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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Time-temperature-stress dependence of boron fiber deformation

The time-dependent deformation of boron fibers over the temperature range from -190 to 800 C is studied by flexural stress relaxation and flexural internal friction techniques on 203-micron diam specimen fibers commercially produced by chemical vapor deposition (CVD) on a 13-micron tungsten substrate. It is shown that up to at least 800 C all nonelastic behavior observed during axial deformation of CVD boron fibers can be explained solely by anelastic mechanisms and that although creep strains are small, boron fiber anelasticity can produce significant mechanical effects which would otherwise be neglected under the elastic approximation. Relations are obtained to demonstrate the considerable effects of anelasticity on such fiber/composite properties as modulus, creep, creep recovery, stress relaxation, and damping capacity. For an elastic-core/anelastic-sheath model, boron fibers on tungsten substrates are shown to have predictable fracture stresses for time-temperature conditions ranging from impact to long-time stress rupture. Possible techniques for altering these stresses are discussed.

Dicarlo, J. A.↗
NASA NTRSDate not supplied

Lessons Learned from the Arecibo Observatory Auxiliary M4N Socket Analysis and Implications for Future Observatory Designs

The Arecibo Observatory collapsed after a progressive structural failure stemming from the failure of a single socket joint cable termination. A failure investigation was conducted to identify the cause of the first failed cable termination socket. Due to the nature of the suspended observatory receiver, the dominant source of loading in the suspension cables was due to deadload, with secondary contributions coming from environmental effects and observatory operation. The high ratio of deadload vs. live load applied to the cable over many years resulted in a creep failure mode of the cast zinc within the socket which further induced stress into the highly stress wires at the entrance of the socket. The design of the socket joint did not explicitly consider socket constituent stress margins and time-dependent damage mechanisms such as creep. Lessons learned from this investigation led to recommendations for revising civil engineering cable standards, and recommendations for manufacturers and designers. The design of cables employing socket terminations should consider the cumulative damage effects from creep and cyclic loading identify their corresponding worst-case inspectable defects or establish a sufficiently high factor of safety so that the design is sufficiently robust to defects and time-dependent failure modes. The design should specify end-of-life capability and set service life inspection intervals with pass/fail inspection criteria. Finally, trial studies on design iterations were conducted to increase the reliability of cables with socket joints.

Pavel Babuska↗
NASA NTRSDate not supplied

Understanding the Durability of SiC Based Ceramic Matrix Composites (CMCs) for Gas Turbine Engine Hot Section Components

Silicon carbide fiber reinforced silicon carbide ceramic matrix composites (SiCf/SiC-CMCs) are being used in the fabrication of gas turbine engine hot section components due to their light weight and excellent thermal and chemical stabilities at high-temperature. These superior properties enable significant enhancements in engine efficiency and reduced fuel burn, emissions, and cooling requirements. In support of NASA’s Aeronautics Mission under various programs at Glenn Research Center, different types of composites have been developed and assessed for a high temperature (2700 °F) SiCf/SiC CMC system for turbine engine applications. These composites have creep-resistant SiC fibers, advanced 3D weaves, 2700 °F-capable hybrid SiC matrices, and durable environmental barrier coatings (EBCs). These efforts have resulted in improvements in the overall CMC thermomechanical and environmental durability. In order to study the role of different constituents and processing variables, a single fiber tow CVI (chemical vapor infiltration) SiC/SiC mini-composite can be considered the basic architectural feature of woven and laminate SiC/SiC CMCs. Moreover, the mini-composite mechanical and tensile creep damage behavior represents the creep behavior of 0° fiber tows in the axial loading direction of a macro-composite or component. In addition, a large number of mini-composite samples can be fabricated at relatively low cost within a short time, which makes them attractive for obtaining a robust set of experimental data and studying constituent behavior. In this presentation, the mini-composite approach to study damage mechanisms that limit CMC life in extreme environments including exposure to steam will be presented. The effects of constituent type and volume fraction on the CMC durability, which will help influence CMC design, will be discussed.

Ceramic Matrix Composites (CMCs)↗
NASA NTRS1993

Probabilistic material degradation under high temperature, fatigue, and creep

A methodology has been developed and embodied in two computer codes for quantitatively characterizing the material strength degradation of aerospace propulsion system structural components that are subjected to various random effects over the course of their service lives. The codes, PROMISS and PROMISC, constitute a material-resistance model that is used in the NESSUS aerospace structural-reliability program. NESSUS addresses the service life-reducing effects of high temperature, mechanical fatigue, and creep.

Boyce, L.↗
NASA NTRS1982

Influence of cobalt, tantalum, and tungsten on the microstructure and mechanical properties of superalloy single crystals

The influence of Co, Ta, and W on the microstructure and mechanical properties of nickel base super-alloy single crystals was investigated. A matrix of alloys was based on Mar-M 247 stripped of C, B, Zr, and Hf. The microstructures of the alloys were examined using optical and electron microscopy, phase extraction, X-ray diffraction, and differential thermal analysis. Tensile and creep-rupture tests were performed at 1000 C. An increase in tensile and creep strength resulted when Co was removed from alloys containing high refractory metal contents, but Co effects were negligible for alloys with lower refractory metal levels. In the composition range studied, W was more effective than Ta in increasing the creep resistance. The mechanical properties are discussed in relation to the microstructures of the alloys.

Nathal, M. V.↗
NASA NTRS2016

Separating the Influence of Environment from Stress Relaxation Effects on Dwell Fatigue Crack Growth in a Nickel-Base Disk Alloy

Both environmental embrittlement and crack tip visco-plastic stress relaxation play a significant role in determining the dwell fatigue crack growth (DFCG) resistance of nickel-based disk superalloys. In the current study performed on the Low Solvus High Refractory (LSHR) disk alloy, the influence of these two mechanisms were separated so that the effects of each could be quantified and modeled. Seven different microstructural variations of LSHR were produced by controlling the cooling rate and the subsequent aging and thermal exposure heat treatments. Through cyclic fatigue crack growth testing performed both in air and vacuum, it was established that four out of the seven LSHR heat treatments evaluated, possessed similar intrinsic environmental resistance to cyclic crack growth. For these four heat treatments, it was further shown that the large differences in dwell crack growth behavior which still persisted, were related to their measured stress relaxation behavior. The apparent differences in their dwell crack growth resistance were attributed to the inability of the standard linear elastic fracture mechanics (LEFM) stress intensity parameter to account for visco-plastic behavior. Crack tip stress relaxation controls the magnitude of the remaining local tensile stresses which are directly related to the measured dwell crack growth rates. It was hypothesized that the environmentally weakened grain boundary crack tip regions fail during the dwells when their strength is exceeded by the remaining local crack tip tensile stresses. It was shown that the classical creep crack growth mechanisms such as grain boundary sliding did not contribute to crack growth, but the local visco-plastic behavior still plays a very significant role by determining the crack tip tensile stress field which controls the dwell crack growth behavior. To account for the influence of the visco-plastic behavior on the crack tip stress field, an empirical modification to the LEFM stress intensity parameter, Kmax, was developed by incorporating into the formulation the remaining stress level concept as measured by simple stress relaxation tests. The newly proposed parameter, Ksrf, did an excellent job in correlating the dwell crack growth rates for the four heat treatments which were shown to have similar intrinsic environmental cyclic fatigue crack growth resistance.

Dwell crack growth↗
NASA NTRS2016

Separating the Influence of Environment from Stress Relaxation Effects on Dwell Fatigue Crack Growth

Seven different microstructural variations of LSHR were produced by controlling the cooling rate and the subsequent aging and thermal exposure heat treatments. Through cyclic fatigue crack growth testing performed both in air and vacuum, it was established that four out of the seven LSHR heat treatments evaluated, possessed similar intrinsic environmental resistance to cyclic crack growth. For these four heat treatments, it was further shown that the large differences in dwell crack growth behavior which still persisted, were related to their measured stress relaxation behavior. The apparent differences in their dwell crack growth resistance were attributed to the inability of the standard linear elastic fracture mechanics (LEFM) stress intensity parameter to account for visco-plastic behavior. Crack tip stress relaxation controls the magnitude of the remaining local tensile stresses which are directly related to the measured dwell crack growth rates. It was hypothesized that the environmentally weakened grain boundary crack tip regions fail during the dwells when their strength is exceeded by the remaining local crack tip tensile stresses. It was shown that the classical creep crack growth mechanisms such as grain boundary sliding did not contribute to crack growth, but the local visco-plastic behavior still plays a very significant role by determining the crack tip tensile stress field which controls the dwell crack growth behavior. To account for the influence of the visco-plastic behavior on the crack tip stress field, an empirical modification to the LEFM stress intensity parameter, Kmax, was developed by incorporating into the formulation the remaining stress level concept as measured by simple stress relaxation tests. The newly proposed parameter, Ksrf, did an excellent job in correlating the dwell crack growth rates for the four heat treatments which were shown to have similar intrinsic environmental cyclic fatigue crack growth resistance.

environmental damage↗
NASA NTRS1975

Hydrogen-environment embrittlement of metals and its control

Types of hydrogen embrittlement are discussed together with characteristics of hydrogen-environment embrittlement, the degree of hydrogen-environment embrittlement of a wide variety of alloys, the effect of hydrogen environments on various properties, (tension, fatigue, creep and fracture mechanics), and the influence of hydrogen exposure parameters on the degree of embrittlement. Design methods for high-pressure hydrogen service and for prevention of hydrogen-environment embrittlement are also covered.

Chandler, W. T.↗
NASA NTRS1982

Life prediction and constitutive models for anisotropic materials

The intent of this program is to develop a basic understanding of cyclic creep-fatigue deformation mechanisms and damage accumulation, a capability for reliable life prediction, and the ability to model the constitutive behavior of anisotropic single crystal (SC) and directionally solidified or recrystallized (DSR) comprise the program, and the work breakdown for each option reflects a distinct concern for two classes of anisotropic materials, SC and DSR materials, at temperatures encountered in the primary gas path (airfoil temperatures), and at temperatures typical of the blade root attachment and shank area. Work directed toward the higher temperature area of concern in the primary gas path includes effects of coatings on the behavior and properties of the materials of interest. The blade root attachment work areas will address the effects of stress concentrations associated with attachment features.

Bill, R. C.↗
NASA NTRS1999

Experimental and Analytical Studies for a Computational Materials Program

The studies supported by Grant NAG1-1780 were directed at providing physical data on polymer behavior that would form the basis for computationally modeling these types of materials. Because of ongoing work in polymer characterization this grant supported part of a larger picture in this regard. Efforts went into two combined areas of their time dependent mechanical response characteristics: Creep properties on the one hand, subject to different volumetric changes (nonlinearly viscoelastic behavior) and time or frequency dependence of dilatational material behavior. The details of these endeavors are outlined sufficiently in the two appended publications, so that no further description of the effort is necessary.

Knauss, W. G.↗
NASA NTRS1998

Strengthening Precipitate Morphologies Fully Quantified in Advanced Disk Superalloys

Advanced aviation gas turbine engines will require disk superalloys that can operate at higher temperatures and stresses than current conditions. Such applications will be limited by the tensile, creep, and fatigue mechanical properties of these alloys. These mechanical properties vary with the size, shape, and quantity of the gamma precipitates that strengthen disk superalloys. It is therefore important to quantify these precipitate parameters and relate them to mechanical properties to improve disk superalloys. Favorable precipitate morphologies and practical processing approaches to achieve them can then be determined. A methodology has been developed at the NASA Lewis Research Center to allow the comprehensive quantification of the size, shape, and quantity of all types of gamma precipitates.

Gabb, Timothy P.↗
NASA NTRS1999

"Ultra"-Fast Fracture Strength of Advanced Structural Ceramic Materials Studied at Elevated Temperatures

The accurate determination of inert strength is important in reliable life prediction of structural ceramic components. At ambient temperature, the inert strength of a brittle material is typically regarded as free of the effects of slow crack growth due to stress corrosion. Therefore, the inert strength can be determined either by eliminating active species, especially moisture, with an appropriate inert medium, or by using a very high test rate. However, at elevated temperatures, the concept or definition of the inert strength of brittle ceramic materials is not clear, since temperature itself is a degrading environment, resulting in strength degradation through slow crack growth and/or creep. Since the mechanism to control strength is rate-dependent viscous flow, the only conceivable way to determine the inert strength at elevated temperatures is to utilize a very fast test rate that either minimizes the time for or eliminates slow crack growth. Few experimental studies have measured the elevated-temperature, inert (or "ultra"-fast fracture) strength of advanced ceramics. At the NASA Lewis Research Center, an experimental study was initiated to better understand the "ultra"-fast fracture strength behavior of advanced ceramics at elevated temperatures. Fourteen advanced ceramics - one alumina, eleven silicon nitrides, and two silicon carbides - have been tested using constant stress-rate (dynamic fatigue) testing in flexure with a series of stress rates including the "ultra"-fast stress rate of 33 000 MPa/sec with digitally controlled test frames. The results for these 14 advanced ceramics indicate that, notwithstanding possible changes in flaw populations as well as flaw configurations because of elevated temperatures, the strength at 33 000 MPa/sec approached the room-temperature strength or reached a higher value than that determined at the conventional test rate of 30 MPa/sec. On the basis of the experimental data, it can be stated that the elevated-temperature, inert strength of an advanced ceramic material can be defined as the strength where no slow crack growth takes place at the temperature.

Choi, Sung R.↗
NASA NTRS2018

Location-Specific Microstructure and the Effect of Heat Treatment on Electron-Beam Melted Ni-Based Superalloy LSHR

Examine set-to-set variability with 4 build sets with the same layout Establish the effect of solutioning heat treatments (HT) on as-fabricated grain structure and microstructure – (gamma)’-solvus= 1156 ºC (1) As-fabricated (2) 2 hour subsolvus HT(-15 ºC) + conventional 2-step aging (3, 4, 5) 0.5 hour, 2 hours, 4 hours supersolvus HT (+18 ºC) + 2 step aging Gain insight into location-specific microstructure to correlate later to mechanical properties (tensile, creep, LCF behavior)

heat treatment↗
NASA NTRS2008

Tensile Creep and Fatigue of Sylramic-iBN Melt-Infiltrated SiC Matrix Composites: Retained Properties, Damage Development, and Failure Mechanisms

An understanding of the elevated temperature tensile creep, fatigue, rupture, and retained properties of ceramic matrix composites (CMC) envisioned for use in gas turbine engine applications are essential for component design and life-prediction. In order to quantify the effect of stress, time, temperature, and oxidation for a state-of-the-art composite system, a wide variety of tensile creep, dwell fatigue, and cyclic fatigue experiments were performed in air at 1204 C for the SiC/SiC CMC system consisting of Sylramic-iBN SiC fibers, BN fiber interphase coating, and slurry-cast melt-infiltrated (MI) SiC-based matrix. Tests were either taken to failure or interrupted. Interrupted tests were then mechanically tested at room temperature to determine the residual properties. The retained properties of most of the composites subjected to tensile creep or fatigue were usually within 20% of the as-produced strength and 10% of the as-produced elastic modulus. It was observed that during creep, residual stresses in the composite are altered to some extent which results in an increased compressive stress in the matrix upon cooling and a subsequent increased stress required to form matrix cracks. Microscopy of polished sections and the fracture surfaces of specimens which failed during stressed-oxidation or after the room-temperature retained property test was performed on some of the specimens in order to quantify the nature and extent of damage accumulation that occurred during the test. It was discovered that the distribution of stress-dependent matrix cracking at 1204 C was similar to the as-produced composites at room temperature; however, matrix crack growth occurred over time and typically did not appear to propagate through thickness except at final failure crack. Failure of the composites was due to either oxidation-induced unbridged crack growth, which dominated the higher stress regime (> 179 MPa) or controlled by degradation of the fibers, probably caused by intrinsic creep-induced flaw growth of the fibers or internal attack of the fibers via Si diffusion through the CVI SiC and/or microcracks at the lower stress regime (< 165 MPa).

Morscher, Greg↗
NASA NTRS1972

Influence of creep damage on the low cycle thermal-mechanical fatigue behavior of two tantalum base alloys

Low cycle fatigue tests have been performed on the tantalum base alloys T-111 and ASTAR 811C with synchronized, independently programmed temperature and strain cycling. The thermal-mechanical cycles applied fell into three basic categories: these were isothermal cycling, in-phase thermal cycling, and out-of-phase thermal cycling. In-phase cycling was defined as tensile deformation associated with high temperature and compressive deformation with low temperature, while out-of-phase thermal cycling was defined as the reverse case. The in-phase thermal cycling had a pronounced detrimental influence on the fatigue life of both alloys, with the life reduction being greater in the solid solution strengthened T-111 alloy than in the carbide strengthened ASTAR 811C alloy. The out-of-phase tests also showed pronounced effects on the fatigue life of both alloys, although not as dramatic.

Sheffler, K. D.↗
NASA NTRS2023

Superior Tensile Creep Behavior of A Novel Oxide Dispersion Strengthened Crconi Multi-Principal Element Alloy

The creep behavior of an additively manufactured (AM) yttrium oxide dispersion strengthened (ODS) CrCoNi multi-principal element alloy (MPEA) is investigated in this study. Tests were conducted over a constant tensile stress range of 40–200 MPa and a temperature range of 973–1173 K. A stress exponent of 6.5 ± 0.1 and activation energies in the range of 335–367 kJ/mol were measured. Compared to its non-ODS counterpart and its parent alloy (CrMnFeCoNi), AM ODS CrCoNi has superior creep resistance at all tested stresses and temperatures. The mechanism contributing to the excellent creep resistance of AM ODS CrCoNi is attributed to the interaction of mixed character dislocations with oxide particles. The creep ductility of AM ODS CrCoNi is higher than its nonODS counterpart due to a large percentage of low angle grain boundaries associated with its columnar grain structure. It was also found that creep ductility is significantly greater at the highest applied stress of 200 MPa compared to lower stresses. The steady state dislocation structure of AM ODS CrCoNi consists of long arrays of dislocations, which is different from that observed in non-ODS CrCoNi which consists of individual curved dislocations.

Seyed Gianmarco Sahragard-Monfared↗
NASA NTRS1977

Creep and tensile properties of several oxide-dispersion-strengthened nickel-base alloys at 1365 K

The tensile properties at room temperature and at 1365 K and the tensile creep properties at low strain rates at 1365 K were measured for several oxide-dispersion-strengthened (ODS) alloys. The alloys examined included ODS Ni, ODS Ni-20Cr, and ODS Ni-16Cr-Al. Metallography of creep tested, large grain size ODS alloys indicated that creep of these alloys is an inhomogeneous process. All alloys appear to possess a threshold stress for creep. This threshold stress is believed to be associated with diffusional creep in the large grain size ODS alloys and normal dislocation motion in perfect single crystal (without transverse low angle boundaries) ODS alloys. Threshold stresses for large grain size ODS Ni-20Cr and Ni-16Cr-Al type alloys are dependent on the grain aspect ratio. Because of the deleterious effect of prior creep on room temperature mechanical properties of large grain size ODS alloys, it is speculated that the threshold stress may be the design limiting creep strength property.

Wittenberger, J. D.↗