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At least 199 records · Page 11

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NASA NTRS1986

Adhesive evaluation of LARC-TPI and a water-soluble version of LARC-TPI

The results of a study to evaluate two Langley Research Center thermoplastic polimide (TPI) materials, identified as TPI/MTC for the material from Mitsui Toatsu Chemicals Inc. and TPI/H2O for the material from United Technologies Research Center, as high temperature thermoplastic adhesives and primers for bonding titanium (6AL-4V) adherends are discussed. A limited characterization of the materials was performed using a Diffuse Reflectance-Fourier Transform Infrared Spectroscopy (DR-FTIR) technique. Thermomechanical Analysis (TMA) and torsional braid techniques were used to determine glass transition temperature. The adhesive's strength, as determined by simple lap shear tests, was used to evaluate the effects of long term thermal exposure (up to 1000 hrs) at 204 deg C and a 72-hour water-boil.

Progar, D. J.↗

NASA NTRS1988

The influence of grain size and composition on 1000 to 1400 K slow plastic flow properties of NiAl

The compressive slow plastic flow behavior of several B2 crystal structure NiAl intermetallics has been studied in air between 1000 and 1400 K. Small grain-sized Ni-48.25 at. pct Al (of about 10 microns) was found to be stronger than the previously studied 17 microns diameter material. While grain refinement improved the strength at all test temperatures, the exact mechanism is not clear. Experiments at lower temperature revealed that composition as well as grain size can be an important factor, since Ni-49.2Al was weaker than Ni-48.25Al. Pronounced yield points were found during slow strain-rate testing at 1000 K; however, continued deformation appears to take place by the same mechanism(s) as found at high temperatures. Small changes in thermomechanical processing (TMP) schedules to fabricate Ni-49.2Al indicated that basic deformation characteristics (stress exponent and activation energy) are not affected; however, the preexponential term could be modified if TMP alters the grain structure.

Whittenberger, J. Daniel↗

NASA NTRS1994

Effects of thermomechanical processing on tensile and long-time creep behavior of Nb-1 percent Zr-0.1 percent C sheet

Effects of thermomechanical processing on the mechanical properties of Nb-1 wt. percent Zr-0.1 wt. percent C, a candidate alloy for use in advanced space power systems, were investigated. Sheet bars were cold rolled into 1-mm thick sheets following single, double, or triple extrusion operations at 1900 K. All the creep and tensile specimens were given a two-step heat treatment 1 hr at 1755 K + 2 hr 1475 K prior to testing. Tensile properties were determined at 300 as well as at 1350 K. Microhardness measurements were made on cold rolled, heat treated, and crept samples. Creep tests were carried out at 1350 K and 34.5 MPa for times of about 10,000 to 19,000 hr. The results show that the number of extrusions had some effects on both the microhardness and tensile properties. However, the long-time creep behavior of the samples were comparable, and all were found to have adequate properties to meet the design requirements of advanced power systems regardless of thermomechanical history. The results are discussed in correlation with processing and microstructure, and further compared to the results obtained from the testing of Nb-1 wt. percent Zr and Nb-1 wt. percent Zr-0.06 wt. percent C alloys.

Titran, Robert H.↗

NASA NTRS1978

High temperature adhesives for bonding Kapton

Experimental polyimide resins have been developed and evaluated as potential high temperature adhesives for bonding Kapton polyimide film. Lap shear strengths of 'Kapton'/'Kapton' bonds were obtained as a function of test temperature, adherend thickness, and long term aging at 575K (575 F) in vacuum. Glass transition temperatures of the polyimide/'Kapton' bondlines were monitored by thermomechanical analysis.

Saint Clair, A. K.↗

NASA NTRS1985

Creep fatigue life prediction for engine hot section materials (isotropic): Third year progress review

This program is designed to investigate fundamental damage processes, identify modeling strategies, and develop practical models which can be used to guide the early design and development of new engines and to increase the durability of existing engines. A review is given of the base program, completed in 1984, which included the comparison and evaluation of several popular high-temperature life prediction approaches as applied to continuously cycled isothermal specimen tests. The option program, of which one year is completed, is designed to develop models which can account for complex cycles and loadings, such as thermomechanical cycling, cumulative damage, multiaxial stress/strain rates, and environmental effects.

Nelson, Richard S.↗

NASA NTRS1988

Ceramic matrix composites in simulated SSME environments

Future Space Shuttle Main Engine (SSME)-type rocket engines can benefit from the use of fiber-reinforced ceramix matrix composites (FRCMC). Ceramics reinforced with long continuous fibers exhibit improved tolerance to severe thermomechanical and environmental exposures. An in-house NASA-Lewis program to evaluate the durability of FRCMC in simulated SSME environments is described. Primary tests involve multiple (one second) exposures of FRCMC specimens in a hydrogen/oxygen rocket test rig. This rig generates surface heating rates of 1000 to 2500 C/second. The FRCMC durability evaluation involves measurement of retained strength as a function of thermal shock severity and number of upshock cycles. Preliminary test results for monolithic silicon nitride (Si3N4) and silicon carbide (SiC), and one type of silicon based FRCMC, are presented. The test data are examined in terms of simple thermal shock theory.

Herbell, Thomas P.↗

NASA NTRS1994

Application of an Energy-Based Life Prediction Model to Bithermal and Thermomechanical Fatigue

The inelastic hysteresis energy applied to the material in a cycle is used as the basis for predicting nonisothermal fatigue life of a wrought cobalt-base superalloy, Haynes 188, from isothermal fatigue data. Damage functions that account for hold-time effects and time-dependent environmental phenomena such as oxidation and hot corrosion are proposed in terms of the inelastic hysteresis energy per cycle. The proposed damage functions are used to predict the bithermal and thermomechanical fatigue lives of Haynes 188 between 316 and 760 C from isothermal fatigue data. Predicted fatigue lives of all but two of the nonisothermal tests are within a factor of 1.5 of the experimentally observed lives.

Radhakrishnan, V. M.↗

NASA NTRS1980

High-temperature adhesives for bonding polyimide film

Experimental polyimide resins were developed and evaluated as potential high temperature adhesives for bonding Kapton polyimide film. Lap shear strengths of Kapton/Kapton bonds were obtained as a function of test temperature, adherend thickness, and long term aging at 575 K (575 F) in vacuum. Glass transition temperatures of the polyimide/"Kapton" bondlines were monitored by thermomechanical analysis.

St.clair, A. K.↗

NASA NTRS1993

Compliant seal development

The compliant metallic seal combines the noncontact feature of the labyrinth seal, the low leakage of a mechanical seal, and the compliant nature of the brush seal. It consists of several thin metallic elements or leaves mounted within a ring which is press fit into the housing, and in form, sort of resembles a lip seal sections wiping the shaft. A second set of overlapping cover leaves are placed on top of the shaft riding leaves which reduces leakage and provides stiffness. The leaves can be straight or angle cut. The shaft riding fingers are designed with mismatched curvature to provide lift off similar to the Rayleigh lift pads in mechanical seals with leading edge clearances nearly twice those of the trailing edge as as shown by Fleming to be optimal for gas flows in convergent seal passages. Leading edge clearances range from 300 to 500 microinches. Balance pockets beneath the leaves provide fluid film feed to the 'Rayleigh lift' surface and the proper balance ratio (mechanical seal) when combined with the static pressure and film pressure. The leaves flex in the radial direction and accommodate thermomechanical behavior as well as axial motion and angular misalignment. In the static mode, there is a net closing force on the leaves. The seals were tested to 70 psi at speeds to 16,000 rpm or surface speeds to 330 fps and temperatures from ambient to 440 F. A slow cycle through the rig critical at 10,000 rpm induced a radial vibration response of 0.004 to 0.005 inch were accommodated by the seal. Preliminary performance data are encouraging demonstrating hydrodynamic liftoff and noncontacting operation at pressure and speeds typical of gas turbine engines. The leakage performance data are significantly better than commercial labyrinth and brush seals which should be expected as this design incorporates the features of the low leakage face or mechanical seal along with the flexibility of the brush configuration.

Hendricks, Robert C.↗

NASA NTRS2001

Thermomechanical Response of Shape Memory Alloy Hybrid Composites

This study examines the use of embedded shape memory alloy (SMA) actuators for adaptive control of the thermomechanical response of composite structures. A nonlinear thermomechanical model is presented for analyzing shape memory alloy hybrid composite (SMAHC) structures exposed to steady-state thermal and dynamic mechanical loads. Also presented are (1) fabrication procedures for SMAHC specimens, (2) characterization of the constituent materials for model quantification, (3) development of the test apparatus for conducting static and dynamic experiments on specimens with and without SMA, (4) discussion of the experimental results, and (5) validation of the analytical and numerical tools developed in the study. Excellent agreement is achieved between the predicted and measured SAMHC responses including thermal buckling, thermal post-buckling and dynamic response due to inertial loading. The validated model and thermomechanical analysis tools are used to demonstrate a variety of static and dynamic response behaviors including control of static (thermal buckling and post-buckling) and dynamic responses (vibration, sonic fatigue, and acoustic transmission). and SMAHC design considerations for these applications. SMAHCs are shown to have significant advantages over conventional response abatement approaches for vibration, sonic fatigue, and noise control.

Turner, Travis L.↗

NASA NTRS1990

Evaluation of thermal and mechanical loading effects on the structural behavior of a SiC/titanium composite

Composite specimens of titanium-15-3 matrix reinforced with continuous SCS-6 silicon carbide fibers were tested under a variety of thermal and mechanical loadings. A combined experimental/finite element approach was used to estimate the effective in-situ modulus of the matrix material and to evaluate changes in modulus due to the applied loads. Several fiber orientations were tested. Results indicate that the effect of the thermal loads on composite stiffness varies with fiber orientation. Applications of this method to test specimens damaged by uniaxial tension, thermal cycling, and isothermal fatigue loadings are used to illustrate that by monitoring overall structural behavior, changes in stiffness caused by thermomechanical loading can be detected.

Grady, Joseph E.↗

NASA NTRS1990

Evaluation of thermal and mechanical loading effects on the structural behavior of a SIC/titanium composite

Composite specimens of titanium-15-3 matrix reinforced with continuous SCS-6 silicon carbide fibers were tested under a variety of thermal and mechanical loadings. A combined experimental/finite element approach was used to estimate the effective in situ modulus of the matrix material and to evaluate changes in modulus due to the applied loads. Several fiber orientations were tested. Results indicate that the effect of thermal loads on composite stiffness varies with fiber orientation. Applications of this method to test specimens damaged by uniaxial tension, thermal cycling, and isothermal fatigue loadings are used to illustrate that by monitoring overall structural behavior, changes in stiffness caused by thermomechanical loading can be detected.

Grady, J. E.↗

NASA NTRS1990

Liquid acquisition devices for superfluid helium transfer

To transfer superfluid helium (He II) in the milli-g or micro-g environment in orbit, it is necessary to provide a reasonably steady supply of liquid to the inlet of the pump in the supply dewar. To accomplish this without providing an artificial gravity through acceleration requires a liquid acquisition device. Fluid swirl and electrostatic devices have been proposed to orientate the fluid. However, the simplest mechanisms appear to be the use of surface tension or the thermomechanical effect. This paper examines four concepts for providing He II to the inlet of a thermomechanical pump. The devices are a distributed thermomechanical pump, a distributed pump with a main thermomechanical pump, a screened channel system and a vane/sponge combination. Calculations on the efficiency of these types of liquid acquisition devices are made using laboratory data from tests involving small scale devices where applicable. These calculations show that the latter two types of liquid acquisition devices are the most efficient. Questions as to the probability of cavitation and the effect of the residual shuttle acceleration on their operation remain to be answered, however.

Dipirro, M. J.↗

NASA NTRS1987

Total strain version of strainrange partitioning for thermomechanical fatigue at low strains

A new method is proposed for characterizing and predicting the thermal fatigue behavior of materials. The method is based on three innovations in characterizing high temperature material behavior: (1) the bithermal concept of fatigue testing; (2) advanced, nonlinear, cyclic constitutive models; and (3) the total strain version of traditional strainrange partitioning.

Halford, G. R.↗

NASA NTRS2023

PMT F7 Silver Standard Material Properties Characterization

PMT F7 epoxy resin was characterized to capture the material's thermochemical and thermo-mechanical properties. DSC testing was performed and the data was used to develop a cure kinetics model which predicts the material cure response and specific heat capacity. TMA and DMA testing were performed and the data was used to develop thermo-mechanical models which predict the material CTE, cure shrinkage, and modulus evolution with cure. Convergent Manufacturing Technologies US determined test parameters, analyzed data, and performed model fitting. NASA completed tests and provided data to Convergent Manufacturing Technologies US.

Epoxy↗

NASA NTRS1996

Stress Determination of Micromembranes Using Laser Vibrometry

A laser vibrometry based dynamic technique for biaxial residual stress determination in micromembranes is reported. Layered, low pressure chemical vapor deposition grown, rectangular shaped micromembranes are ultrasonically excited. The resulting micromembrane displacement, as a function of frequency and position, is recorded to yield resonance frequencies and associated vibrational mode numbers. The stress is determined from this information. Vibrations forced from ultrasonic pressure (-25 Pa) resulted in peak displacements of tens of nanometers at resonance. For the sample set studied, the (3,1) mode resonance peak displayed the least damping in atmospheric testing and was used to establish stress levels approximately 10(exp 8)N/m2 . A trend for this stress showing an overall decrease by approximately 40% as a result of thermomechanical cycling was identified.

Biswas, Abhijit↗

NASA NTRS1989

Tensile and fatigue behavior of tungsten/copper composites

Work on W/Cu unidirectional composites was initiated to study the behavior of this ductile-ductile composite system under thermomechanical fatigue and to examine the applicability of fatigue-life prediction methods for thermomechanical fatigue of this metal matrix composite. The first step was to characterize the tensile behavior of four ply, 10 vol. percent W/Cu plates at room and elevated temperatures. Fatigue tests were conducted in load control on 0 degree specimens at 260 C. The maximum cyclic stress was varied but the minimum cyclic stress was kept constant. All tests were performed in vacuum. The strain at failure increased with increasing maximum cyclic stress.

Verrilli, Michael J.↗

NASA NTRS2005

Materials Chemistry Issues in the Development of a Single-Crystal Solar/Thermal Refractive Secondary Concentrator

A translucent crystal concentrates and transmits energy to a heat exchanger, which in turn heats a propellant gas, working gas of a dynamic power system, or a thermopile. Materials are the limiting issue in such a system. Central is the durability of the crystal, which must maintain the required chemical, physical/optical, and mechanical properties as it is heated and cooled. This report summarizes available data to date on the materials issues with this system. We focus on the current leading candidate materials, which are sapphire (Al2O3) for higher temperatures and silica (SiO2) for lower temperatures. We use data from thermochemical calculations; laboratory coupon tests with silica and sapphire; and system tests with sapphire. The required chemical properties include low-vapor pressure and interfacial stability with supporting structural materials. Optical properties such as transmittance and index of refraction must be maintained. Thermomechanical stability is a major challenge for a large, single-crystal ceramic and has been discussed in another report. In addition to the crystal, other materials in the proposed system include refractory metals (Nb, Ta, Mo, W, and Re), carbon (C), and high-temperature ceramic insulation. The major issue here is low levels of oxygen, which lead to volatile refractory metal oxides and rapid consumption of the refractory metal. Interfacial reactions between the ceramic crystal and refractory metal are also discussed. Finally, high-temperature ceramic insulating materials are also likely to be used in this system. Outgassing is a major issue for these materials. The products of outgassing are typically reactive with the refractory metals and must be minimized.

Jacobson, Nathan S.↗