Michael J. Verrilli

Ceramic Matrix Composite Vane Subelement Testing in a Gas Turbine Environment

Vane subelements were fabricated from a silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composite and were coated with an environmental barrier coating (EBC). A test configuration for the vanes in a gas turbine environment was designed and fabricated. Prior to testing, finite element analyses were performed to predict the temperatures and stress conditions present in vane during rig testing. This paper discusses the test configuration, the finite element analysis predictions, and results of the vane testing.Copyright

Ceramic Matrix Composite Vane Subelement Fabrication

Vane subelements were fabricated from a silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composite and were coated with an environmental barrier coating (EBC). In order to address realistic critical design features of a turbine airfoil, the vane subelement cross section was derived from an existing production aircraft engine vane. A new fabrication technique has been developed at NASA Glenn Research Center that enables ceramic composite vanes to be constructed using stoichiometric SiC fiber in the form of a two dimensional cloth. A unique woven cloth configuration was used to provide a sharp trailing edge with continuous fiber reinforcement. Fabrication of vanes with a sharp trailing edge was considered to be one of the more challenging features for fabricating a ceramic composite vane. The vanes were densified through the chemical vapor infiltration/slurry cast/silicon melt-infiltration process. Both NDE inspection and metallographic examinations revealed that the final as-fabricated composite quality of the vanes was consistent with that typically obtained for the same composite material fabricated into flat panels. Two vane configurations were fabricated. One consisted of a thin wall (1.5 mm) shell with a continuously reinforced sharp trailing edge. The second vane configuration included a reinforcing web bridging the pressure and suction-side vane walls and the same reinforced sharp trailing edge. This paper will discuss the vane fabrication and characterization efforts.Copyright

Creep-Rupture Behavior of a Nicalon/SiC Composite

A study of the high temperature tensile creep rupture behavior of a commercially available woven [0/90] Nicalon/SiC composite was performed. Tests were performed at temperatures of 500 to 1149°C. At a creep stress of 83 MPa, lives of less than 40 hours for temperatures above 600°C in air were obtained. At the reduced stress level of 69 MPa, lives were twice as long. However, a test conducted at 83 MPa and temperature of 982°C in vacuum yielded a run-out life of more than 1000 hours. Also, the tests performed in air have significantly shorter lives than those reported by the composite manufacturer. Post test investigations focused on identifying the mechanisms responsible for the unexpected difference in life and results are presented. Analytical modeling and supplemental testing revealed an unexpected environmentally assisted material degradation at an intermediate temperature range of 700 to 800°C. Microscopic and chemical analyses indicate that a combination of applied stress and temperature produces an oxidation-embrittlement damage mechanism. The role of the selected specimen geometry and testing conditions, which was instrumental in the detection of this failure mechanism, is thoroughly discussed.

Application of a Thermal Fatigue Life Prediction Model to High-Temperature Aerospace Alloys B1900 + Hf and Haynes 188

The results of the application of a newly proposed thermomechanical fatigue (TMF) life prediction method to a series of laboratory TMF results on two high-temperature aerospace engine alloys are presented. The method, referred to as TMF/TS-SRP, is based on three relatively recent developments: the total strain version of the method of Strainrange Partitioning (TS-SRP), the bithermal testing technique for characterizing TMF behavior, and advanced viscoplastic constitutive models. The high-temperature data reported in a companion publication are used to evaluate the constants in the model and to provide the TMF verification data to check its accuracy. Predicted lives are in agreement with the experimental lives to within a factor of approximately 2.

Characterization of Ceramic Matrix Composite Fasteners Exposed in a Combustor Linear Rig Test

Combustion tests on SiC/SiC CMC components were performed in an aircraft combustion environment using the rich-burn, quick-quench, lean-burn (RQL) sector rig. SiC/SiC fasteners were used to attach several of these components to the metallic rig structure. The effect of combustion exposure on the fastener material was characterized via microstructural examination. Fasteners were also destructively tested, after combustion exposure, and the failure loads of fasteners exposed in the sector rig were compared to those of as-manufactured fasteners. Combustion exposure reduced the average fastener failure load by 50% relative to the as-manufactured fasteners for exposure times ranging from 50 to 260 hours. The fasteners exposed in the combustion environment demonstrated failure loads that varied with failure mode. Fasteners that had the highest average failure load, failed in the same manner as the unexposed fasteners.

Microstructural and Defect Characterization in Ceramic Composites Using an Ultrasonic Guided Wave Scan System

In this study, an ultrasonic guided wave scan system was used to characterize various microstructural and flaw conditions in two types of ceramic matrix composites, SiC/SiC and C/SiC. Rather than attempting to isolate specific lamb wave modes to use for characterization (as is desired for many types of guided wave inspection problems), the guided wave scan system utilizes the total (multi‐mode) ultrasonic response in its inspection analysis. Several time‐ and frequency‐domain parameters are calculated from the ultrasonic guided wave signal at each scan location to form images. Microstructural and defect conditions examined include delamination, density variation, cracking, and pre/post‐infiltration. Results are compared with thermographic imaging methods. Although the guided wave technique is commonly used so scanning can be eliminated, applying the technique in the scanning mode allows a more precise characterization of defect conditions.

Thermomechanical and bithermal fatigue behavior of cast B1900 + Hf and wrought Haynes 188

A thermomechanical fatigue (TMF) high-temperature life prediction method has been evaluated using the experimental data. Bithermal fatigue (BTF), bithermal creep-fatigue (BTC-F), and TMF experiments were performed using two aerospace structural alloys, cast B1900 + Hf and wrought Haynes 188. The method which is based on the total strain version of strain range partitioning and unified cyclic constitutive modeling requires, as an input, information on the flow and failure behavior of the material of interest. Bithermal temperatures of 483 and 871 C were used for the cast B1900 + Hf nickel-base alloy and 316 and 760 C for the wrought Haynes 188 cobalt-base alloy. Maximum and minimum temperatures were also used in both TMF and BTF tests. Comparisons were made between the results of these tests and isothermal tensile and fatigue test data obtained previously. Qualitative correlations were observed between tensile and isothermal fatigue tests.

Characterization of C/Enhanced SiC Composite During Creep-Rupture Tests Using an Ultrasonic Guided Wave Scan System

Summary An ultrasonic guided wave scan system was used to nondestructively monitor damage over time and position in a C/enhanced SiC sample that was creep tested to failure at 1200 o C in air at a stress of 69 MPa (10 ksi). The use of the guided wave scan system for mapping evolving oxidation profiles (via porosity gradients resulting from oxidation) along the sample length and predicting failure location was explored. The creep-rupture tests were interrupted for ultrasonic evaluation every two hours until failure at ~17.5 cumulative hours. Introduction Ceramic matrix composites (CMC) are being developed for advanced aerospace propulsion applications in order to save weight, to improve reuse capability, and to increase performance. C/SiC materials have shown some promise in these applications but are extremely susceptible to oxidation damage. C/Enhanced SiC (the “enhancement” is in boron carbide, which is put in the composite matrix in order to protect the carbon fibers from oxidation) have shown increased lifetimes over the conventional C/SiC materials.

Stressed Oxidation Life for a Carbon Fiber Reinforced Silicon Carbide Matrix Composite

Stress-rupture life and residual strength results obtained for a carbon fiber reinforced silicon carbide matrix composite are presented. The material used in this study had a chemical vapor infiltration silicon carbide matrix reinforced by T-300 carbon fibers in a two-dimensional woven cloth, with a plain weave architecture. Tests were conducted in a reduced oxygen environment of 1000 ppm O2 /Ar at 1200 °C. The results from two sets of twenty creep rupture tests performed at a fixed stress are presented and discussed. In the first test set, specimens were run to failure. In the second set, rupture testing was interrupted after a fixed time interval and retained strengths were then measured. The results of this study reveal a simple deterministic relationship between the retained strength and residual life of the ceramic composite when exposed to the oxidative environment. Creep life was also observed to increase for specimens with wider tests section widths. Microstructural examination revealed damage mechanisms in the form of fiber oxidation and this points to the need for developing environmental models for predicting life of a component manufactured with this material.Copyright

Characterization of Ceramic Matrix Composite Fasteners Exposed in a Combustor Liner Rig Test

NASA Glenn Research Center Cleveland, OH ABSTRACT Combustion tests on SiC/SiC CMC components were performed in an aircraft combustion environment using the Rich-burn, Quick-quench, !:ean-burn (RQL) sector rig. SiC/SiC fasteners were used to attach several of these components to the metallic rig structure. The effect of combustion exposure on the fastener material was characterized via microstructural examination. Fasteners were also destructively tested, after combustion exposure, and the failure loads of fasteners exposed in the sector rig were compared to those of as-manufactured fasteners. Combustion exposure reduced the fastener failure load by 50% relative to the as-manufactured fasteners for exposure times ranging from 50 to 260 hours. The fasteners exposed in the combustion environment demonstrated failure loads that varied with fail ure mode. Fasteners that had the highest average failure load, failed in the same manner as the unexposed fasteners. INTRODUCTION A major focus of NASAs Enabling Propulsion Materials (EPM) program was development of an advanced ceramic matrix composites (CMCs) for turbine engine combustor liners. CMCs offer great potential to improve turbine engine performance by reducing cooling requirements and NOx emissions by operating at higher temperatures than materials used for hot structures, such as Ni-base superalloys. A melt-infiltrated SiC fiber reinforced SiC matrix material (MI SiC/SiC) was the result of the collaborative efforts of NASA, General Electric, and Pratt