Distinguishing Primary and Secondary Loads to Support Gas Turbine Blades and Vanes Design

Abstract

Creep mechanisms are present in heavy duty gas turbine blades and vanes due to the simultaneous presence of high temperature and high stresses. Therefore, the microstructural phenomena (dislocation movement and atomic diffusion) that occur and accumulate during service are able to convert part of the initial elastic field of strain into permanent creep strain. This also induces a global redistribution of stresses. The progressive accumulation of creep strain can, in some extreme cases, produce changes and damage in the material (gamma prime rafting, porosity) and can eventually lead to component failure. This work shows how the understanding of the nature of the load significantly affect the capability of creep strain to produce damage. In fact, it is shown how both primary (non-self-limiting) and secondary (self-limiting) loads are both capable to generate a significant amount of creep strain, but the microstructural damage is more easily generated by relentless primary loads, generated by external forces such as the rotor blade centrifugal force (or, in other components, external gas pressure, dead weight). In the case of turbine blades and vanes, due to the complexity of the component, it is challenging to quantitatively distinguish relentless primary from self-limiting secondary stresses or simply thermal from mechanical contributions. This work is aimed to provide the designer with tools to perform such distinction and support the interpretation of the creep calculations. The proposed methodologies are developed to improve the accuracy of the prediction of the creep damage in turbine blades and vanes, but they can also be used for other purposes (e.g. predict the hysteresis cycle shift, support the estimation of the plastic strain on the basis of an elastic FE calculation), as illustrated in the paper.