Experiments and Modelling of the Cyclic Behaviour of Haynes 282

In this contribution the mechanical behaviour of the Ni-based superalloy Haynes 282, developed for high-temperature applications in aero and land based gas turbine engines, is studied. Experiments for cyclic loading have been performed at room temperature and elevated temperature. To capture the cyclic hardening/softening of the material at the different temperatures, a plasticity model has been calibrated against experimental data. The robustness and the uniqueness of the identified material parameters are ensured by performing sensitivity and correlation analyses. A criterion based on the strain energy density, is used for LCF life predictions of Haynes 282. The criterion has been tuned to fit test data for the different temperatures and it has been evaluated with respect to both cyclic experimental data and with respect to model response. The influence of uncertainties in experimental data on identified material parameters, fatigue life predictions and finite element predictions has been investigated

Influence of carbide distribution on ductility of Haynes®282® forgings

Haynes®282®, a relatively new superalloy is used in gas turbines in form of sheets, plates and forgings. Forgings undergo a series of deformation steps at high temperatures to form complex shapes of components. The deformation on forgings, changes the microstructural features and their distribution, and any change in distribution of microstructural features can affect the mechanical properties of the material. The present study is undertaken to investigate the possible causes of anisotropy in mechanical properties of a Haynes®282® forging through optical and electron microscopy. Microscopic investigations show that ductility is anisotropic and changes from 15% to 21%. The electron backscattered diffraction (EBSD) investigation reveals that the presence of carbide stringers (banding of MC and M6C carbides) is associated with fine grains, thereby giving a bimodal distribution of grain size. Carbide stringers follow the complexity of forgings and are identified as the primary cause for the anisotropic behavior in ductility. Furthermore, micromechanical simulations of carbide stringers in association with a bimodal grain structure was seen to qualitatively correspond to the experimental observation indicating improved ductility with banding along the tensile axis

The importance of livestock demography and infrastructure in driving Foot and Mouth disease dynamics

Transboundary animal diseases, such as foot and mouth disease (FMD) pose a significant and ongoing threat to global food security. Such diseases can produce large, spatially complex outbreaks. Mathematical models are often used to understand the spatio-temporal dynamics and create response plans for possible disease introductions. Model assumptions regarding transmission behavior of premises and movement patterns of livestock directly impact our understanding of the ecological drivers of outbreaks and how to best control them. Here, we investigate the impact that these assumptions have on model predictions of FMD outbreaks in the U.S. using models of livestock shipment networks and disease spread. We explore the impact of changing assumptions about premises transmission behavior, both by including within-herd dynamics, and by accounting for premises type and increasing the accuracy of shipment predictions. We find that the impact these assumptions have on outbreak predictions is less than the impact of the underlying livestock demography, but that they are important for investigating some response objectives, such as the impact on trade. These results suggest that demography is a key ecological driver of outbreaks and is critical for making robust predictions but that understanding management objectives is also important when making choices about model assumptions

British press attitudes towards the EU's global presence:from the Russian-Georgian War to the 2009 Copenhagen Summit

This article surveys the way in which British print media have presented the European Union (EU)'s global presence in the international arena by analysing two case studies which reflect two very distinctive areas of EU foreign policy: global climate change policy and the policy towards Russia. It employs frame analysis, allowing for the identification of the way in which the discourse of the press was categorized around a series of central opinions and ideas. Frames underscore the connections made by journalists between different events, policies or phenomena and their possible interpretations. The analysis highlights that acting through the common framework of the EU rather than unilaterally was a strategy preferred by the British press. These findings are in stark contrast with the deep Euroscepticism which characterizes press attitudes towards most policy areas, and is often considered to be rooted in the British political culture, media system, public opinion or the longstanding tradition of viewing the European continent as the other

Modelling of cyclic and viscous behaviour of materials for gas turbine applications - with a view towards fatigue life predictions

Critical jet engine components are exposed to high temperatures. These components are also required to have low weight in order to reduce fuel consumption and allow for heavier payload, higher speed, and longer flight distance of the aircraft. The combined requirements of high temperature resistance and low weight raise the demands on the material strength and durability properties, as well as the demands on a robust and accurate component design methodology including thermal, structural and fatigue analyses. Uncertainties in these analyses entails that higher safety factors and more conservative designs must be utilised. Therefore, it is of great importance to continuously improve the accuracy of these analyses by e.g. taking into account experimentally observed mechanical behaviour of the material.The aim of this thesis is to tailor material models that can capture the observed physical deformation and failure mechanisms in the studied high temperature alloys; thereby providing a basis for an accurate and robust design methodology. The focus is placed on calibrating and evaluating the material models with regard to isothermal strain controlled low-cycle fatigue tests of a Ni-based superalloy, Haynes 282, and a high temperature Ti-alloy, Ti-6242. Different cyclic and viscous material phenomena are studied, such as: cyclic plasticity, cyclic hardening/softening, mean stress relaxation, and stress relaxation.The material models are formulated to accurately capture the stress-strain behaviour; thereby, they may also serve as a basis for crack initiation fatigue analysis.A criterion for predicting the crack initiation phase of the fatigue life, accounting for the evolution of the cyclic material behaviour and viscous effects, is adopted in the thesis and the influence of the material model on the predicted fatigue life is investigated. To be able to account for the evolution of the cyclic material behaviour, many load cycles need to be simulated, which is computationally demanding. Therefore, methods for improving the computational efficiency of such simulations are assessed. Moreover, the complexity of the material parameter identification of the formulated material models is highlighted, and methodologies for improving and simplifying this identification process are discussed. The sensitivity of experimental scatter on the identified material parameter values and the resulting model response is also evaluated.Furthermore, the intergranular fracture of Haynes 282 forgings is investigated using micromechanical models. In these models, the nonuniform distribution of grain size and carbides along the grain boundaries results in an anisotropic macroscopic tensile ductility similar to that observed in experiments

Modelling of the cyclic behaviour of superalloys

In the aviation industry today there is a trend of increasing operating temperatures in turbine engines. The advantages with higher temperatures in the engines are improved performance and efficiency. During a flight cycle the engine components are exposed to cyclically varying thermal and mechanical loads, which might cause thermo-mechanical fatigue (TMF) of the material. For prediction of the TMF life it is important to have a material model that can cover the temperature range of interest for the analysed component. The material model should also capture the decisive material mechanisms active in the component during the flight. Therefore the constitutive model should be able to mimic phenomena such as cyclic hardening/softening, the Bauschinger effect, ratchetting, shake down, creep and stress relaxation. The objective of this thesis is to investigate and develop modelling of cyclic plasticity phenomena observed for superalloys in low-cycle fatigue (LCF) experiments at different temperatures.In the first paper, the mechanical behaviour of the Ni-based superalloy Haynes 282 is studied at room temperature and 650℃. A Chaboche-type of plasticity model is chosen as a base model and calibrated against LCF experiments. The robustness and uniqueness of the identified material parameters are ensured by performing sensitivity analyses. The plasticity model is extended to include several kinematic hardening variables and it is studied how this affects its response. Furthermore, the influence of uncertainties in experimental data on identified material parameters, fatigue life predictions and finite element (FE) predictions is investigated.The modelling of the cyclic behaviour of Haynes 282 is continued in the second paper where the base model is modified to account for the cyclic softening observed at high temperatures. The material model is calibrated for a range of elevated temperatures and a temperature dependence is established for the material parameters. The temperature dependence is validated against experiments with good results. In addition, an FE example is given to illustrate the consequences when including cyclic softening in the material model. The slow evolution of the cyclic softening requires many loading cycles to develop and therefore a technique for cycle extrapolation is incorporated in the FE analysis to increase the efficiency of the computations.In the third paper, the base model is used to capture the initial mean stress relaxation observed for a high strength Titanium alloy. The calibrated material model is included in FE analyses of LCF crack growth experiments. Correlation between experimental and FE results indicates a potential for the prediction of crack length based on the measured load drop during LCF testing