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NASA NTRS1981

Effects of high energy radiation on the mechanical properties of epoxy/graphite fiber composites

Studies on the effects of high energy radiation on graphite fiber reinforced composites are summarized. Studies of T300/5208 and C6000/PMR15 composites, T300 fibers and the resin system MY720/DDS (tetraglycidyl-4,4'-diaminodiphenyl methane cured with diaminodiphenyl sulfone) are included. Radiation dose levels up to 8000 Mrads were obtained with no deleterious effects on the breaking stress or modulus. The effects on the structure and morphology were investigated using mechanical tests, electron spin resonance, X-ray diffraction, and electron spectroscopy for chemical analysis (ESCA or X-ray photoelectron spectroscopy). Details of the experiments and results are given. Studies of the fracture surfaces of irradiated samples were studied with scanning electron microscopy; current results indicate no differences in the morphology of irradiated and control samples.

Fornes, R. E.↗

NASA NTRS1982

Anharmonic properties of solids from measurements of the stress acoustic constant

The equations of elastic motion and their solutions are generalized in order to include nonzero homogeneous initial stresses and redefine the stress acoustic constants to include the effect of initial stress. In deriving the relationship between the stress acoustic constants and the strain-generalized Gruneisen parameters, implications in material anharmonicity and nonlinear thermoelasticity are discovered. It is found that the linear change in velocity obtained from these measurements is a nonlinear process resulting from anharmonicity in the interatomic potential. In addition, the thermal expansion coefficient can also be expressed in terms of linear combinations.

Cantrell, J. H., Jr.↗

NASA NTRS1983

Effect of crack curvature on stress intensity factors for ASTM standard compact tension specimens

The stress intensity factors (SIF) are calculated using the method of lines for the compact tension specimen in tensile and shear loading for curved crack fronts. For the purely elastic case, it was found that as the crack front curvature increases, the SIF value at the center of the specimen decreases while increasing at the surface. For the higher values of crack front curvatures, the maximum value of the SIF occurs at an interior point located adjacent to the surface. A thickness average SIF was computed for parabolically applied shear loading. These results were used to assess the requirements of ASTM standards E399-71 and E399-81 on the shape of crack fronts. The SIF is assumed to reflect the average stress environment near the crack edge.

Alam, J.↗

NASA NTRS1984

The effect of diffusion induced lattice stress on the open-circuit voltage in silicon solar cells

It is demonstrated that diffusion induced stresses in low resistivity silicon solar cells can significantly reduce both the open-circuit voltage and collection efficiency. The degradation mechanism involves stress induced changes in both the minority carrier mobility and the diffusion length. Thermal recovery characteristics indicate that the stresses are relieved at higher temperatures by divacancy flow (silicon self diffusion). The level of residual stress in as-fabricated cells was found to be negligible in the cells tested.

Weizer, V. G.↗

NASA NTRS1984

Mechanical behavior of carbon-carbon composites

A general background, test plan, and some results of preliminary examinations of a carbon-carbon composite material are presented with emphasis on mechanical testing and inspection techniques. Experience with testing and evaluation was gained through tests of a low modulus carbon-carbon material, K-Karb C. The properties examined are the density - 1.55 g/cc; four point flexure strength in the warp - 137 MPa (19,800 psi) and the fill - 95.1 MPa (13,800 psi,) directions; and the warp interlaminar shear strength - 14.5 MPa (2100 psi). Radiographic evaluation revealed thickness variations and the thinner areas of the composite were scrapped. The ultrasonic C-scan showed attenuation variations, but these did not correspond to any of the physical and mechanical properties measured. Based on these initial tests and a survey of the literature, a plan has been devised to examine the effect of stress on the oxidation behavior, and the strength degradation of coated carbon-carbon composites. This plan will focus on static fatigue tests in the four point flexure mode in an elevated temperature, oxidizing environment.

Rozak, G. A.↗

NASA NTRS1983

An endochronic model for the response of unidirectional composites under off-axis tensile load

A methodology of modelling both nonlinear elastic and dissipative response of transversely isotropic fibrous composites is developed and illustrated with the aid of the observed response of graphite-polyimide off-axis coupons. The methodology is based on the internal variable formalism employed within the text of classical irreversible thermodynamics and entails extension of Valanis' endochronic theory to transversely isotropic media. Applicability of the theory to prediction of various response characteristics of fibrous composites is illustrated by accurately modelling such often observed phenomena as: stiffening reversible behavior along fiber direction; dissipative response in shear and transverse tension characterized by power-laws with different hardening exponents; permanent strain accumulation; nonlinear unloading and reloading; and stress-interaction effects.

Pindera, M. J.↗

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 NTRS1984

Development of high efficiency solar cells on silicon web

Web base material is being improved with a goal toward obtaining solar cell efficiencies in excess of 18% (AM1). Carrier loss mechanisms in web silicon was investigated, techniques were developed to reduce carrier recombination in the web, and web cells were fabricated using effective surface passivation. The effect of stress on web cell performance was also investigated.

Rohatgi, A.↗

NASA NTRS1985

Stress rate and proof-testing of silicon wafers

Fracture mechanics test methods were applied to evaluate the proof-test characteristics of single-crystal silicon wafers. The results indicate that the strength distribution of silicon wafers is truncated by proof-testing. No subcritical crack growth occurred during proof-loading, as inferred from the lack of a stress-rate effect on strength. Mechanical proof-testing appears to be an effective method for eliminating weak samples before cell processing.

Chen, C. P.↗

NASA NTRS1984

The effect of diffusion induced lattice stress on the open-circuit voltage in silicon solar cells

Weizer, V. G.↗

NASA NTRS1984

Methods for assessing the structural reliability of brittle materials

Failure from contact-induced surface flaws is considered along with controlled indentation flaws for construction of toughness and fatigue master maps, fatigue properties of ceramics with natural and controlled flaws, and a statistical analysis of size and stress state effects on the strength of an alumina ceramic. Attention is also given to dynamic and static fatigue of a machinable glass ceramic, the effect of multiregion crack growth on proof testing, and a fracture mechanics analysis of defect sizes. Other topics explored are related to the effect of temperature and humidity on delayed failure of optical glass fibers, subthreshold indentation flaws in the study of fatigue properties of ultrahigh-strength glass, the lifetime prediction for hot-pressed silicon nitride at high temperatures, static fatigue in high-performance ceramics, and requirements for flexure testing of brittle materials.

Freiman, S. W.↗

NASA NTRS1987

On the modelling of non-reactive and reactive turbulent combustor flows

A study of non-reactive and reactive axisymmetric combustor flows with and without swirl is presented. Closure of the Reynolds equations is achieved by three models: kappa-epsilon, algebraic stress and Reynolds stress closure. Performance of two locally nonequilibrium and one equilibrium algebraic stress models is analyzed assuming four pressure strain models. A comparison is also made of the performance of a high and a low Reynolds number model for combustor flow calculations using Reynolds stress closures. Effects of diffusion and pressure-strain models on these closures are also investigated. Two models for the scalar transport are presented. One employs the second-moment closure which solves the transport equations for the scalar fluxes, while the other solves the algebraic equations for the scalar fluxes. In addition, two cases of non-premixed and one case of premixed combustion are considered. Fast- and finite-rate chemistry models are applied to non-premixed combustion. Both show promise for application in gas turbine combustors. However, finite rate chemistry models need to be examined to establish a suitable coupling of the heat release effects on turbulence field and rate constants.

Nikjooy, Mohammad↗

NASA NTRS1987

Analysis of plastic deformation in silicon web crystals

Numerical calculation of 111-plane 110-line slip activity in silicon web crystals generated by thermal stresses is in good agreement with etch pit patterns and X-ray topographic data. The data suggest that stress redistribution effects are small and that a model, similar to that proposed by Penning (1958) and Jordan (1981) but modified to account for dislocation annihilation and egress, can be used to describe plastic flow effects during silicon web growth.

Spitznagel, J. A.↗

NASA NTRS1988

Effect of temperature, microstructure, and stress state on the low cycle fatigue behavior of Waspaloy

Specimens of Waspaloy of two different microstructures were tested in uniaxial and torsional low-cycle fatigue at 24 and 649 C. For all specimens, deformation and failure mechanisms are found to be independent of stress state at 24 C; in both microstructures, failure is associated with the formation of shear cracks. At 649 C, deformation and failure mechanisms for the fine-grain large gamma-prime specimens are independent of stress state, and the mechanisms are similar to those observed at 24 C. For the coarse-grain small gamma-prime specimens, however, failure occurs on principal planes in torsion and on shear plane in uniaxial tension. The results are interpreted in terms of deformation mode and microstructural instability.

Stahl, D. R.↗

NASA NTRS1988

On the pulsational overstability in narrowly confined viscous rings

Viscous accretion disks and rings are subject to pulsational overstability if the effective viscous stress is a rapidly increasing function of the disks' (or rings') surface density. Such a phenomenon is induced by the transfer of energy from shear to normal mode oscillations through viscous stress. The paper examines the global response of viscous accretion disks or rings to pulsational overstabilities. The analytic and preliminary numerical results indicate that all modes with wavelength longer than the scale height of the disk or ring are overstable. From the results, some examples of the complex time-dependent structure which may be induced by this overstability are illustrated. Finally, results are applied to narrow planetary rings to address the issues of maintenance of uniform precession and origin of eccentricity.

Papaloizou, J. C. B.↗

NASA NTRS1990

Nonlinear response of unidirectional boron/aluminum

Experimental results obtained for unidirectional boron/aluminum subjected to combined loading using off-axis tension, compression and Iosipescu shear specimens are correlated with a nonlinear micromechanics model. It is illustrated that the nonlinear response in the principal material directions is markedly influenced by the different loading modes and different ratios of the applied stress components. The observed nonlinear response under pure and combined loading is discussed in terms of initial yielding, subsequent hardening, stress-interaction effects and unloading-reloading characteristics. The micromechanics model is based on the concept of a repeating unit cell representative of the composite-at-large and employs the unified theory of Bodner and Partom to model the inelastic response of the matrix. It is shown that the employed micromechanics model is sufficiently general to predict the observed nonlinear response of unidirectional boron/aluminum with good accuracy.

Pindera, M.-J.↗

NASA NTRS1991

Temperature effects on the deformation and fracture of Al-Li-Cu-In alloys

The crack initiation and growth fracture resistance of Al-Cu-Li and Al-Cu-Li-In alloys were characterized and optimized for cryogenic tank applications. Presently, the effects of stress state and temperature is being determined on the fracture toughness and fracture mechanisms of commercially available Vintage 3 2090-T81 and experimental 2090+In-T6. Precracked J-integral specimens of both alloys were tested at ambient and cryogenic temperatures in the plane stress and plane strain conditions. Considering ambient temperature, results showed that 2090-T81 exhibited the highest toughness in both plane strain and plane stress conditions. For the plane strain condition, reasonable crack initiation and growth toughness of 1090-T81 are associated with a significant amount of delamination and transgranular fracture. Plane stress toughnesses were higher and fracture was characterized by shear cracking with minimal delaminations. In comparisons, the fracture behavior of 2090+In-T6 is significantly degraded by subgrain boundary precipitation. Toughness is low and characterized by intersubgranular fracture with no delamination in the plane stress or plane strain conditions. Intersubgranular cracking is a low energy event which presumably occurs prior to the onset of slip band cracking. Copious grain boundary precipitation is atypical of commercially available 2090. At cryogenic temperatures, both alloys exhibit increased yield strength, toughness, and amount of delamination and shear cracking. The change in fracture mode of 2090+In-T6 from intersubgranular cracking at ambient temperature to a combination of intersubgranular cracking, shear cracking, and delamination at cryogenic temperature is the subject of further investigation.

Wagner, John A.↗

NASA NTRS1991

Recent developments in nickel electrode analysis

Three aspects of nickel electrode analysis for Nickel-Hydrogen and Nickel-Cadmium battery cell applications are addressed: (1) the determination of active material; (2) charged state nickel (as NiOOH + CoOOH); and (3) potassium ion content in the electrode. Four deloading procedures are compared for completeness of active material removal, and deloading conditions for efficient active material analyses are established. Two methods for charged state nickel analysis are compared: the current NASA procedure and a new procedure based on the oxidation of sodium oxalate by the charged material. Finally, a method for determining potassium content in an electrode sample by flame photometry is presented along with analytical results illustrating differences in potassium levels from vendor to vendor and the effects of stress testing on potassium content in the electrode. The relevance of these analytical procedures to electrode performance is reviewed.

Whiteley, Richard V.↗