Creep-behavior-based material selection for a clamping spring of ceramic-matrix composite inner-shroud in utility and industrial gas-turbine engines

This paper addresses the problem of materials selection for springs used to clamp an inner shroud segment to the outer shroud block in utility and industrial gas turbine engines. Clamping is achieved through the application of an initial compressive load to the spring. However, since the spring is subjected to high temperature and oxidizing conditions, it experiences creep and surface oxidation. Both of these processes result in the loss of the compressive load within the spring with time. A material selection procedure is developed, which identifies optimum materials (design variables), with respect to the minimum loss in the clamping-spring load (objective function) for a given set of geometrical constraints (i.e. maximum size of the spring is constrained by the outer-shroud cavity which houses the spring) and functional constraints (force retention should persist over the expected life of the inner-shroud segment). Two material selection procedures are devised: (a) one, fairly rigorous and computationally intensive, based on the use of a finite element analysis; and (b) the other, less rigorous but computationally less expensive, based on the use of a simplified analytical/numerical procedure. In the absence of oxidation, the two approaches yielded different, but mutually consistent, results with identical ranking of the clamping-force candidate materials. The inclusion of the oxidation effects showed that oxidation-induced loss in the spring material increases the extent of clamping-force relaxation and may affect the ranking of the candidate materials.