Ratchetting and Creep Failure in Twin-Wall Turbine Blades Experiencing Severe Thermal and Centrifugal Loading

Abstract

Twin-wall structures can be cooled both externally and internally, raising great potential for use in high temperature applications. However, their increased geometric complexity imposes a range of potential failure mechanisms for consideration in design. The primary aim of this study is to identify the nature of such mechanisms by constructing Bree type interaction diagrams for idealised double wall systems under cyclic thermomechanical loading which show the combination of loading conditions for which cyclic plasticity (leading to fatigue failure)-creep-ratchetting occur. Through an extension of the classical Bree analysis we determine analytical boundaries between different regimes of behaviour. We also quantify the effects of wall thickness ratio, temperature field as well as yield and creep material properties. Local cyclic plasticity is shown to dominate over structural/global ratchetting when the yield strength reduces with temperature and/or when the temperature gradient through the hot wall thickness dominates over the temperature difference between the walls. Thus, we conclude that global ratchetting is unlikely to occur in the practical loading range of Nickel based twin-wall turbine blades, but instead these systems suffer from local fatigue at cooling holes and excessive creep deformation. This is verified by 3D cyclic Finite Element (FE) simulations, demonstrating that the analytical approach provides a powerful, cost-effective strategy for: providing physical insight into possible deformation mechanisms in a range of thin-walled components; highlighting the key trade-offs to be considered in design; and directing the use of computer methods towards more detailed calculations.