Date of Award

5-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Committee Member

Dr. Mica Grujicic, Committee Chair

Committee Member

Dr. Chenning Tong

Committee Member

Dr. Jay Ochterbeck

Committee Member

Dr. Rajendra Singh

Abstract

Ever increasing world energy need and growing environmental concerns have resulted in rising efficiency and reduced emissions requirements from the energy industry. Current gas turbines, widely used for power generation, have reached a plateau in efficiency. To further boost their efficiency and reduce emissions it is imperative to increase the operating temperatures. This necessitates the advent of new materials which have higher temperature capability than the existing super alloys, used to manufacture current gas turbines. Ceramic Matrix Composites (CMCs) are such a class of material, which have very high melting points and are extremely light weight in comparison to the superalloys. The CMCs are made from ceramic constituents that are inherently brittle; however, the CMCs show metal-like ductile behavior. The present work focuses on a non-oxide class of CMCs which are made SiC fibers and SiC matrix. A room temperature multi-length scale constitutive material model has been developed by homogenization at two characteristic microstructural Length Scales (LS), fiber/tow LS and ply/lamina LS. The results obtained from virtual mechanical tests on representative volume elements for the two LS are homogenized to generate a component length scale material model which exhibits the characteristic elastic and inelastic behavior of CMCs. This material model is implemented as a user subroutine for a commercial finite element package ABAQUS. Being a relatively new class of material, the CMCs are targeted initially for manufacturing low stress bearing stationary components in the hot-section of the gas turbines. Hence, the material model is tested by conducting a foreign object impact test on a typical stationary gas turbine hot-section component, namely the inner shroud. The effect of fiber architecture (cross-ply vs. plain weave) and strength of the fiber-matrix bond on the impact resistance of the inner shroud is demonstrated. In the hot-section of the gas turbine, the CMC components experience significant in-service high temperature environmental degradation. To capture this degradation four environmental effects: (a) grain growth and porosity growth; (b) creep; (c) dry oxidation; and (d) wet oxidation, have been identified. Using experimental data reported in open literature, the component length scale CMC material model properties are modified to be a function of the nature, duration and extent of the environmental exposure. Again, foreign object impact tests are conducted to measure the CMC material degradation after exposing it to the four environmental conditions. Out of the four environmental effects considered the wet oxidation results in highest material degradation, at a given time and temperature exposure. After the commercial success of stationary CMC components is established, more hot-section components like turbine blades are expected to be made from CMCs to further extend the efficiency benefits offered by the use of CMCs in gas turbines. Creep is a primary failure mechanism for rotating components like blade, which experience high in-service temperature. A generalized anisotropic 3-D creep deformation and creep rupture model is developed for SiC/SiC CMCs subjected to multi-axial stresses. Experimental results from open literature are used to parameterize and validate the creep deformation and rupture model for the SiC/SiC CMCs. This model is then used in a finite element package ABAQUS to predict the gas turbine operation time associated with the first blade-tip rub and eventual creep rupture (at the root) of a CMC blade used in the low pressure turbine of a gas turbine engine. The gas turbine engine maintenance schedule and life time of CMC blades, which are governed by the engine operation time associated with blade-tip rub and creep rupture events, are predicted using the results of this analysis. Lastly, the issue of attaching the stationary CMC component (inner shroud) to the metallic components in the gas turbine has been addressed. Traditional fastening techniques are not suitable since the CMCs have a very low thermal expansion coefficient in comparison to the surrounding metallic components. Hence, a floating type assembly is used to attach the inner shroud to the outer casing. It consists of pre-compressed spring to provide clamping force to the inner shroud. The metallic spring undergoes creep and oxidation since it is located in the hot-section of the gas turbine, resulting in a loss of clamping force. This is a potential life limiting mechanism for the CMC inner shroud. Material selection procedures are developed for the metallic spring using rigorous finite element method and relatively simplified analytical technique. The objective is to minimize the loss in spring clamping force, subjected to geometric constraints (spring dimensions are limited by the size of the cavity that houses it) and functional constraint (maximum allowable drop in spring clamping force over the expected inner shroud life time). Both the procedures generate consistent ordering of candidate materials for the spring in the case of creep. However, consideration of oxidation alters the results among the two procedures. The computational procedures and the results from this dissertation are intended to complement the ongoing and future experimental CMC development efforts by reducing the associated time and cost.

Share

COinS