Engineering topics

Yun, Hee Mann

Publications and source records attributed to Yun, Hee Mann.

18 records

NASA NTRS2007

In-Plane Cracking Behavior and Ultimate Strength for 2D Woven and Braided Melt-Infiltrated SiC/SiC Composites Tensile Loaded in Off-Axis Fiber Directions

The tensile mechanical properties of ceramic matrix composites (CMC) in directions off the primary axes of the reinforcing fibers are important for architectural design of CMC components that are subjected to multi-axial stress states. In this study, 2D-woven melt-infiltrated (MI) SiC/SiC composite panels with balanced fiber content in the 0 degree and 90 degree directions were tensile loaded in-plane in the 0 degree direction and at 45 degree to this direction. In addition, a 2D triaxially-braided MI composite panel with balanced fiber content in the plus or minus 67 degree bias directions and reduced fiber content in the axial direction was tensile loaded perpendicular to the axial direction tows (i.e., 23 degrees from the bias fibers). Stress-strain behavior, acoustic emission, and optical microscopy were used to quantify stress-dependent matrix cracking and ultimate strength in the panels. It was observed that both off-axis loaded panels displayed higher composite onset stresses for through-thickness matrix cracking than the 2D-woven 0/90 panels loaded in the primary 0 degree direction. These improvements for off-axis cracking strength can in part be attributed to higher effective fiber fractions in the loading direction, which in turn reduces internal stresses on critical matrix flaws for a given composite stress. Also for the 0/90 panel loaded in the 45 degree direction, an improved distribution of matrix flaws existed due to the absence of fiber tows perpendicular to the loading direction. In addition, for the +67/0/-67 braided panel, the axial tows perpendicular to the loading direction were not only low in volume fraction, but were also were well separated from one another. Both off-axis oriented panels also showed relatively good ultimate tensile strength when compared to other off-axis oriented composites in the literature, both on an absolute strength basis as well as when normalized by the average fiber strength within the composites. Initial implications are discussed for constituent and architecture design to improve the directional cracking of SiC/SiC CMC components with MI matrices.

Morscher, Gregory N.↗

NASA NTRS2005

SiC/SiC Ceramic Matrix Composites Developed for High-Temperature Space Transportation Applications

Researchers at the NASA Glenn Research Center have been developing durable, high-temperature ceramic matrix composites (CMCs) with silicon carbide (SiC) matrices and SiC or carbon fibers for use in advanced reusable launch vehicle propulsion and airframe applications in the Next Generation Launch Technology (NGLT) Program. These CMCs weigh less and are more durable than competing metallic alloys, and they are tougher than silicon-based monolithic ceramics. Because of their high specific strength and durability at high temperatures, CMCs such as C/SiC (carbon- fiber-reinforced silicon carbide) and SiC/SiC (silicon-carbide-fiber-reinforced silicon carbide) may increase vehicle performance and safety significantly and reduce the cost of transporting payloads to orbit.

Kiser, J. Douglas↗

NASA NTRS2004

High-Performance SiC/SiC Ceramic Composite Systems Developed for 1315 C (2400 F) Engine Components

As structural materials for hot-section components in advanced aerospace and land-based gas turbine engines, silicon carbide (SiC) ceramic matrix composites reinforced by high performance SiC fibers offer a variety of performance advantages over current bill-of-materials, such as nickel-based superalloys. These advantages are based on the SiC/SiC composites displaying higher temperature capability for a given structural load, lower density (approximately 30- to 50-percent metal density), and lower thermal expansion. These properties should, in turn, result in many important engine benefits, such as reduced component cooling air requirements, simpler component design, reduced support structure weight, improved fuel efficiency, reduced emissions, higher blade frequencies, reduced blade clearances, and higher thrust. Under the NASA Ultra-Efficient Engine Technology (UEET) Project, much progress has been made at the NASA Glenn Research Center in identifying and optimizing two highperformance SiC/SiC composite systems. The table compares typical properties of oxide/oxide panels and SiC/SiC panels formed by the random stacking of balanced 0 degrees/90 degrees fabric pieces reinforced by the indicated fiber types. The Glenn SiC/SiC systems A and B (shaded area of the table) were reinforced by the Sylramic-iBN SiC fiber, which was produced at Glenn by thermal treatment of the commercial Sylramic SiC fiber (Dow Corning, Midland, MI; ref. 2). The treatment process (1) removes boron from the Sylramic fiber, thereby improving fiber creep, rupture, and oxidation resistance and (2) allows the boron to react with nitrogen to form a thin in situ grown BN coating on the fiber surface, thereby providing an oxidation-resistant buffer layer between contacting fibers in the fabric and the final composite. The fabric stacks for all SiC/SiC panels were provided to GE Power Systems Composites for chemical vapor infiltration of Glenn designed BN fiber coatings and conventional SiC matrices. Composite panels with system B were heat treated at Glenn, and the pores that remained open were filled by silicon melt infiltration (MI). Panels with system A and the other SiC/SiC systems were not heat treated, and remaining open pores in these systems were filled with SiC slurry and silicon MI.

DiCarlo, James A.↗

NASA NTRS2003

Matrix Cracking in 3D Orthogonal Melt-Infiltrated SiC/SiC Composites with Various Z-Fiber Types

The occurrence of matrix cracks in melt-infiltrated SiC/SiC composites with a 3D orthogonal architecture was determined at room temperature for specimens tested in tension oriented in the X-direction (parallel to Z-bundle weave direction) and Y-direction (perpendicular to Z-bundle weave direction) and Y-direction (perpendicular to Z-bundle weave direction). The fiber-types were Sylramic and Sylramic-IBN in the X and Y-directions and lower modulus ZMI, T300, and rayon in the Z-direction. Acoustic emission (AE) was used to monitor the matrix cracking activity. For Y-direction composites, the AE data was used to determine the exact (+/- 0.25 mm) location where matrix cracks occurred in the 3D orthogonal architecture. This enabled the determination of the stress-dependent matrix crack distributions for small but repeatable matrix rich 'unidirectional' and the matrix poor 'cross-ply' regions within the architecture. It was found that matrix cracking initiated at very low stresses (approx. 40 MPa) in the 'unidirectional' regions for the largest z-direction fiber tow composites. Decreasing the size of the z-fiber bundle, increased the stress for matrix cracking in the 'unidirectional' regions. Matrix cracking in the 'cross-ply' regions always occurred at higher stresses than in 'unidirectional' regions, and the stress-dependent matrix crack distribution of the 'cross-ply' regions was always over a wider stress-range than the 'unidirectional' regions. For composites tested in the X-direction, a lower elastic modulus and a narrower and lower stress-range for matrix cracking were observed compared to composites tested in the Y-direction.

Morscher, Gregory N.↗

NASA NTRS2002

New High-Performance SiC Fiber Developed for Ceramic Composites

Sylramic-iBN fiber is a new type of small-diameter (10-mm) SiC fiber that was developed at the NASA Glenn Research Center and was recently given an R&D 100 Award for 2001. It is produced by subjecting commercially available Sylramic (Dow Corning, Midland, MI) SiC fibers, fabrics, or preforms to a specially designed high-temperature treatment in a controlled nitrogen environment for a specific time. It can be used in a variety of applications, but it currently has the greatest advantage as a reinforcement for SiC/SiC ceramic composites that are targeted for long-term structural applications at temperatures higher than the capability of metallic superalloys. The commercial Sylramic SiC fiber, which is the precursor for the Sylramic-iBN fiber, is produced by Dow Corning, Midland, Michigan. It is derived from polymers at low temperatures and then pyrolyzed and sintered at high temperatures using boron-containing sintering aids (ref. 1). The sintering process results in very strong fibers (>3 GPa) that are dense, oxygen-free, and nearly stoichiometric. They also display an optimum grain size that is beneficial for high tensile strength, good creep resistance, and good thermal conductivity (ref. 2). The NASA-developed treatment allows the excess boron in the bulk to diffuse to the fiber surface where it reacts with nitrogen to form an in situ boron nitride (BN) coating on the fiber surface (thus the product name of Sylramic-iBN fiber). The removal of boron from the fiber bulk allows the retention of high tensile strength while significantly improving creep resistance and electrical conductivity, and probably thermal conductivity since the grains are slightly larger and the grain boundaries cleaner (ref. 2). Also, as shown in the graph, these improvements allow the fiber to display the best rupture strength at high temperatures in air for any available SiC fiber. In addition, for CMC applications under oxidizing conditions, the formation of an in situ BN surface layer creates a more environmentally durable fiber surface not only because a more oxidation-resistant BN is formed, but also because this layer provides a physical barrier between contacting fibers with oxidation-prone SiC surface layers (refs. 3 and 4). This year, Glenn demonstrated that the in situ BN treatment can be applied simply to Sylramic fibers located within continuous multifiber tows, within woven fabric pieces, or even assembled into complex product shapes (preforms). SiC/SiC ceramic composite panels have been fabricated from Sylramic-iBN fabric and then tested at Glenn within the Ultra-Efficient Engine Technology Program. The test conditions were selected to simulate those experienced by hot-section components in advanced gas turbine engines. The results from testing at Glenn demonstrate all the benefits expected for the Sylramic-iBN fibers. That is, the composites displayed the best thermostructural performance in comparison to composites reinforced by Sylramic fibers and by all other currently available high-performance SiC fiber types (refs. 3 and 5). For these reasons, the Ultra-Efficient Engine Technology Program has selected the Sylramic-iBN fiber for ongoing efforts aimed at SiC/SiC engine component development.

DiCarlo, James A.↗

NASA NTRS2002

Architectures for High-Performance Ceramic Composites Being Improved

A major thrust of the Ultra-Efficient Engine Technology (UEET) Program at the NASA Glenn Research Center is to develop advanced hot-section engine components using SiC/SiC ceramic matrix composites (CMC's) with thermostructural capability to 2400 F (1315 C). In previous studies, UEET determined that the higher the ultimate tensile strength (UTS) of the as-fabricated CMC, the greater its structural performance at 2400 F. Thus efforts have been ongoing within UEET to understand and develop fiber architecture approaches that can improve the UTS of SiC/SiC CMC's. Under UEET, SiC/SiC test panels and demonstration engine components are currently produced by the multi-ply layup of two-dimensional fabric pieces. The fabric is typically formed of multifilament tows containing high-performance Sylramic (Dow Corning) SiC fiber that is woven into two-dimensional five-harness satin fabric with 20 ends per inch in the 0 degree and 90 degree directions. In some cases, fabric pieces containing woven Sylramic fiber tows are thermally treated at NASA to form Sylramic-iBN fibers that contain a very thin in-situ-grown boron nitride layer on their surfaces. The final SiC/SiC panels and components are fabricated at the CMC vendor by compressing the fabric pieces in tools and then depositing a thin BN interphase coating on the fibers by chemical vapor deposition. The last step at the vendor is to infiltrate the BN-coated fiber architecture with SiC and silicon matrix constituents to form a dense product. Because the as-produced Sylramic fiber tows are sized with a thin polymer coating to facilitate handling and weaving, the individual fibers within the tows and fabric are in close contact with each other. This contact is further increased during fabric compression. One important recent finding is that increasing Sylramic fiber tow width in a fabric increases the UTS of the final SiC/SiC CMC. This effect is presumably related to minimizing fiber/fiber contact, which can be detrimental to CMC strength because of the boron-rich chemistry and roughness of the Sylramic fiber surface. Tows can be spread by mechanically agitating the Sylramic fabric prior to CMC fabrication or by simply thermally treating the Sylramic fabric as in the formation of the Sylramic-iBN fibers. However, CMC's with the treated Sylramic-iBN fabric are even stronger than CMC's with mechanically spread Sylramic tows. The extra strength capability is presumably related to the in situ BN on the fiber surface, which adds compliance to the fiber surfaces and is more resistant to oxygen impurities introduced during the chemical vapor deposition BN process. As shown, another important finding is that the use of fabric with tows having less than the standard of 20 ends per inch provides advantages in terms of reduced ply height and increased ply and CMC strength. The reduced ply height provides more control of part thickness by allowing more plies for a given thickness and by reducing interlaminar residual stresses between plies. The increased ply strength is presumably related to a reduced number of interlaced 90 tows, which, in turn, reduces the crimp angle on the high-modulus fibers in the 0 degree tows. Also, as shown, although fabric with fewer ends per inch reduced the maximum fiber fraction in an eight-ply CMC panel; CMC UTS actually increased because of increased ply strength. Thus, using fabric with fewer ends per inch has several advantages, including providing a significantly higher strength per fiber fraction in the CMC. Consequently, ongoing UEET efforts will attempt to use architectural approaches for components that minimize fiber-fiber contacts and fiber bending within the final composite microstructure.

Yun, Hee Mann↗

NASA NTRS2002

Fracture Mechanisms For SiC Fibers And SiC/SiC Composites Under Stress-Rupture Conditions at High Temperatures

The successful application of SiC/SiC ceramic matrix composites as high-temperature structural materials depends strongly on maximizing the fracture or rupture life of the load-bearing fiber and matrix constituents. Using high-temperature data measured under stress-rupture test conditions, this study examines in a mechanistic manner the effects of various intrinsic and extrinsic factors on the creep and fracture behavior of a variety of SiC fiber types. It is shown that although some fiber types fracture during a large primary creep stage, the fiber creep rate just prior to fracture plays a key role in determining fiber rupture time (Monkman-Grant theory). If it is assumed that SiC matrices rupture in a similar manner as fibers with the same microstructures, one can develop simple mechanistic models to analyze and optimize the stress-rupture behavior of SiC/SiC composites for applied stresses that are initially below matrix cracking.

DiCarlo, James A.↗

NASA NTRS2002

High Temperature Si-doped BN Interphases for Woven SiC/SiC Composites

The hydrolytic stability of high-temperature deposited Si-doped BN has been shown in the past to be superior in comparison to "pure" BN processed at similar or even higher temperatures. This type of material would be very desirable as a SiC/SiC composite interphase that is formed by chemical infiltration into multi-ply woven preform. However, due to rapid deposition on the preform outer surface at the high processing temperature, this has proven very difficult. To overcome this issue, single plies of woven fabric were infiltrated with Si-doped BN. Three composite panels of different SiC fiber types were fabricated with Si-doped BN interphases including Sylramic, Hi-Nicalon Type S and Sylramic-iBN fiber-types. The latter fiber-type possesses a thin in-situ grown BN layer on the fiber surface. High Si contents (approx. 7 to 10 a/o) and low oxygen contents (less than 1 a/o) were achieved. All three composite systems demonstrated reasonable debonding and sliding properties. The coated Sylramic fabric and composites were weak due to fiber degradation apparently caused during interphase processing by the formation of TiN crystals on the fiber surface. The Hi-Nicalon Type S composites with Si-doped BN interphase were only slightly weaker than Hi-Nicalon Type S composites with conventional BN when the strength on the load-bearing fibers at failure was compared. On the other hand, the Sylramic-iBN fabric and composites with Si-doped BN showed excellent composite and intermediate temperature stress-rupture properties. Most impressive was the lack of any significant interphase oxidation on the fracture surface of stress-ruptured specimens tested well above matrix cracking at 815C.

Morscher, Gregory N.↗

NASA NTRS2002

Progress in SiC/SiC Ceramic Composite Development for Gas Turbine Hot-Section Components under NASA EPM and UEET Programs

The successful application of ceramic matrix composites as hot-section components in advanced gas turbine engines will require the development of constituent materials and processes that can provide the material systems with the key thermostructural properties required for long-term component service. Much initial progress in identifying these materials and processes was made under the former NASA Enabling Propulsion Materials Program using stoichiometric Sylramic (trademark) silicon-carbide (SiC) fibers, 2D (two dimensional)-woven fiber architectures, chemically vapor-infiltrated (CVI) BN fiber coatings (interphases), and SiC-based matrices containing CVI SiC interphase over-coatings, slurry-infiltrated SiC particulate, and melt-infiltrated (MI) silicon. The objective of this paper is to discuss the property benefits of this SiC/SiC composite system for high-temperature engine components and to elaborate on further progress in SiC/SiC development made under the new NASA Ultra Efficient Engine Technology Program. This progress stems from the recent development of advanced constituent materials and manufacturing processes, including specific treatments at NASA that improve the creep, rupture, and environmental resistance of the Sylramic fiber as well as the thermal conductivity and creep resistance of the CVI SiC over-coatings. Also discussed are recent observations concerning the detrimental effects of inadvertent carbon in the fiber-BN interfacial region and the beneficial effects of certain 2D-architectures for thin-walled SiC/SiC panels.

DiCarlo, J. A.↗

NASA NTRS2001

Overview of NASA Studies on High-Temperature Ceramic Fibers

NASA, DOD (Department of Defense), and DOE (Department of Energy) are currently looking to the NASA UEET (Ultra-Efficient Engine Technology) Program to develop ceramic matrix composites (CMC) for hot-section components in advanced power and propulsion systems. Success will greatly depend on developing ceramic fibers with a variety of key thermostructural properties, in particular, high as-produced tensile strength and retention of a large fraction of this strength for long times under the anticipated CMC service conditions. Current UEET approaches center on selecting the optimum fiber type from commercially available fibers since the costs for development of advanced fibers are high and the markets for high-temperature CMC have yet to be established.

DiCarlo, James A.↗

NASA NTRS2001

Overview of NASA Studies on High-Temperature Ceramic Fibers

NASA, DOD, and DOE are currently looking to the NASA UEET Program to develop ceramic matrix composites (CMC) for hot-section components in advanced power and propulsion systems - Success will depend strongly on developing ceramic fibers with a variety of key thermostructural properties, in particular, high as-produced tensile strength and retention of a large fraction of this strength for long times under the anticipated CMC service conditions. - Current UEET approach centers on selecting the optimum fiber type from commercially available fibers since the costs for development of advanced fibers are high and the markets for high-temperature CMC have yet to be established.

DiCarlo, James A.↗

NASA NTRS2000

Constituent-Based Life Models Being Developed for SiC/SiC Composites

For the successful utilization of ceramic matrix composites (CMC) as hot-section components in advanced aeropropulsion engines, the CMC constituents will need to be tailored and optimized to meet all the critical property demands of each component. Under the High-Speed Research (HSR) and Advanced High-Temperature Engine Materials Technology (HITEMP) Programs, the NASA Glenn Research Center at Lewis Field initiated research to develop mechanistic models for key CMC thermostructural properties. These models would describe the effects of different constituent factors (composition, geometry, and volume fraction) and of potential application conditions (stress, time, temperature, and environment) on these properties. Particular focus was placed on both analytical and numerical modeling of state-of-the-art SiC/SiC composites where the primary load-bearing constituents are stoichiometric SiC fibers in a complex multiphase SiC matrix produced by chemical vapor infiltration and melt infiltration. Recent studies have resulted in computer-based numerical models for the elastic modulus, thermal expansion, and thermal conductivity properties of the SiC/SiC system. Additional studies have generated analytical and empirical models for the time dependence of composite rupture strength at temperatures above 2200 F (1200 C), where CMC's have an important thermostructural advantage over current nickel-based superalloys. These life models utilize thermal activation theory and fiber stress-rupture results measured at Glenn to generate Larson-Miller (L M) plots of fiber rupture strength versus q, a single time- and temperature-dependent parameter. Assuming a worse case in which the SiC matrix is cracked, rupture is then controlled by the time-dependent fracture characteristics of the fiber bundles bridging the matrix cracks. With this as the controlling mechanism, one can then use simple composite theory and the fiber L M plots to predict CMC rupture strength versus the q parameter. The dashed line shows the predicted rupture strength of a SiC/SiC composite that is reinforced by a state-of-the-art stoichiometric SiC fiber. For the q parameter, time is in hours and temperature in degrees kelvin. To generate these predictions, a two-dimensional 0 /90 composite with approximately 16 percent fiber in the applied stress direction and an air test environment were assumed. As such, it is possible to compare the model predictions against limited stress-rupture data for this CMC as shown by the data points. The good agreement confirms the rupture model at least for the selected CMC and test conditions. Thus for this particular SiC/SiC composite, one can estimate a 1000-hr rupture strength of approximately 12 ksi at 2400 F (T = 1588 K and q = 39 700 K). At lower CMC application stresses, the SiC matrix is typically uncracked, so both the fiber and matrix constituents share the composite load. In this case, CMC rupture is controlled by the constituent with the longest rupture time based on the creep rate of the composite. Measured Monkman-Grant plots of rupture time versus creep rate for the two SiC constituents have been used to develop CMC life models for this important application condition. NASA and DOD are currently using this information to establish application and material goals for more advanced CMC's that can be used at even higher temperatures.

DiCarlo, James A.↗

NASA NTRS1997

Fiber Contraction Approaches for Improving CMC Proportional Limit

The fact that the service life of ceramic matrix composites (CMC) decreases dramatically for stresses above the CMC proportional limit has triggered a variety of research activities to develop microstructural approaches that can significantly improve this limit. As discussed in a previous report, both local and global approaches exist for hindering the propagation of cracks through the CMC matrix, the physical source for the proportional limit. Local approaches include: (1) minimizing fiber diameter and matrix modulus; (2) maximizing fiber volume fraction, fiber modulus, and matrix toughness; and (3) optimizing fiber-matrix interfacial shear strength; all of which should reduce the stress concentration at the tip of cracks pre existing or created in the matrix during CMC service. Global approaches, as with pre-stressed concrete, center on seeking mechanisms for utilizing the reinforcing fiber to subject the matrix to in-situ compressive stresses which will remain stable during CMC service. Demonstrated CMC examples for the viability of this residual stress approach are based on strain mismatches between the fiber and matrix in their free states, such as, thermal expansion mismatch and creep mismatch. However, these particular mismatch approaches are application limited in that the residual stresses from expansion mismatch are optimum only at low CMC service temperatures and the residual stresses from creep mismatch are typically unidirectional and difficult to implement in complex-shaped CMC.

DiCarlo, James A.↗

NASA NTRS1997

Time/Temperature Dependent Tensile Strength of SiC and Al2O3-Based Fibers

In order to understand and model the thermomechanical behavior of fiber-reinforced composites, stress-rupture, fast-fracture, and warm-up rupture studies were conducted on various advanced SiC and Al2O3-based fibers in the,temperature range from 20 to 1400 C in air as well as in inert environments. The measured stress-rupture, fast fracture, and warm-up rupture strengths were correlated into a single master time/temperature-dependent strength plot for each fiber type using thermal activation and slow crack growth theories. It is shown that these plots are useful for comparing and selecting fibers for CMC and MMC reinforcement and that, in comparison to stress rupture tests, the fast-fracture and warm-up tests can be used for rapid generation of these plots.

Yun, Hee Mann↗

NASA NTRS1997

Thermomechanical Behavior of Advanced SiC Fiber Multifilament Tows

In order to relate single fiber behavior to multiple fiber behavior in composites, fast-fracture tensile strength, creep, and stress-rupture studies were conducted on advanced SiC fiber multifilament tows in the temperature range from 20 to 1400 C in air as well as in inert environments. For conditions of small fiber creep (short times and low temperatures), the tow results of this study confirm the ability of limited single fiber data to model the strength behavior of multiple fibers in a bundle. For conditions of high creep (long times and high temperatures), further studies are needed to explain tow rupture behavior being better than average single fiber behavior.

Yun, Hee Mann↗

NASA NTRS1993

Tensile and stress-rupture behavior of hafnium carbide dispersed molybdenum and tungsten base alloy wires

The tensile strain rate sensitivity and the stress-rupture strength of Mo-base and W-base alloy wires, 380 microns in diameter, were determined over the temperature range from 1200 K to 1600 K. Three molybdenum alloy wires; Mo + 1.1w/o hafnium carbide (MoHfC), Mo + 25w/o W + 1.1w/o hafnium carbide (MoHfC+25W) and Mo + 45w/o W + 1.1w/o hafnium carbide (MoHfC+45W), and a W + 0.4w/o hafnium carbide (WHfC) tungsten alloy wire were evaluated. The tensile strength of all wires studied was found to have a positive strain rate sensitivity. The strain rate dependency increased with increasing temperature and is associated with grain broadening of the initial fibrous structures. The hafnium carbide dispersed W-base and Mo-base alloys have superior tensile and stress-rupture properties than those without HfC. On a density compensated basis the MoHfC wires exhibit superior tensile and stress-rupture strengths to the WHfC wires up to approximately 1400 K. Addition of tungsten in the Mo-alloy wires was found to increase the long-term stress rupture strength at temperatures above 1400 K. Theoretical calculations indicate that the strength and ductility advantage of the HfC dispersed alloy wires is due to the resistance to recrystallization imparted by the dispersoid.

Yun, Hee Mann↗

NASA NTRS1993

Stress-rupture behavior of small diameter polycrystalline alumina fibers

Continuous length polycrystalline alumina fibers are candidates as reinforcement in high temperature composite materials. Interest therefore exists in characterizing the thermomechanical behavior of these materials, obtaining possible insights into underlying mechanisms, and understanding fiber performance under long term use. Results are reported on the time-temperature dependent strength behavior of Nextel 610 and Fiber FP alumina fibers with grain sizes of 100 and 300 nm, respectively. Below 1000 C and 100 hours, Nextel 610 with the smaller grain size had a greater fast fracture and rupture strength than Fiber FP. The time exponents for stress-rupture of these fibers were found to decrease from approximately 13 at 900 C to below 3 near 1050 C, suggesting a transition from slow crack growth to creep rupture as the controlling fracture mechanism. For both fiber types, an effective activation energy of 690 kJ/mol was measured for rupture. This allowed stress-rupture predictions to be made for extended times at use temperatures below 1000 C.

Yun, Hee Mann↗

NASA NTRS1989

Tensile Behavior of Tungsten/Niobium Composites at 1300 to 1600 K

The tensile behavior of continuous tungsten fiber reinforced niobium composites (W/Nb), fabricated by an arc-spray process, was studied in the 1300 to 1600 K temperature range. The tensile properties of the fiber and matrix components as well as of the composites were measured and were compared to rule of mixtures (ROM) predictions. The deviation from the ROM was found to depend upon the chemistry of the tungsten alloy fibers, with positive deviations for ST300/Nb (i.e., stronger composite strength than the ROM) and negative or zero deviations for 218/Nb.

Yun, Hee Mann↗