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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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Implementation of Improved Transverse Shear Calculations and Higher Order Laminate Theory Into Strain Rate Dependent Analyses of Polymer Matrix Composites

A numerical procedure has been developed to investigate the nonlinear and strain rate dependent deformation response of polymer matrix composite laminated plates under high strain rate impact loadings. A recently developed strength of materials based micromechanics model, incorporating a set of nonlinear, strain rate dependent constitutive equations for the polymer matrix, is extended to account for the transverse shear effects during impact. Four different assumptions of transverse shear deformation are investigated in order to improve the developed strain rate dependent micromechanics model. The validities of these assumptions are investigated using numerical and theoretical approaches. A method to determine through the thickness strain and transverse Poisson's ratio of the composite is developed. The revised micromechanics model is then implemented into a higher order laminated plate theory which is modified to include the effects of inelastic strains. Parametric studies are conducted to investigate the mechanical response of composite plates under high strain rate loadings. Results show the transverse shear stresses cannot be neglected in the impact problem. A significant level of strain rate dependency and material nonlinearity is found in the deformation response of representative composite specimens.

Zhu, Lin-Fa↗
NASA NTRS1970

The Varestraint test for refractory metals, task 2

The principles of the Varestraint test were applied to the evaluation of refractory metal alloys. This required development of a special test apparatus employing a modest specimen size for improved economy and compatible with the high purity inert atmosphere required for welding these alloys. Minimum specimen size is desirable because of the high cost of these alloys and also to permit timely testing of limited production developmental alloys. The Varestraint apparatus satisfies these requirements by providing a timely and economic test for hot crack sensitivity. This test can be used both for alloy development and production surveillance. The test apparatus was designed for use in vacuum purged dry boxes and is completely compatible with high purity inert atmosphere. Augmented strain can be varied continuously from 1/4% thru 4%. In addition to varying the augmented strain, the primary Varestraint variable, the effect on hot cracking of varying other weld parameters such as unit weld length heat input have also been studied. Test reproducibility, in terms of bending or strain rate, was demonstrated using high speed photography.

Lessmann, G. G.↗
NASA NTRS1982

Strain rate characterization of unidirectional graphite/epoxy composite

For an investigation of problems of composites at high strain rates with dominant wave propagation effects, such as dynamically loaded composite jet engine blades, the longitudinal, transverse, and in-plane shear properties of unidirectional SP 288/AS graphite/epoxy were measured at various strain rates ranging from quasi-static to over 500/s. The test specimens were rings 10.16 cm in diameter, 2.54 cm long, 6 to 8 plies thick, and with fibers at 0, 90, and 10 deg off-axis. Quasi-static testing was conducted in a fixture which applies hydraulic pressure to the ring specimens, and dynamic testing was conducted by applying an internal pressure pulse through a liquid in a special fixture. Dynamic stress-strain curves are presented. For the 0-deg specimens, the modulus increases with strain up to 20 percent of the static value. The modulus and strength increase sharply in the 90-deg rings with strain rate reaching values two to three times the corresponding static rate.

Daniel, I. M.↗
NASA NTRS1993

The experimental behavior of spinning pretwisted laminated composite plates

The purpose of the research is to gain an understanding of the material and geometric couplings present in advanced composite turbo-propellers. Twelve pre-twisted laminated composite plates are tested. Three different ply lay-ups (2 symmetric and 1 asymmetric) and four different geometries (flat and 30x pre-twist about the mid-chord, quarter-chord, and leading edge) distinguish each plate from one another. Four rotating and non-rotating tests are employed to isolate the material and geometric couplings of an advanced turbo propeller. The first series of tests consist of non-rotating static displacement, strain, and vibrations. These tests examine the effects of ply lay-up and geometry. The second series of tests consist of rotating displacement, strain, and vibrations with various pitch and sweep settings. These tests utilize the Dynamic Spin Rig Facility at the NASA Lewis Research Center. The rig allows the spin testing of the plates in a near vacuum environment. The tests examine how the material and plate geometry interact with the pitch and sweep geometry of an advanced turbo-propeller.

Kosmatka, John B.↗
NASA NTRS2008

Multiscale Computer Simulation of Failure in Aerogels

Aerogels have been of interest to the aerospace community primarily for their thermal properties, notably their low thermal conductivities. While such gels are typically fragile, recent advances in the application of conformal polymer layers to these gels has made them potentially useful as lightweight structural materials as well. We have previously performed computer simulations of aerogel thermal conductivity and tensile and compressive failure, with results that are in qualitative, and sometimes quantitative, agreement with experiment. However, recent experiments in our laboratory suggest that gels having similar densities may exhibit substantially different properties. In this work, we extend our original diffusion limited cluster aggregation (DLCA) model for gel structure to incorporate additional variation in DLCA simulation parameters, with the aim of producing DLCA clusters of similar densities that nevertheless have different fractal dimension and secondary particle coordination. We perform particle statics simulations of gel strain on these clusters, and consider the effects of differing DLCA simulation conditions, and the resultant differences in fractal dimension and coordination, on gel strain properties.

Good, Brian S.↗
NASA NTRS1971

Finite stretching of an annular plate.

The problem of the finite stretching of an annular plate which is bonded to a rigid inclusion at its inner edge is considered. The material is assumed to be isotropic and incompressible with a Mooney-type constitutive law. It is shown that the inclusion of the effect of the transverse normal strain leads to a rapid variation in thickness which is confined to a narrow edge zone. The explicit solutions to the boundary layer equations, which govern the behavior of the plate near the edges, are presented.

Biricikoglu, V.↗
NASA NTRS1973

The effects of plasticity in adhesive fracture

An energy-balance analysis is presented for adhesive failure in end loaded cantilever beams. The analysis includes the effects of input work, stored strain energy, dissipated plastic energy, and specific adhesive surface energy. Experimental results obtained with 6061-T6 aluminum are presented as evidence for the validity of the approach.-

Chang, M. D.↗
NASA NTRS1982

Micromechanical predictions of crack propagation and fracture energy in a single fiber boron/aluminum model composite

The axisymmetric finite element model and associated computer program developed for the analysis of crack propagation in a composite consisting of a single broken fiber in an annular sheath of matrix material was extended to include a constant displacement boundary condition during an increment of crack propagation. The constant displacement condition permits the growth of a stable crack, as opposed to the catastropic failure in an earlier version. The finite element model was refined to respond more accurately to the high stresses and steep stress gradients near the broken fiber end. The accuracy and effectiveness of the conventional constant strain axisymmetric element for crack problems was established by solving the classical problem of a penny-shaped crack in a thick cylindrical rod under axial tension. The stress intensity factors predicted by the present finite element model are compared with existing continuum results.

Adams, D. F.↗
NASA NTRS1985

Structure and properties of polyethylene films used in heavy lift balloons

The following features of five polyethylene films used by NASA in the construction of heavy lift balloons have been examined: molecular weight, molecular weight distribution, branching, melting behavior, density, surface texture, birefringence, orientation of crystalline regions, unlaxial deformation in the machine and transverse directions, and the effect of sample geometry and strain rate on deformation behavior. The goal of this exploratory study was to determine whether there are significant differences in any of the above mentioned features, or combination of features between the films. The acquisition of such information is a first step towards determining whether there are any specific correlations between film characteristics and the incidence of catastrophic failure of balloons during ascent through the troposphere. This exploratory study has resulted in the identification of similarities and differences between various features of the films.

Khoury, F.↗
NASA NTRS1984

The effect of microstructure, temperature, and hold-time on low-cycle fatigue of As HIP P/M Rene 95

The effects of microstructure, temperature, plastic strain range, and hold time on the low-cycle fatigue (LCF) life were studied for Rene 95, an important Ni base superalloy used in jet engine disks. It was shown that the life could be varied by approximately an order of magnitude at elevated temperatures by simple heat treatments. The life was largest for the microstructure that promoted the most homogeneous deformation mode. The results are explained using the concept of a synergistic interaction between the deformation mode and boundary oxidation.

Bashir, S.↗
NASA NTRS1986

Estimation of high temperature low cycle fatigue on the basis of inelastic strain and strainrate

Fatigue life at elevated temperature can be predicted by introducing parametric values obtained from monotonic constitutive behavior into the Universal-Slopes Equation. For directionally solidified MAR-M200+HF at 975 C, these parameters are the maximum stress achievable under entirely plastic (time-independent) and purely creep (time-dependent) conditions and the corresponding inelastic strains, as well as the elastic modulus. For materials which exhibit plasticity/creep interaction, two more pairs of monotonic parameters must be evaluated for fatigue life prediction. This life-prediction method based on the Universal-Slopes Equation, resulted from a constitutive model characterizing monotonic and cyclic data as inelastic strainrate as a function of inelastic strain. Characterizing monotonic data is this way, permitted distinction between different material responses such as strain-hardening, strain-softening, and dynamic recovery effects. Understanding and defining the region of influence of each of these effects facilitated formulation of the constitutive model in relation to the mechanical and microstructural processes occurring in the material under cyclic loading.

Berkovits, A.↗
NASA NTRS1988

Head-on collision of viscous vortex rings

The head-on collision of two identical axisymmetric viscous vortex rings is studied through direct simulations of the incompressible Navier-Stokes equations. The initial vorticity distributions considered are those of Hill's spherical vortex and of rings with circular Gaussian cores, each at Reynolds numbers of about 350 and 1000. The Reynolds number is defined by Gamma/Nu, the ratio of circulation to viscosity. As the vortices approach each other by self-induction, the radii increase by mutual induction, and vorticy cancels through viscous cross-diffusion across the collision plane. Following contact, the vorticity distribution in the core forms a head-tail structure (for the cases considered). The characteristic time of vorticity annihilation is compared with that of a 3D collision experiment and 3D numerical simulations. It is found that the annihilation time is somewhat longer in the axisymmetric case than it is in the symmetry plane of the experiment and 3D numerical simulation. By comparing the annihilatiom time with a viscous timescale and a circulation timescale, it is deduced that both the strain rate due to local effects and to 3D vorticity realignment are important.

Stanaway, S.↗
NASA NTRS1946

Correlation between strength properties in standard test specimens and molded phenolic parts

This report describes an investigation of the tensile, flexural, and impact properties of 10 selected types of phenolic molding materials. The materials were studied to see in what ways and to what extent their properties satisfy some assumptions on which the theory of strength of materials is based: namely, (a) isotropy, (b) linear stress-strain relationship for small strains, and (c) homogeneity. The effect of changing the dimensions of tensile and flexural specimens and the span-depth ratio in flexural tests were studied. The strengths of molded boxes and flexural specimens cut from the boxes were compared with results of tests on standard test specimens molded from the respective materials. The nonuniformity of a material, which is indicated by the coefficient of variation, affects the results of tests made with specimens of different sizes and tests with different methods of loading. The strength values were found to depend on the relationship between size and shape of the molded specimen and size and shape of the fillers. The most significant variations observed within a diversified group of materials were found to depend on the orientation of fibrous fillers. Of secondary importance was the dependence of the variability of test results on the pieces of filler incorporated into the molding powder as well as on the size of the piece. Static breaking strength tests on boxes molded from six representative phenolic materials correlated well with falling-ball impact tests on specimens cut from molded flat sheets. Good correlation was obtained with Izod impact tests on standard test specimens prepared from the molding materials. The static breaking strengths of the boxes do not correlate with the results of tensile or flexural tests on standard specimens.

MATERIALS, MONSANTO RESINOX↗
NASA NTRS1994

Elastic-plastic models for multi-site damage

This paper presents recent developments in advanced analysis methods for the computation of stress site damage. The method of solution is based on the p-version of the finite element method. Its implementation was designed to permit extraction of linear stress intensity factors using a superconvergent extraction method (known as the contour integral method) and evaluation of the J-integral following an elastic-plastic analysis. Coarse meshes are adequate for obtaining accurate results supported by p-convergence data. The elastic-plastic analysis is based on the deformation theory of plasticity and the von Mises yield criterion. The model problem consists of an aluminum plate with six equally spaced holes and a crack emanating from each hole. The cracks are of different sizes. The panel is subjected to a remote tensile load. Experimental results are available for the panel. The plasticity analysis provided the same limit load as the experimentally determined load. The results of elastic-plastic analysis were compared with the results of linear elastic analysis in an effort to evaluate how plastic zone sizes influence the crack growth rates. The onset of net-section yielding was determined also. The results show that crack growth rate is accelerated by the presence of adjacent damage, and the critical crack size is shorter when the effects of plasticity are taken into consideration. This work also addresses the effects of alternative stress-strain laws: The elastic-ideally-plastic material model is compared against the Ramberg-Osgood model.

Actis, Ricardo L.↗
NASA NTRS1996

Constitutive Models for Shape Memory Alloy Polycrystals

Shape memory alloys (SMA) exhibiting the superelastic or one-way effects can produce large recoverable strains upon application of a stress. In single crystals this stress and resulting strain are very orientation dependent. We show experimental stress/strain curves for a Ni-Al single crystal for various loading orientations. Also shown are model predictions; the open and closed circles indicate recoverable strains obtained at various stages in the transformation process. Because of the strong orientation dependence of shape memory properties, crystallographic texture can be expected to play an important role in the mechanical behavior of polycrystalline SMA. It is desirable to formulate a constitutive model to better understand and exploit the unique properties of SMA.

Comstock, R. J., Jr.↗
NASA NTRS2000

Determining a Prony Series for a Viscoelastic Material From Time Varying Strain Data

In this study a method of determining the coefficients in a Prony series representation of a viscoelastic modulus from rate dependent data is presented. Load versus time test data for a sequence of different rate loading segments is least-squares fitted to a Prony series hereditary integral model of the material tested. A nonlinear least squares regression algorithm is employed. The measured data includes ramp loading, relaxation, and unloading stress-strain data. The resulting Prony series which captures strain rate loading and unloading effects, produces an excellent fit to the complex loading sequence.

Tzikang, Chen↗
NASA NTRS2001

Effect of Stitching on Debonding in Composite Structural Elements

Stitched multiaxial warp knit materials have been suggested as viable alternatives to laminated prepreg materials for large aircraft structures such as wing skins. Analyses have been developed to quantify the effectiveness of stitching for reducing strain energy release rates in skin-stiffener debond, lap joint and sandwich debond configurations. Strain energy release rates were computed using the virtual crack closure technique. In all configurations, the stitches were shown to significantly reduce the strain energy release rate.

Raju, I. S.↗