engine usually suffer from fatigue load in company with a HTG environment in service. The corrosive substances in HTG can deposit on the surface of turbine components and induce accelerated damage known as hot corrosion. In this study, an experimental system is designed and built up to conduct LCF tests of superalloy in HTG environment. The influence of HTG on the LCF behavior of the nickel-base superalloy GH4169 at 650°C is studied. According to the test results, the average fatigue life of the specimens in HTG environment (22270 cycles) is about 31.5% less than that in air (32496 cycles). The protective oxide film on the surface of the specimen can be destroyed by the electrochemical reaction between HTG and oxide film. Compared with the specimen tested in air, there are more fatigue sources in the specimen tested in HTG environment, and the transgranular–intergranular transition occurs in the crack growth area.

Abstract: TBCs (Thermal Barrier Coatings) is deposited on gas turbine blades to protect the substrate from a combustion gas flow. One of the serious problems occurred in gas turbine is TBC delamination which is caused by startup, steady and stop operation in service. TBC delamination results from subjecting to both cyclic thermal stress and evolution of internal stress due to thermally grown oxide (TGO). In this study, the finite element code which can simulate thermal and internal stress fields generated in TBC was developed. The developed code involves the follows: inelastic constitutive equation for ceramic coating, bilinear-type constitutive equation for bond coating and Chaboche-type inelastic constitutive equation for the substrate, and mass transfer equation in consideration of oxygen diffusion and chemical reaction with aluminum. Thermal cycling simulation was conducted using the developed code. It was confirmed that maximum stress and its location in the ceramic coating/bond coating interface were matched with the associated experimental results.

Abstract: A delamination of thermal barrier coatings (TBC) applied to turbine blades in gas turbine could be caused by a high-velocity impingement of various foreign objects. It is important to accurately predict the size of interfacial crack for safety operation of gas turbine. In this study, in order to establish a practical equation for prediction of the length of interfacial crack, a high velocity impingement test and a finite element analysis (FEA) based on a cohesive model were conducted. As the result, the length of interfacial crack is linearly increased with the impact velocity. In addition, it was confirmed that it was accurately estimated by the FEA. The equation for prediction of the length of interfacial crack was formulated based on these results and the energy conservation before and after impingement. Finally, the applicability of the equation was demonstrated in a wide range of impact velocity through a comparison with the experimental results.

Abstract: The fan blades and turbine blades in a jet engine are seriously damaged by high velocity impingements of various foreign objects. In this study, a prediction method of indentation size formed by a high-velocity impingement of a solid sphere (PMIS) was developed from a theoretical model based on an expanding cavity model and energy conservation before and after impingement. The Johnson-Cook constitutive equation was employed to introduce effects of work hardening, strain rate hardening and thermal softening into the cavity model. As a result, the distribution of equivalent plastic strain, equivalent plastic strain rate, temperature and equivalent von Mises stress estimated using the expanding cavity model was in good agreement with the data obtained from the finite element analysis. In addition, it has been demonstrated that PMIS can accurately predict the radius of indentation formed on various metallic materials subjected to the impingement of a solid sphere with the radii of 0.75, 1.5 and 3 mm at several impact velocities from 50 to 300 m/s.