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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Damage mechanisms in three-dimensional woven reinforced ceramic matrix composites under tensile and flexural loading at room and elevated temperatures

Three-dimensional Nicalon SiC/SiC matrix composites made through a chemical vapor infiltration process were studied under tensile and flexural loading at 23, 1200, and 1550 C in air. In situ damage accumulation monitoring NDE techniques were utilized to identify failure mechanisms in these materials. The effectiveness and durability of a chemical vapor deposition SiC surface coating were also evaluated in severe oxidizing environment. Results show that the failure response was very similar for the 23 and 1200 C specimens, while at 1550 C there were significant changes in both the composite mechanical behavior and the matrix microstructure. Extensive fiber pull-out was observed only in the 1550 C specimen. It is also found that the SiC surface coating can protect the composite up to the critical matrix cracking strength.

Chulya, Abhisak↗
NASA NTRS1992

Load redistribution considerations in the fracture of ceramic matrix composites

Using a macroscopic viewpoint, composite laminae are homogeneous orthotropic solids whose directional strengths are random variables. Incorporation of these random variable strengths into failure models, either interactive or noninteractive, allows for the evaluation of the lamina reliability under a given stress state. Using a noninteractive criterion for demonstration purposes, laminate reliabilities are calculated assuming previously established load sharing rules for the redistribution of load as the failure of laminae occur. The matrix cracking predicted by ACK theory is modeled to allow a loss of stiffness in the fiber direction. The subsequent failure in the fiber direction is controlled by a modified bundle theory. Results are compared with previous models which did not permit separate consideration of matrix cracking, as well as to results obtained from experimental data. The effects of variations from the ideal physical geometry which is normally used to depict the matrix cracking are also studied.

Thomas, David J.↗
NASA NTRS1992

Fiber coating/matrix reactions in silicon-base ceramic matrix composites

The Knudsen cell technique and coupons of carbon coated Si3N4 and BN coated SiC were employed to study the possible reactions at the SiC/C/Si3N4 and SiC/BN/SiC interface. Carbon reacts with Si3N4 to form gaseous N2 and solid SiC. Solid SiC acts as a physical barrier to the reaction, which prevents the generation of high N2 pressure predicted from thermochemical calculations. Thus, deleterious effects of the reaction to the composite are limited. Limited reactions between BN and C-rich SiC was observed. However, the vapor pressure was so low that it is not likely to cause any interfacial instability. The predicted formation of a BN-C solid solution was not observed.

Lee, K. N.↗
NASA NTRS1993

Ceramic matrix composites (continuous fiber reinforced) thermal protection systems

Concluded remarks are: (1) advanced carbon-carbon (ACC) substrate fabrication technology in good shape; (2) ACC coating improvements satisfactory but additional work needed; (3) non-destructive test techniques to monitor hardware during operational life needed; and (4) cost reduction approaches a high priority.

Riccitiello, Salvatore R.↗
NASA NTRS1991

Mechanics of interfaces in fiber reinforced SiC/RBSN ceramic matrix composites

The mechanical behavior of continuous fiber reinforced SiC/RBSN composites with strong and weak interface characteristics is evaluated. Both catastrophic and noncatastrophic failures are observed in tensile specimens. Effects of fiber/matrix interface debonding (splitting) parallel to the fibers are discussed. Micromechanical models incorporating residual stresses to calculate the critical matrix cracking strength, ultimate strength and work of pull-out are reviewed and used to predict composite response. Experimental results are compared to analytical predictions.

Chulya, Abhisak↗
NASA NTRS1992

Combustor materials requirements and status of ceramic matrix composites

The HSCT combustor will be required to operate with either extremely rich or lean fuel/air ratios to reduce NO(x) emission. NASA High Speed Research (HSR) sponsored programs at Pratt & Whitney (P&W) and GE Aircraft Engines (GEAE) have been studying rich and lean burn combustor design approaches which are capable of achieving the aggressive HSCT NO(x) emission goals. In both of the combustor design approaches under study, high temperature (2400-3000 F) materials are necessary to meet the HSCT emission goals of 3-8 gm/kg. Currently available materials will not meet the projected requirements for the HSCT combustor. The development of new materials is an enabling technology for the successful introduction to service of the HSCT.

Hecht, Ralph J.↗
NASA NTRS1995

Chemical Stability of the Fiber Coating/Matrix Interface in Silicon-Based Ceramic Matrix Composites

Carbon and boron nitride are used as fiber coatings in silicon-based composites. In order to assess the long-term stability of these materials, reactions of carbon/Si3N4 and BN/SiC were studied at high temperatures with Knudsen effusion, coupon tests, and microstructural examination. In the carbon/Si3N4 system, carbon reacted with Si3N4 to form gaseous N2 and SiC. The formation of SiC limited further reaction by physically separating the carbon and Si3N4. Consequently, the development of high p(N2) at the interface, predicted from thermochemical calculations, did not occur, thus limiting the potential deleterious effects of the reaction on the composite. Strong indications of a reaction between BN and SiC were shown by TEM and SIMS analysis of the BN/SiC interface. In long-term exposures, this reaction can lead to a depletion of a BN coating and/or an unfavorable change of the interfacial properties, limiting the beneficial effects of the coating.

Lee, Kang N.↗
NASA NTRS1996

Non-Contact Determination of Antisymmetric Plate Wave Velocity in Ceramic Matrix Composites

A 13 mJ NdYAG 1064 nm, 4 ns, laser pulse was employed to produce ultrasonic plate waves in 20 percent porous SiC/SiC composite tensile specimens of three different architectures. An air coupled 0.5 MHz transducer was used to detect and collect the waveforms which contained first antisymmetric plate wave pulses for determining the shear wave velocity (VS). These results were compared to VS values determined on the same specimens with 0.5 MHz ultrasonic transducers with contact coupling. Averages of four noncontact determinations on each of 18 specimens were compared to averages of four contact values. The noncontact VS's fall in the same range as the contact. The standard deviations for the noncontact VS's averaged 2.8 percent. The standard deviations for the contact measurements averaged 2.3 percent, indicating similar reproducibility. Repeated laser pulsing at the same location always lead to deterioration of the ulu-"nic signal. The signal would recover in about 24 hr in air however, indicating that no permanent damage was produced.

Kautz, Harold E.↗
NASA NTRS1997

Durability and Life of Ceramic Matrix Composites in Combustion Environment

It is now generally recognized that the next frontier in the commercial air travel will be achieved by an optimum combination of the concepts being used for the air and space transports. It will depend upon the new engines specifically designed to meet the demanding strength, temperature, environment and economy requirements. The successful development of an advanced propulsion system for a 21st Century High Speed Civil Transport will require key advances in the combustor system. Minimizing nitrous oxide (NO(x)) emission is critical, since the deleterious interaction within the ozone layer would occur at the proposed vehicle cruise level of about 75,000 feet. In order to achieve the low NOx levels an efficient combustor operating at controlled equivalence ratios and high combustion temperatures are required. This requires a combustor, whose one portion will be operating in an oxidizing environment while the other portion will operate in a reducing environment. This rather unusual requirement has introduced considerable challenge to the materials scientists and engineers, the development of material systems capable of long-life at high temperatures (up to 3000 F) in an oxidizing and/or reducing environment.

Tewari, Surendra↗
NASA NTRS1999

Oxygen Diffusion and Reaction Kinetics in Continuous Fiber Ceramic Matrix Composites

Previous stressed oxidation tests of C/SiC composites at elevated temperatures (350 C to 1500 C) and sustained stresses (69 MPa and 172 MPa) have led to the development of a finite difference cracked matrix model. The times to failure in the samples suggest oxidation occurred in two kinetic regimes defined by the rate controlling mechanisms (i.e. diffusion controlled and reaction controlled kinetics). Microstructural analysis revealed preferential oxidation along as-fabricated, matrix microcracks and also suggested two regimes of oxidation kinetics dependent on the oxidation temperature. Based on experimental results, observation, and theory, a finite difference model was developed. The model simulates the diffusion of oxygen into a matrix crack bridged by carbon fibers. The model facilitates the study of the relative importance of temperature, the reaction rate constant, and the diffusion coefficient on the overall oxidation kinetics.

Halbig, Michael C.↗
NASA NTRS1999

Durability of Ceramic Matrix Composites in Combustion Environments

SiO2 scale volatility and the resulting recession of SiC has been previously identified for CVD, as well as sintered, SiC and Si3N4 materials. Linear weight loss and surface recession of SiC was observed as a result of SiO2 volatility for both fuel-lean and fuel-rich gas mixtures. A strong Arrhenius temperature dependence exist and parametric studies using multiple linear regression were used to develop recession relationships as a function of e(sup -Q/RT) central dot P(sup X)V(sup y) for both lean and rich combustion conditions. Recession of 0.2 - 2.0 micron/hr is predicted for typical combustion conditions at 1200 - 1400 C, with a somewhat higher absolute recession rate and activation energy for rich combustion versus lean.

Robinson, Raymond C.↗
NASA NTRS2000

Nondestructive Characterization Techniques Used for Ceramic Matrix Composite Life Determination

Recent results indicate that the specific damping capacity and resonant frequency measurements taken periodically during a component's lifetime is able to quantify the mechanical fatigue of CMCS. This gives hope for the potential of determining the actual and residual life of CMC materials using a combination of nondestructive techniques. If successful, then this new paradigm for life prediction of CMCs could revolutionize the approach for designing and servicing CMC components, thereby significantly reducing costs for design, development, health monitoring, and maintenance of CMC components and systems. The Nondestructive Characterization (NDC) life prediction approach would complement life prediction using micromechanics and continuum finite element models. This paper reports on the initial concept of NDC life prediction, a review of the C/SiC blisk damping data, and how changes in the specific damping capacity & ultrasonic elastic modulus data have established the concept as a possibility.

Effinger, Michael↗
NASA NTRS2001

The Oxidation Kinetics of Continuous Carbon Fibers in a Cracked Ceramic Matrix Composite

Experimental observations and results suggest two primary regimes as a function of temperature, i.e., diffusion and reaction controlled kinetics. Thermogravimetric analysis of carbon fiber in flowing oxygen gave an activation energy of 64.1 kJ/mol in the temperature range of 500 to 600 C and an apparent activation energy of 7.6 kJ/mol for temperatures from 600 to 1400 C. When C/SiC composite material was unstressed, matrix effects at temperatures from 900 to 1400 C protected the internal fibers. When under stress, self-protection was not observed. Increasing the stress from 10 to 25 ksi caused a 67 to 82 percent reduction in times to failure at temperatures from 750 to 1500 C. Based on experimental results, observation, and theory, a finite difference model was developed, which simulates the diffusion of oxygen into a matrix crack that is bridged by carbon fibers. The model allows the influence of important variables on oxidation kinetics to be studied systematically, i.e., temperature, reaction rate constant, diffusion coefficient, environment, and sample geometry.

Halbig, Michael C.↗
NASA NTRS2001

Development of Reliability Based Life Prediction Methods for Thermal and Environmental Barrier Coatings in Ceramic Matrix Composites

Literature survey related to the EBC/TBC (environmental barrier coating/thermal barrier coating) fife models, failure mechanisms in EBC/TBC and the initial work plan for the proposed EBC/TBC life prediction methods development was developed as well as the finite element model for the thermal/stress analysis of the GRC-developed EBC system was prepared. Technical report for these activities is given in the subsequent sections.

Shah, Ashwin↗
NASA NTRS2001

Life Prediction Issues in Thermal/Environmental Barrier Coatings in Ceramic Matrix Composites

Issues and design requirements for the environmental barrier coating (EBC)/thermal barrier coating (TBC) life that are general and those specific to the NASA Ultra-Efficient Engine Technology (UEET) development program have been described. The current state and trend of the research, methods in vogue related to the failure analysis, and long-term behavior and life prediction of EBCITBC systems are reported. Also, the perceived failure mechanisms, variables, and related uncertainties governing the EBCITBC system life are summarized. A combined heat transfer and structural analysis approach based on the oxidation kinetics using the Arrhenius theory is proposed to develop a life prediction model for the EBC/TBC systems. Stochastic process-based reliability approach that includes the physical variables such as gas pressure, temperature, velocity, moisture content, crack density, oxygen content, etc., is suggested. Benefits of the reliability-based approach are also discussed in the report.

Shah, Ashwin R.↗
NASA NTRS2002

Effect of Environment on Fatigue Behavior of a Nicalon(TM)/Si-N-C Ceramic Matrix Composite

The effect of environmental exposure on the fatigue life of Nicalon(TM) /Si-N-C composite was investigated in this study. Test specimens with arrays of 1.8 mm diameter holes and two different open areas, 25 and 35%, were machined. Three environmental conditions were studied: 1) continuous fatigue cycling in air, 2) fatigue cycling in air alternating with humidity exposure, and 3) fatigue cycling in air alternating with exposure to a salt-fog environment. All fatigue testing on specimens with holes was performed with a load ratio, R = 0.05, and at a temperature of 910 C. In general, fatigue lives were shortest for specimens subjected to salt-fog exposure and longest for specimens subjected to continuous fatigue cycling in air. The fatigue data generated on the specimens with holes were compared with fatigue data generated in air on specimens with no holes. Fatigue strength reduction factors for different environmental conditions and open areas investigated in the study were calculated for the Nicalon(TM) /Si-N-C composite.

Kalluri, Sreeramesh↗