Investigations Into Creep Behavior of Gas Turbine Component Assemblies

Modern day gas turbines are very complex in construction and consist of a very large number of smaller parts and subassemblies. Hence the most vital parts of the entire assembly are the mechanical devices which are deployed to connect and keep them together. In gas turbines two approaches are normally used in the assembly process. They are the threaded fasteners such as bolt and nut and shrunk-fit or interference-fit assemblies. In the high temperature regions of the gas turbines the effect of creep on the integrity of such fastening arrangements needs to be assessed at the design stage. A problem commonly faced pertains to lack of creep data which would facilitate detailed nonlinear analysis. The available data invariably exhibit scatter. In this paper parametric studies are undertaken. Creep curves are chosen so that both primary and secondary stages are accounted for. The coefficients are chosen to meet the design needs. The performance of bolted joints and shrunk-fit assemblies get affected over time due to stress relaxation leading to loss of bolt pretension or the effective interference. The bolt preload as well as the interference is to be optimally chosen. Higher the preload or the interference the more effective is the joint. At the same time the stress levels are higher and hence the stresses will relax to a greater extent. For a design stage assessment of the behavior of assemblies there is need for correlation among the various operating parameters such as stress, temperature and time. For individual components one normally uses empirical correlations such as Larson-Miller to predict rupture life and also creep growth. For assemblies in which relaxation is the main design issue, such parameters are usually not available. There is need to carry out detailed nonlinear analysis. Typical bolted flange and shrink-fit assemblies are chosen for study. Parametric studies are carried out. Using creep properties as described earlier, nonlinear structural responses are studied. The purpose is to correlate the creep properties, in terms of creep strain with respect to time, stress and temperature, with the joint behavior. The key joint behavior indices are the bolt tensile stress in the case of threaded fastening and the compressive force of “effective interference” in the case of shrunk-fit assemblies. The studies have established the need for rigorous creep analysis of components having interference fits or threaded fasteners. Once the operational requirements are known, the approach presented helps in material selection.