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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 343 records · Page 19

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NASA NTRS1984

Predicting electrochemical breakdown in terrestrial photovoltaic modules

Electrochemical corrosion of PV cells can cause a cell-to-frame short circuit with two undesirable resuits: (1) catastrophic loss of source-circuit power; and (2) high field maintenance (module replacement) costs. Parameter dependencies are determined and encapsulated two-cell coupons are exposed to a range of voltage differences, temperatures, and relative humidities. Material relative performance is assessed. Nine different metallization-encapsulation combinations are tested.

Mon, G. R.↗

NASA NTRS1985

Fatigue criterion to system design, life and reliability

A generalized methodology to structural life prediction, design, and reliability based upon a fatigue criterion is advanced. The life prediction methodology is based in part on work of Weibull and Lundberg and Palmgren. The approach incorporates the computed life of elemental stress volumes of a complex machine element to predict system life. The results of coupon fatigue testing can be incorporated into the analysis allowing for life prediction and component or structural renewal rates with reasonable statistical certainty.

Zaretsky, E. V.↗

NASA NTRS1986

Fatigue Criterion for System Design

Report discusses principles of structural-life prediction. Generalized methodology developed for structural life prediction, design, and reliability, based upon fatigue criterion. Approach incorporates computed life of elemental stress volumes of complex machine elements to predict system life. Results of coupon fatigue testing incorporated into analysis, allowing for life prediction and component or structural renewal rates, with reasonable statistical certainty.

Zeretsky, E. V.↗

NASA NTRS1986

Flight service evaluation of composite components on the Bell helicopter model 206L

This is the second annual report on the flight service components for the Bell Model 206L JetRanger helicopter. The components have been placed in service in the Continental United States, Canada, and Alaska. The report covers the period from 1 August 1983 to 1 January 1986. The status of 34 sets of components is discussed in this report. Approximately 73,000 flight hours were accumulated on the components as of 1 January 1986. Three years of coupon tests on the components are presented with test results. A vertical fin had been struck by lightning and the results of its examination and tests are included.

Wilson, H. E., II↗

NASA NTRS1985

A normal stress criterion for crack extension direction in orthotropic composite materials

A criterion for predicting the direction of crack extension in orthotropic composite materials is presented. The criterion is based upon the normal stress and the anisotropic tensile strength on arbitrary planes about the tip of a crack. Results are obtained, via finite element solutions, for: (1) isotropic mixed mode fracture, (2) cracks in unidirectional off-axis slotted composite tensile coupons and (3) cracks in cross plied laminates. Comparisons are made with other theories and experimental results.

Buczek, M. B.↗

NASA NTRS1985

Orthotropic fracture using a singular isoparametric element

The six noded quarter point natural isoparametric triangular element is employed to obtain displacement and stress distributions in the vicinity of the crack tip in a center cracked tensile coupon of unidirectional graphite epoxy. The material is considered to be homogeneous, elastic and orthotropic. The finite element results are compared to the analytical solution of anisotropic elasticity. Convergence as a function of mesh parameters is studied for isotropic and orthotropic materials. It is shown that displacements converge faster than stresses and that meshes which are convergent for isotropic and orthotropic materials. It is shown that displacements converge faster than stresses and that meshes which are convergent for isotropic materials are not completely convergent for the highly orthotropic graphite epoxy.

Gregory, M. A.↗

NASA NTRS1986

Shear characterization of unidirectional composites with the off-axis tension test

The influence of end constraints on accurate determination of the intralaminar shear modulus G(12) from an off-axis tension test is examined both analytically and experimentally. The Pagano-Halpin (1968) model is employed to illustrate that, when the effect of end constraints is properly considered, the exact expression for G(12) is obtained. When the effect of end constraints is neglected, expressions for the apparent shear modulus G(12)-asterisk and apparent Young's modulus Exx-asterisk are obtained. Numerical comparison for various off-axis configurations and aspect ratios is carried out using typical material properties for graphite/polyimide unidirectional composites. It is demonstrated that the end-constraint effect influences accurate determination of G(12) more adversely than it affects the laminate Young's modulus E(xx) in the low off-axis range. Experimental results obtained from off-axis tests on unidirectional graphite/polyimide specimens confirm the above. Based on the presented analytical and experimental evidence, the 45-deg off-axis coupon is proposed for the determination of the intralaminar shear modulus G(12).

Pindera, M.-J.↗

NASA NTRS1985

Crack growth in unidirectional graphite-epoxy under biaxial loading

An anisotropic elasticity crack tip stress analysis is implemented using three crack extension direction criteria (the normal stress ratio, the tensor polynominal and the strain energy density) to predict the direction of crack extension in unidirectional of axis graphite-epoxy. The theoretical predictions of crack extension direction are then compared with experimental results for 15 deg off axis tensile coupons with center cracks. Specimens of various aspect ratios and crack orientations are analyzed. It is shown that only the normal stress ratio criterion predicts the correct direction of crack growth.

Beuth, J. L., Jr.↗

NASA NTRS1985

Crack growth direction in unidirectional off-axis graphite-epoxy

An anisotropic elasticity crack tip stress analysis is implemented using three crack extension direction criteria (the normal stress ratio, the tensor polynominal and the strain energy density) to predict the direction of crack extension in unidirectional off axis graphite-epoxy. The theoretical predictions of crack extension direction are then compared with experimental results for 15 deg off axis tensile coupons with center cracks. Specimens of various aspect ratios and crack orientations are analyzed. It is shown that only the normal stress ratio criterion predicts the correct direction of crack growth.

Herakovich, C. T.↗

NASA NTRS1986

Experimental study of a metal-matrix composite

A metal-matrix specimen was investigated using moire interferometry with 2400 lines/m (60,960 l/in.). The specimen was a 6-ply (0/ + or - 45)s boron-aluminum tensile coupon with a central slot. The unequal deformations in fibers and matrix were evident. Special attention was given to a plastic slip zone in the matrix. Normal and shear strains were calculated in the slip zone; the shear-strain concentration factor rose dramatically with the onset of plastic slip and continued to rise gradually with load to 95 percent of the failure load. The precipitous change from maximum tensile strain in one fiber to zero tensile strain in the neighboring fiber was accommodated by the very high shear strains of the matrix in the slip zone. Experimental considerations are discussed: shear strains are independent of rigid-body rotations; simplified strain calculations are justified; averaging across the narrow slip region and the influence of finite thickness of the specimen grating contribute to underestimation of peak strains.

Post, D.↗

NASA NTRS1986

Flight service evaluation of composite helicopter components

This report presents an assessment of composite helicopter tail rotor spars and horizontal stabilizers, exposed to the effects of the environment, after up to five and a half years of commercial service. This evaluation is supported by test results of helicopter components and panels which have been exposed to outdoor environmental effects since September 1979. Full scale static and fatigue tests have been conducted on graphite/epoxy and Kevlar/epoxy composite components obtained from Sikorsky Model S-76 helicopters in commercial operations in the Gulf Coast region of Louisiana. Small scale static and fatigue tests are being conducted on coupons obtained from panels under exposure to outdoor conditions in Stratford, Connecticut and West Palm, Florida. The panel layups are representative of the S-76 components. Additionally, this report discusses the results of moisture absorption evaluations and strength tests on the S-76 components and composite panels with up to five years of outdoor exposure.

Mardoian, G. H.↗

NASA NTRS1987

An analytical and experimental study of crack extension in center-notched composites

The normal stress ratio theory for crack extension in anisotropic materials is studied analytically and experimentally. The theory is applied within a microscopic-level analysis of a single center notch of arbitrary orientation in a unidirectional composite material. The bulk of the analytical work of this study applies an elasticity solution for an infinite plate with a center line to obtain critical stress and crack growth direction predictions. An elasticity solution for an infinite plate with a center elliptical flaw is also used to obtain qualitative predictions of the location of crack initiation on the border of a rounded notch tip. The analytical portion of the study includes the formulation of a new crack growth theory that includes local shear stress. Normal stress ratio theory predictions are obtained for notched unidirectional tensile coupons and unidirectional Iosipescu shear specimens. These predictions are subsequently compared to experimental results.

Beuth, Jack L., Jr.↗

NASA NTRS1987

Development of a strain/temperature gauge and attachment system for use on carbon composites at elevated temperature

The difficulties encountered when instrumenting solid rocket motors for acquiring strain data are reviewed, emphasizing the strong temperature dependence due to apparent strain effects. The development of a strain/temperature gage to overcome some of these problems is discussed. This gage is designed to produce low apparent strain when attached to a carbon-carbon substrate. Characterization and performance data for gages attached with ceramic cement to carbon-carbon tensile coupons are presented, and the effect of a flame-sprayed installation process is discussed.

Lanius, S. J.↗

NASA NTRS1988

Ply-level failure analysis of a graphite/epoxy laminate under bearing-bypass loading

A combined experimental and analytical study was conducted to investigate and predict the failure modes of a graphite/epoxy laminate subjected to combined bearing and bypass loading. Tests were conducted in a test machine that allowed the bearing-bypass load ratio to be controlled while a single-fastener coupon was loaded to failure in either tension or compression. Onset and ultimate failure modes and strengths were determined for each test case. The damage-onset modes were studied in detail by sectioning and micrographing the damaged specimens. A two-dimensional, finite-element analysis was conducted to determine lamina strains around the bolt hole. Damage onset consisted of matrix cracks, delamination, and fiber failures. Stiffness loss appeared to be caused by fiber failures rather than by matrix cracking and delamination. An unusual offset-compression mode was observed for compressive bearing-bypass laoding in which the specimen failed across its width along a line offset from the hole. The computed lamina strains in the fiber direction were used in a combined analytical and experimental approach to predict bearing-bypass diagrams for damage onset from a few simple tests.

Naik, R. A.↗

NASA NTRS1988

Characterization of elastic-plastic properties of AS4/APC-2 thermoplastic composite

Elastic and inelastic properties of AS4/APC-2 composites were characterized with respect to temperature variation by using a one-parameter orthotropic plasticity model and a one parameter failure criterion. Simple uniaxial off-axis tension tests were performed on coupon specimens of unidirectional AS4/APC-2 thermoplastic composite at various temperatures. To avoid the complication caused by the extension-shear coupling effect in off-axis testing, new tabs were designed and used on the test specimens. The experimental results showed that the nonlinear behavior of constitutive relations and the failure strengths can be characterized quite well using the one parameter plasticity model and the failure criterion, respectively.

Sun, C. T.↗

NASA NTRS1987

Fracture surfaces of irradiated composites

Electron microscopy was used to analyze the fracture surfaces of T300/934 graphite/epoxy unidirectional off-axis tensile coupons which were subjected to 1.0-MeV electron radiation at a rate of 50 Mrad/h for a total dose of 10 Grad. Fracture surfaces from irradiated and nonirradiated specimens tested at 116 K, room temperature, and 394 K were analyzed to assess the influence of radiation and temperature on the mode of failure and variations in constituent material as a function of environmental exposure. Micrographs of fracture surfaces indicate that irradiated specimens are more brittle than nonirradiated specimens at low temperatures. However, at elevated temperatures the irradiated specimens exhibit significantly more plasticity than nonirradiated specimens.

Milkovich, Scott M.↗

NASA NTRS1987

Consequences of bacterial resistance to disinfection by iodine in potable water

This study was done to quantify the sensitivity of bacteria to iodine under controlled laboratory conditions. When exposed to 1 mg/1 I2 for 1 min, bacteria isolated from the Shuttle were more resistant than a P. aeruginosa isolated from a povidine-iodine solution. Cultures grown in rich media were more sensitive than those grown in low nutrient solutions. The P. aeruginosa and a P. cepacia isolated from the Shuttle were resuspended in PBW after exposure to iodine. Iodinated cells recovered better than uniodinated controls. Pseudomonads in biofilms developed on coupons of stainless steel were more resistant to iodine than cells suspended in buffered water. Although resistant bacteria may colonize spacecraft water systems, multiple treatment barriers should provide adequate control of these contaminants.

Mcfeters, Gordon A.↗

NASA NTRS1987

Encapsulant free-surfaces and interfaces: Critical parameters in controlling cell corrosion

Results are reported of studying the interaction of moisture with photovoltaic (PV) modules and custom-built test coupons in both laboratory and outdoor ambient environments. Characteristics measured included module cell-to-frame leakage currents, encapsulant bulk and surface currents, glass surface currents, and encapsulant/glass interface currents as functions of temperature and humidity. Using a computer model of interelectrode conduction, preferential conduction paths in PVB- and EVA-encapsulated modules are identified. In comparing the results of field and laboratory test environments on identical samples, comments are made about the greater severity of the outdoor environment. Several aspects of water-module interaction, including design strategies and continuing research, are discussed.

Mon, G.↗