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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Parametric Study on the Response of Compression-Loaded Composite Shells With Geometric and Material Imperfections

The results of a parametric study of the effects of initial imperfections on the buckling and postbuckling response of three unstiffened thinwalled compression-loaded graphite-epoxy cylindrical shells with different orthotropic and quasi-isotropic shell-wall laminates are presented. The imperfections considered include initial geometric shell-wall midsurface imperfections, shell-wall thickness variations, local shell-wall ply-gaps associated with the fabrication process, shell-end geometric imperfections, nonuniform applied end loads, and variations in the boundary conditions including the effects of elastic boundary conditions. A high-fidelity nonlinear shell analysis procedure that accurately accounts for the effects of these imperfections on the nonlinear responses and buckling loads of the shells is described. The analysis procedure includes a nonlinear static analysis that predicts stable response characteristics of the shells and a nonlinear transient analysis that predicts unstable response characteristics.

Hilburger, Mark W.↗
NASA NTRS1940

The Effect of Initial Displacement of the Center Support on the Buckling of a Column Continuous over Three Supports

The test indicate that "an indiscriminate single application of the Southwell method (for analyzing Exp. Observations in problems of elastic stability)--can result in definite and measurable errors" The test also indicate "that the effect of curvature due to bending on the critical load for the compression flange material of a box beam is probably small and can be neglected." We have not found this to be true in our tests. It is believed that the effect of curvature, together with a small amount of fixity at the ribs, tends to force the stiffeners to bow in each bay thus effectively increasing their end fixity and thereby raising their allowable loads.

Lunquist, Eugene E.↗
NASA NTRS2004

Buckling and Failure of Compression-Loaded Composite Cylindrical Shells With Geometric and Material Imperfections

The results of an experimental and numerical study of the effects of initial imperfections on the buckling response and failure of unstiffened thin-walled compression-loaded graphite-epoxy cylindrical shells are presented. The shells considered in the study have six different orthotropic or quasi-isotropic shell-wall laminates and two different shell-radius-to-thickness ratios. The numerical results include the effects of geometric shell-wall mid-surface imperfections, shell-wall thickness variations, local shell-wall ply-gaps associated with the fabrication process, shell-end geometric imperfections, nonuniform end loads, and the effects of elastic boundary conditions. Selected cylinder parameter uncertainties were also considered. Results that illustrate the effects of imperfections and uncertainties on the nonlinear response characteristics, buckling loads and failure the shells are presented. In addition, a common failure analysis is used to predict material failures in the shells.

Hilburger, Mark W.↗
NASA NTRS1985

Short-wavelength buckling and shear failures for compression-loaded composite laminates

The short-wavelength buckling (or the microbuckling) and the interlaminar and inplane shear failures of multi-directional composite laminates loaded in uniaxial compression are investigated. A laminate model is presented that idealizes each lamina. The fibers in the lamina are modeled as a plate, and the matrix in the lamina is modeled as an elastic foundation. The out-of-plane w displacement for each plate is expressed as a trigonometric series in the half-wavelength of the mode shape for laminate short-wavelength buckling. Nonlinear strain-displacement relations are used. The model is applied to symmetric laminates having linear material behavior. The laminates are loaded in uniform end shortening and are simply supported. A linear analysis is used to determine the laminate stress, strain, and mode shape when short-wavelength buckling occurs. The equations for the laminate compressive stress at short-wavelength buckling are dominated by matrix contributions.

Shuart, M. J.↗
NASA NTRS1992

Cytherean crustal bending at Salme Dorsa

The Salme ridge belt can be interpreted as being the leading edge of a venusian crustal unit that moved against the highland foreland unit. It is indicative of a compressional zone, with a thrust front facing west. The Salme ridge belt with adjoining structures is an evident indication of lateral stresses and adjoining crustal movements on Venus. It supports the idea of southeast compression against and over the foreland planitia, which has bent under the load and/or lateral stress, resulting in trough and bulge formation in front of the ridge belt. The origin of the driving force for the movements remains masked. Laima Tessera is located in the direction from which the thrust is thought to apply but there are no appropriate candidates for a rift zone although a thrust from the southeast would be in good agreement with structures of Laima Tessera. The temperature gradient suggests that the lithosphere is approximately 12 km thick, while its elastic layer is approximately 3 km thick based either on the load-induced flexure model or on the compressional buckling model.

Raitala, J.↗
NASA NTRS1942

Stability of elastically supported columns

A criterion is developed for the stiffness required of elastic lateral supports at the ends of a compression member to provide stability. A method based on this criterion is then developed for checking the stability of a continuous beam-column. A related method is also developed for checking the stability of a member of a pin-jointed truss against rotation in the plane of the truss.

COMPRESSION MEMBERS-STABILITY↗
NASA NTRS1936

Tests for the determination of the stress condition in tension fields

The present experiments treat the stress of actual tension fields within the elastic range. They give the magnitude of the flexural stresses due to wrinkling. They also disclose, particularly by slightly exceeded buckling load, the marked unloading - as compared with the tension-field theory - of the uprights as a result of the flexural stiffness of the web plate. The test sheets were clamped at the edges and brought to buckling through shearing and compressive stresses applied in the direction of the long sides.

Lahde, R↗
NASA NTRS1989

Stress, vibration, and buckling of multilayered cylinders

An efficient computational method for lowering the cost of the free vibration, stress, and buckling analyses of multilayered composite cylinders is presented. The analytical methodology is based on the linear three-dimensional theory of elasticity where the cylinders are assumed to have simply supported curved edges, and the fibers of the different layers are either in the circumferential or in the longitudinal direction. The full equations of the finite element model are solved for a single pair of Fourier harmonics, and the response that corresponds to the other Fourier harmonics is generated utilizing a reduced system with considerably fewer degrees of freedom.

Noor, Ahmed K.↗
NASA NTRS1961

Consistent Pl Analysis of Aqueous Uranium-235 Critical Assemblies

The lethargy-dependent equations of the consistent Pl approximation to the Boltzmann transport equation for slowing down neutrons have been used as the basis of an IBM 704 computer program. Some of the effects included are (1) linearly anisotropic center of mass elastic scattering, (2) heavy element inelastic scattering based on the evaporation model of the nucleus, and (3) optional variation of the buckling with lethargy. The microscopic cross-section data developed for this program covered 473 lethargy points from lethargy u = 0 (10 Mev) to u = 19.8 (0.025 ev). The value of the fission neutron age in water calculated here is 26.5 square centimeters; this value is to be compared with the recent experimental value given as 27.86 square centimeters. The Fourier transform of the slowing-down kernel for water to indium resonance energy calculated here compared well with the Fourier transform of the kernel for water as measured by Hill, Roberts, and Fitch. This method of calculation has been applied to uranyl fluoride - water solution critical assemblies. Theoretical results established for both unreflected and fully reflected critical assemblies have been compared with available experimental data. The theoretical buckling curve derived as a function of the hydrogen to uranium-235 atom concentration for an energy-independent extrapolation distance was successful in predicting the critical heights of various unreflected cylindrical assemblies. The critical dimensions of fully water-reflected cylindrical assemblies were reasonably well predicted using the theoretical buckling curve and reflector savings for equivalent spherical assemblies.

Fieno, Daniel↗
NASA NTRS1996

Differences in Mechanical Properties of the Human and Monkey Tibia

A method which uses an instrument that detects the response of a long bone to a vibratory stimulus to quantify mechanical properties non-invasively was revised and validated for use in the tibia. Stored data from healthy men was reanalyzed and compared with values from non-human primates. The analysis uses the relationship K(sub b) = 48 EI/L(sup 3) where K(sub b) is the lateral stiffness of a beam with force applied midspan, E is the elastic modulus, I the geometric moment of inertia and L, the limb length. Values for stiffness (EI, Nm(sup2)), the Euler buckling load (P(sub cr) = EI (pi/L)(sup 2)), and bone sufficiency (S) which represents the axial load the bone can support, adjusted to BW (S=P(sub cr)/BW) were obtained. The interest precision of the method in relaxed men, 5.8%, and in sedated male monkeys, 4.3%, was based on repeated measures in the same subjects at 1 month intervals. The R tibias of 40 men, aged 38.6 +/- 7.3 yrs with BW 78.9 +/- 7.9 kg, showed average (+/- SD) L to be 35 +/- 2 cm, EI 222 +/- 71 Nm(sup 2), P(sub cr) 18.1 +/- 4.9 kN, and S 23.4 +/- 5.7 N. The R tibias of 24 Rhesus monkeys ranging in age from 2-12 years, BW 4.9 +/- 3 kg, showed L to be 14.7 +/- 1.9 cm, EI 6.0 +/- 4.8 Nm(sup 2), P(sub cr) 2.51 +/- 1.2 kN and S 57.3 N. These measurements indicate that the tibia of a terrestrial non-human primate, M. mulatta, has higher load carrying capacity for the level of body weights in the species than the human bone.

Arnaud, Sara B.↗
NASA NTRS1983

Transverse vibration and buckling of a cantilevered beam with tip body under constant axial base acceleration

The planar transverse bending behavior of a uniform cantilevered beam with rigid tip body subject to constant axial base acceleration was analyzed. The beam is inextensible and capable of small elastic transverse bending deformations only. Two classes of tip bodies are recognized: (1) mass centers located along the beam tip tangent line; and (2) mass centers with arbitrary offset towards the beam attachment point. The steady state response is studied for the beam end condition cases: free, tip mass, tip body with restricted mass center offset, and tip body with arbitrary mass center offset. The first three cases constitute classical Euler buckling problems, and the characteristic equation for the critical loads/accelerations are determined. For the last case a unique steady state solution exists. The free vibration response is examined for the two classes of tip body. The characteristic equation, eigenfunctions and their orthogonality properties are obtained for the case of restricted mass center offset. The vibration problem is nonhomogeneous for the case of arbitrary mass center offset. The exact solution is obtained as a sum of the steady state solution and a superposition of simple harmonic motions.

Storch, J.↗
NASA NTRS1942

Critical Compressive Stress for Outstanding Flanges

A chart is presented for the values of the coefficient in the formula for the critical compressive stress at which buckling may be expected to occur in outstanding flanges. These flanges are flat rectangular plates supported along the loaded edges, supported and elastically restrained along one unloaded edge, and free along the other unloaded edge. The mathematical derivations of the formulas required for construction of the chart are given.

Lundquist, Eugene E↗
NASA NTRS1941

Critical Compressive Stress for Outstanding Flanges

A chart is presented for the values of the coefficient in the formula for the critical compressive stress at which buckling may be expected to occur in outstanding flanges. These flanges are flat rectangular plates supported along the Loaded edges, supported and elastically restrained along one unloaded edge, and free along the other unloaded edge. The mathematical derivations of the formulas required for the construction of the chart are given.

Lundquist, Eugene E.↗
NASA NTRS2020

Investigating Local Buckling and Plastic Collapse in Multifunctional Sandwich Composite Cores using a Unit Cell Method

The primary goal of developing analytical models of composite sandwich cores has been to obtain simple expressions for elastic and failure response to improve the rapid analysis of composite structures. An earlier investigation by the authors was made to verify the accuracy of widely-used analytical models that calculate equivalent elastic moduli of a uniform homogeneous single layer of material in place of the complex honeycomb core properties in sandwich composites. This transformation of elastic properties permits a coarsening of the finite element model (FEM) of the core to a fraction of the computational cost in simulating the original complex core architecture. In addition, the core was assumed constructed of multi-layered isotropic walls to support potential additional functionality beyond simple load-carrying capability.

Erik Saether↗
NASA NTRS1988

A micromechanics model of the stiffness and strength of laminates with fiber waviness

A mathematical model based on the Euler-Bernoulli beam theory is proposed for predicting the effective Young's moduli of piecewise isotropic composite laminates with local ply curvatures in the main load-carrying layers. Strains in corrugated layers, In-Phase layers, Out-of-Phase layers are predicted for various geometries and material configurations by assuming matrix layers as elastic foundations of different spring constants. The effective Young's moduli measured from corrugated aluminum specimens and aluminum/epoxy specimens with In-Phase and Out-of-Phase wavy patterns coincide very well with the model predictions. Moire fringe analysis of an In-Phase specimen and an Out-of-Phase specimen are also presented confirming the main assumption of the model related to the elastic constraint due to the matrix layers. The present model is also compared with the experimental results and other models, including the micro-buckling models, published in the literature. The results of the present study show that even a very small scale local ply curvature produces a noticeable effect on the mechanical constitutive behavior of a laminated composite.

Harris, Charles E.↗
NASA NTRS1939

Local instability of centrally loaded columns of channel section and Z-section

Charts are presented for the coefficients in formulas for the critical compressive stress at which cross-sectional distortion begins in a thin-wall member with either a channel section or a Z-section with identical flanges. The energy method of Timoshenko was used in the theoretical calculations required for the construction of the charts. The deflection equations were carefully selected to give good accuracy. The calculation of the critical compressive stress at stresses beyond the elastic range is briefly discussed. In order to demonstrate the use of the formulas and the charts in engineering calculations, two illustrative problems are included.

ELASTICITY↗