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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Implementation of Free-Formulation-Based Flat Shell Elements into NASA Comet Code and Development of Nonlinear Shallow Shell Element

This study presents a transient nonlinear finite element analysis within the realm of a multi-body dynamics formulation for determining the dynamic response of a moderately thick laminated shell undergoing a rapid and large rotational motion and nonlinear elastic deformations. Nonlinear strain measure and rotation, as well as 'the transverse shear deformation, are explicitly included in the formulation in order to capture the proper motion-induced stiffness of the laminate. The equations of motion are derived from the virtual work principle. The analysis utilizes a shear deformable shallow shell element along with the co-rotational form of the updated Lagrangian formulation. The shallow shell element formulation is based on the Reissner-Mindlin and Marguerre theory.

Barut, A.↗
NASA NTRS2006

Chemical, Physical, and Mechanical Characterization of Isocyanate Cross-linked Amine-Modified Silica Aerogels

We describe a new mechanically strong lightweight porous composite material obtained by encapsulating the skeletal framework of amine-modified silica aerogels with polyurea. The conformal polymer coating preserves the mesoporous structure of the underlying silica framework and the thermal conductivity remains low at 0.041 plus or minus 0.001 W m(sup -1 K(sup -1). The potential of the new cross-linked silica aerogels for load-carrying applications was determined through characterization of their mechanical behavior under compression, three-point bending, and dynamic mechanical analysis (DMA). A primary glass transition temperature of 130 C was identified through DMA. At room temperature, results indicate a hyperfoam behavior where in compression cross-linked aerogels are linearly elastic under small strains (less than 4%) and then exhibit yield behavior (until 40% strain), followed by densification and inelastic hardening. At room temperature the compressive Young's modulus and the Poisson's ratio were determined to be 129 plus or minus 8 MPa and 0.18, respectively, while the strain at ultimate failure is 77% and the average specific compressive stress at ultimate failure is 3.89 x 10(exp 5) N m kg(sup -1). The specific flexural strength is 2.16 x 10(exp 4) N m kg(sup -1). Effects on the compressive behavior of strain rate and low temperature were also evaluated.

Katti, Atul↗
NASA NTRS2007

Mesh Deformation Based on Fully Stressed Design: The Method and Two-Dimensional Examples

Mesh deformation in response to redefined boundary geometry is a frequently encountered task in shape optimization and analysis of fluid-structure interaction. We propose a simple and concise method for deforming meshes defined with three-node triangular or four-node tetrahedral elements. The mesh deformation method is suitable for large boundary movement. The approach requires two consecutive linear elastic finite-element analyses of an isotropic continuum using a prescribed displacement at the mesh boundaries. The first analysis is performed with homogeneous elastic property and the second with inhomogeneous elastic property. The fully stressed design is employed with a vanishing Poisson s ratio and a proposed form of equivalent strain (modified Tresca equivalent strain) to calculate, from the strain result of the first analysis, the element-specific Young s modulus for the second analysis. The theoretical aspect of the proposed method, its convenient numerical implementation using a typical linear elastic finite-element code in conjunction with very minor extra coding for data processing, and results for examples of large deformation of two-dimensional meshes are presented in this paper. KEY WORDS: Mesh deformation, shape optimization, fluid-structure interaction, fully stressed design, finite-element analysis, linear elasticity, strain failure, equivalent strain, Tresca failure criterion

Hsu, Su-Yuen↗
NASA NTRS2005

Nonlinear Visco-Elastic Response of Composites via Micro-Mechanical Models

Micro-mechanical models for a study of nonlinear visco-elastic response of composite laminae are developed and their performance compared. A single integral constitutive law proposed by Schapery and subsequently generalized to multi-axial states of stress is utilized in the study for the matrix material. This is used in conjunction with a computationally facile scheme in which hereditary strains are computed using a recursive relation suggested by Henriksen. Composite response is studied using two competing micro-models, viz. a simplified Square Cell Model (SSCM) and a Finite Element based self-consistent Cylindrical Model (FECM). The algorithm is developed assuming that the material response computations are carried out in a module attached to a general purpose finite element program used for composite structural analysis. It is shown that the SSCM as used in investigations of material nonlinearity can involve significant errors in the prediction of transverse Young's modulus and shear modulus. The errors in the elastic strains thus predicted are of the same order of magnitude as the creep strains accruing due to visco-elasticity. The FECM on the other hand does appear to perform better both in the prediction of elastic constants and the study of creep response.

Gates, Thomas S.↗
NASA NTRS1982

Elastic-plastic finite-element analyses of thermally cycled double-edge wedge specimens

Elastic-plastic stress-strain analyses were performed for double-edge wedge specimens subjected to thermal cycling in fluidized beds at 316 and 1088 C. Four cases involving different nickel-base alloys (IN 100, Mar M-200, NASA TAZ-8A, and Rene 80) were analyzed by using the MARC nonlinear, finite element computer program. Elastic solutions from MARC showed good agreement with previously reported solutions obtained by using the NASTRAN and ISO3DQ computer programs. Equivalent total strain ranges at the critical locations calculated by elastic analyses agreed within 3 percent with those calculated from elastic-plastic analyses. The elastic analyses always resulted in compressive mean stresses at the critical locations. However, elastic-plastic analyses showed tensile mean stresses for two of the four alloys and an increase in the compressive mean stress for the highest plastic strain case.

Kaufman, A.↗
NASA NTRS1976

Nonlinear effects in thermal stress analysis of a solid propellant rocket motor

Direct characterization procedures were used to determine the relaxation modulus as a function of time, temperature, and state of strain. Using the quasi-elastic method of linearviscoelasticity, these properties were employed in a finite element computer code to analyze a thick-walled, nonlinear viscoelastic cylinder in the state of plane strain bonded to a thin (but stiff) elastic casing and subjected to slow thermal cooling. The viscoelastic solution is then expressed as a sequence of elastic finite element solutions. The strain-dependent character of the relaxation modulus is included by replacing the single relaxation curve used in the linear viscoelastic theory by a family of relaxation functions obtained at various strain levels. These functions may be regarded as a collection of stress histories or responses to specific loads (in this case, step strains) with which the cooldown solution is made to agree by iterations on the modulus and strain level.

Francis, E. C.↗
NASA NTRS1991

Inelastic Strain Analysis of Solder Joint in NASA Fatigue Specimen

The solder fatigue specimen designed by NASA-GSFC/UNISYS is analyzed in order to obtain the inelastic strain history during two different representative temperature cycles specified by UNISYS. In previous reports (dated July 25, 1990, and November 15, 1990), results were presented of the elastic-plastic and creep analysis for delta T = 31 C cycle, respectively. Subsequent results obtained during the current phase, from viscoplastic finite element analysis of the solder fatigue specimen for delta T = 113 C cycle are summarized. Some common information is repeated for self-completeness. Large-deformation continuum formulations in conjunction with a standard linear solid model is utilized for modeling the solder constitutive creep-plasticity behavior. Relevant material properties are obtained from the literature. Strain amplitudes, mean strains, and residual strains (as well as stresses) accumulated due to a representative complete temperature cycle are obtained as a result of this analysis. The partitioning between elastic strains, time-independent inelastic (plastic) strains, and time-dependent inelastic (creep) strains is also explicitly obtained for two representative cycles. Detailed plots are presented for two representative temperature cycles. This information forms an important input for fatigue damage models, when predicting the fatigue life of solder joints under thermal cycling

Dasgupta, Abhijit↗
NASA NTRS1997

Microgravity Foam Structure and Rheology

To exploit rheological and multiple-light scattering techniques, and ultimately microgravity conditions, in order to quantify and elucidate the unusual elastic character of foams in terms of their underlying microscopic structure and dynamics. Special interest is in determining how this elastic character vanishes, i.e. how the foam melts into a simple viscous liquid, as a function of both increasing liquid content and shear strain rate. The unusual elastic character of foams will be quantified macroscopically by measurement of the shear stress as a function of static shear strain, shear strain rate, and time following a step strain; such data will be analyzed in terms of a yield stress, a static shear modulus, and dynamical time scales. Microscopic information about bubble packing and rearrangement dynamics, from which these macroscopic non-Newtonian properties presumably arise, will be obtained non-invasively by novel multiple-light scattering diagnostics such as Diffusing-Wave Spectroscopy (DWS). Quantitative trends with materials parameters, such as average bubble size, and liquid content, will be sought in order to elucidate the fundamental connection between the microscopic structure and dynamics and the macroscopic rheology.

Durian, Douglas J.↗
NASA NTRS2002

Foam Optics and Mechanics

The Foam Optics and Mechanics (FOAM) project will exploit the microgravity environment to more accurately measure the rheological and optical characteristics of wet aqueous foams. Using both rheology and laser light scattering diagnostics, the goal is to quantify the unusual elastic character of foams in terms of their underlying microscopic structure and dynamics. Of particular interest is determining how the elastic character vanishes, i.e., how the foam 'melts' into a simple viscous liquid, as a function of both increasing liquid content and increasing shear strain rate. The unusual elastic character of foams will be quantified macroscopically by measurement of the shear stress as a function of shear strain rate and of time following a step strain. Such data will be analyzed in terms of a yield stress, shear moduli, and dynamical time scales. Microscopic information about bubble packing and rearrangement dynamics, from which the macroscopic non-Newtonian properties ultimately arise, will be obtained non-invasively by multiple-light scattering: diffuse transmission spectroscopy (DTS) and diffusing wave spectroscopy (DWS). Quantitative trends with materials parameters, most importantly average bubble size and liquid content, will be sought in order to elucidate the fundamental connection between the microscopic structure and dynamics and the macroscopic rheology.

Durian, Douglas J.↗
NASA NTRS1974

A model of convergent plate margins based on the recent tectonics of Shikoku, Japan

A viscoelastic finite element plate tectonic model is applied to displacement data for the island of Shikoku, Japan. The flow properties and geometry of the upper portions of the earth are assumed known from geophysical evidence, and the loading characteristics are determined from the model. The nature of the forces acting on the Philippine Sea plate, particularly in the vicinity of the Nankai trough, is determined. Seismic displacement data related to the 1946 Nankaido earthquake are modeled in terms of a thick elastic plate overlying a fluidlike substratum. The sequence of preseismic and seismic displacements can be explained in terms of two independent processes operating on elastic lithospheric plates: a strain accumulation process caused by vertical downward forces acting on or within the lithosphere in the vicinity of the trench, and a strain release process caused by plate failure along a preexisting zone on weakness. This is a restatement of Reid's elastic rebound theory in terms of elastic lithospheric plates.

Bischke, R. E.↗
NASA NTRS1999

Anisotropic Plasticity of BN Nanotubes

Plastic collapse of compressed BN nanotubes are investigated and compared with carbon nanotubes of similar nature. Using a generalized tight-binding molecular dynamics (GTBMD) method for system containing B, N and C atoms we compute stiffness and plastic collapse of BN and C nanotubes under axial compression. For small compressional strain, BN nanotubes are found to be about 92% as stiff as similar C nanotubes. Due to BN bond buckling effect, however, the elastic limit of BN nanotubes is found to be more than C nanotubes. A route to plasticity is explored in which we find that at elastic limit the accumulated strain is released by a local plastic deformation of the nanotube. The mechanism of strain release and the resulting plastic deformation, however. are anisotropic in nature. The strain is released preferentially towards N as leading edge of a buckled BN bond and the tube, compressed at both ends, plastically collapses preferentially towards one end. Details of the anisotropic plasticity and prospective applications will be discussed in this presentation.

Madhu, Menon↗
NASA NTRS1990

Thickness and orientational design for a maximum stiff membrane

Recent results from sensitivity analysis for strain energy with anisotropic elasticity are applied to thickness and orientational design of laminated membranes. Primarily, the first order gradients of the total elastic energy are used in an optimality criteria based method. This traditional method is shown to give slow convergence with respect to design parameters, although the convergence of strain energy is very good. To get a deeper insight into this rather general characteristic, second order derivatives are included and it is shown how they can be obtained by first order sensitivity analysis. Examples of only thickness design, only orientational design, and combined thickness--orientational design are presented.

Pedersen, Pauli↗
NASA NTRS1993

Three-dimensional elastic-plastic finite-element analyses of constraint variations in cracked bodies

Three-dimensional elastic-plastic (small-strain) finite-element analyses were used to study the stresses, deformations, and constraint variations around a straight-through crack in finite-thickness plates for an elastic-perfectly plastic material under monotonic and cyclic loading. Middle-crack tension specimens were analyzed for thicknesses ranging from 1.25 to 20 mm with various crack lengths. Three local constraint parameters, related to the normal, tangential, and hydrostatic stresses, showed similar variations along the crack front for a given thickness and applied stress level. Numerical analyses indicated that cyclic stress history and crack growth reduced the local constraint parameters in the interior of a plate, especially at high applied stress levels. A global constraint factor alpha(sub g) was defined to simulate three-dimensional effects in two-dimensional crack analyses. The global constraint factor was calculated as an average through-the-thickness value over the crack-front plastic region. Values of alpha(sub g) were found to be nearly independent of crack length and were related to the stress-intensity factor for a given thickness.

Newman, J. C., Jr.↗
NASA NTRS1975

Modeling damaged wings: Element selection and constraint specification

The NASTRAN analytical program was used for structural design, and no problems were anticipated in applying this program to a damaged structure as long as the deformations were small and the strains remained within the elastic range. In this context, NASTRAN was used to test three-dimensional analytical models of a damaged aircraft wing under static loads. A comparison was made of calculated and experimentally measured strains on primary structural components of an RF-84F wing. This comparison brought out two sensitive areas in modeling semimonocoque structures. The calculated strains were strongly affected by the type of elements used adjacent to the damaged region and by the choice of multipoint constraints sets on the damaged boundary.

Stronge, W. J.↗
NASA NTRS1993

Mechanical properties of composite materials

A composite material incorporates high strength, high modulus fibers in a matrix (polymer, metal, or ceramic). The fibers may be oriented in a manner to give varying in-plane properties (longitudinal, transverse-stress, strain, and modulus of elasticity). The lay-up of the composite laminates is such that a center line of symmetry and no bending moment exist through the thickness. The laminates are tabbed, with either aluminum or fiberglass, and are ready for tensile testing. The determination of the tensile properties of resin matrix composites, reinforced by continuous fibers, is outlined in ASTM standard D 3039, Tensile Properties of Oriented Fiber Composites. The tabbed flat tensile coupons are placed into the grips of a tensile machine and load-deformation curves plotted. The load-deformation data are translated into stress-strain curves for determination of mechanical properties (ultimate tensile strength and modulus of elasticity).

Thornton, H. Richard↗