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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Effects of Nesting on Compression-Loaded 2-D Woven Textile Composites

Layer nesting was investigated in five harness satin weave textile composite laminates under static compression loading. Two carbon/epoxy material systems, AS4/3501-6 and IM7/8551-7A were considered. Laminates were fabricated with three idealized nesting cases: stacked, split-span and diagonal. Similar compression strength reductions due to the effects of idealized nesting were identified for each material. The diagonal nesting geometry produced the largest reduction in static strength when compared to the compression strength of a conventional textile composite. All three nesting cases produced reductions in strength and ultimate strain due to the effects of idealized nesting. Finite element results showed consistent strength reduction trends for the idealized nesting cases, however the magnitudes of compressive strengths were overpredicted.

Adams, Daniel OHare↗
NASA NTRS1990

Buckling and postbuckling behavior of square compression-loaded graphite-epoxy plates with circular cutouts

An experimental study of the postbuckling behavior of square compression-loaded graphite-epoxy plates and isotropic plates with a central circular cutout is presented. Results are presented for unidirectional (0 sub 10)s and (90 sub 10)s plates, (0/90 sub 5)s plates, and for aluminum plates. Results are also presented for (+ or - O sub 6)s angle-ply plates for values of O = 30, 46, and 60 degrees. The experimental results indicate that the change in axial stiffness of a plate at buckling is strongly dependent upon cutout size and plate orthotropy. The presence of a cutout gives rise to an internal load distribution that changes, sometimes dramtically, as a function of cutout size coupled with the plate orthotropy. In the buckled state, the role of orthotropy becomes more significant since bending in addition to membrane orthotropy is present. Most of the plates with cutouts exhibited less postbuckling stiffness than the corresponding plate without a cutout, and the postbuckling stiffness decreased with increasing cutout size. However, some of the highly orthotropic plates with cutouts exhibited more postbuckling stiffness than the corresponding plate without a cutout.

Nemeth, Michael P.↗
NASA NTRS1990

Buckling and postbuckling behavior of compression-loaded isotropic plates with cutouts

An experimental study of the buckling and postbuckling behavior of square and rectangular compression loaded aluminum plates with centrally located circular, square, and elliptical cutouts is presented. Experimental results indicate that the plates exhibit overall trends of increasing buckling strain and decreasing initial postbuckling stiffness with increasing cutout width. Corresponding plates with circular and square cutouts of the same width buckle at approximately the same strain level, and exhibit approximately the same initial postbuckling stiffness. Results show that the reduction in initial postbuckling stiffness due to a cutout generally decreases as the plate aspect ratio increases. Other results presented indicate that square plates with elliptical cutouts having a large cutout-width-to-plate-width ratio generally lose prebuckling and initial postbuckling stiffness as the cutout height increases. However, the plates buckle at essentially the same strain level. Results also indicate that postbuckling stiffness is more sensitive to changes in elliptical cutout height than are prebuckling stiffness and buckling strain.

Nemeth, Michael P.↗
NASA NTRS1990

Buckling and postbuckling behavior of compression-loaded isotropic plates with cutouts

An experimental study of buckling and postbuckling behavior of square and rectangular compression-loaded aluminum plates with centrally located circular, square, and elliptical cutouts is presented. Experimental results indicate that the plates exhibit overall trends of increasing buckling strain and decreasing initial postbuckling stiffness with increasing cutout width. Corresponding plates with circular and square cutouts of the same width buckle at approximately the same strain level, and exhibit approximately the same initial postbuckling stiffness. Results show that the reduction in initial postbuckling stiffness due to a cutout generally decreases as the plate aspect ratio increases. Other results presented in this paper indicate that square plates with elliptical cutouts having a large cutout-width-to-plate-width ratio generally lose prebuckling and initial postbuckling stiffness as the cutout height increases. However, the plates buckle at essentially the same strain level. Results also indicate that postbuckling stiffness is more sensitive to changes in elliptical cutout height than are prebuckling stiffness and buckling strain.

Nemeth, Michael P.↗
NASA NTRS1986

Observation of the initiation and progression of damage in compressively loaded composite plates containing a cutout

A series of experiments was carried out to determine the mechanism of failure in compressively loaded laminated plates with a circular cutout. Real time holographic interferometry and photomicrography are used to observe the progression of failure. These observations together with post experiment plate sectioning and deplying for interior damage observation provide useful information for modelling the failure process. It is revealed that the failure is initiated as a localised instability in the zero layers, at the hole surface. With increasing load extensive delamination cracking is observed. The progression of failure is by growth of these delaminations induced by delamination buckling. Upon reaching a critical state, catastrophic failure of the plate is observed. The levels of applied load and the rate at which these events occur depend on the plate stacking sequence.

Waas, A.↗
NASA NTRS2005

Identifying and Characterizing Discrepancies Between Test and Analysis Results of Compression-Loaded Panels

Results from a study to identify and characterize discrepancies between validation tests and high-fidelity analyses of compression-loaded panels are presented. First, potential sources of the discrepancies in both the experimental method and corresponding high-fidelity analysis models were identified. Then, a series of laboratory tests and numerical simulations were conducted to quantify the discrepancies and develop test and analysis methods to account for the discrepancies. The results indicate that the discrepancies between the validation tests and high-fidelity analyses can be attributed to imperfections in the test fixture and specimen geometry; test-fixture-induced changes in specimen geometry; and test-fixture-induced friction on the loaded edges of the test specimen. The results also show that accurate predictions of the panel response can be obtained when these specimen imperfections and edge conditions are accounted for in the analysis. The errors in the tests and analyses, and the methods used to characterize these errors are presented.

Thornburgh, Robert P.↗
NASA NTRS1979

Effects of truncation of a predominantly compression load spectrum on the life of a notched graphite/epoxy laminate

The fatigue behavior of a notched, graphite/epoxy (T300/5208) laminate subjected to predominantly compressive loading was explored in a series of constant-amplitude and transport wing spectrum tests. Results of these tests indicate that (1) the amount of local (near the notch) buckling allowed in the tests significantly affected fatigue life; (2) spectrum truncation of either the high- or low-load end of the spectrum produced lives greater than those obtained in the baseline, complete-spectrum test, but life was much more sensitive to truncations at the high-load end; and (3) the Palmgren-Miner linear cumulative damage theory always predicted lives much longer than the actual spectrum loading test lives.

Phillips, E. P.↗
NASA NTRS1980

Behavior of damaged graphite/epoxy laminates under compression loading

The influence of three different resin systems on the damage tolerance of graphite/polymer laminates was evaluated. Testing consisted of both static compression and cyclic compression evaluation of 10.2 by 15.2 by 0.5 cm (4 by 6 by 0.2 in) laminates with circular holes, simulated delaminations, and low velocity impact. Damage growth under steadily increasing compression and cyclic compression loading was monitored. Damage size and impact-induced failures for the three materials were compared. Of the three material systems evaluated, the one most tolerant to impact damage exhibited the least delamination within the cross section due to impact, the highest transverse tension strain to failure, and the largest crack opening force, as determined from double cantilever beam tests.

Byers, B. A.↗
NASA NTRS2002

Structural Response of Compression-Loaded, Tow-Placed, Variable Stiffness Panels

Results of an analytical and experimental study to characterize the structural response of two compression-loaded variable stiffness composite panels are presented and discussed. These variable stiffness panels are advanced composite structures, in which tows are laid down along precise curvilinear paths within each ply and the fiber orientation angle varies continuously throughout each ply. The panels are manufactured from AS4/977-3 graphite-epoxy pre-preg material using an advanced tow placement system. Both variable stiffness panels have the same layup, but one panel has overlapping tow bands and the other panel has a constant-thickness laminate. A baseline cross-ply panel is also analyzed and tested for comparative purposes. Tests performed on the variable stiffness panels show a linear prebuckling load-deflection response, followed by a nonlinear response to failure at loads between 4 and 53 percent greater than the baseline panel failure load. The structural response of the variable stiffness panels is also evaluated using finite element analyses. Nonlinear analyses of the variable stiffness panels are performed which include mechanical and thermal prestresses. Results from analyses that include thermal prestress conditions correlate well with measured variable stiffness panel results. The predicted response of the baseline panel also correlates well with measured results.

Wu, K. Chauncey↗
NASA NTRS1991

The fatigue behaviour of orthotropic laminates under tension-compression loading

The fatigue behavior of orthotropic laminates (0, +/-45, 0 deg)2s and (90, +/-45, 90 deg)2s, has been evaluated under alternating tension-compression loading. Even though the first laminate is much stronger than the second, both started to fail by delamination. Visual damage started to show only at the very end of the fatigue life but measurement of the stiffness showed that degradation starts at about 80 percent of the fatigue life. The first laminate failed in compression after delamination between the 0 and the 45 deg laminae, while the second failed in tension after delamination between the +45 and -45 deg laminae. It is shown that the interlaminar fatigue strength of both laminate structures can be correlated to the applied fatigue load.

Rotem, A.↗
NASA NTRS1998

Damage Characteristics and Residual Strength of Composite Sandwich Panels Impacted with and Without Compression Loading

The results of an experimental study of the impact damage characteristics and residual strength of composite sandwich panels impacted with and without a compression loading are presented. Results of impact damage screening tests conducted to identify the impact-energy levels at which damage initiates and at which barely visible impact damage occurs in the impacted facesheet are discussed. Parametric effects studied in these tests include the impactor diameter, dropped-weight versus airgun-launched impactors, and the effect of the location of the impact site with respect to the panel boundaries. Residual strength results of panels tested in compression after impact are presented and compared with results of panels that are subjected to a compressive preload prior to being impacted.

McGowan, David M.↗
NASA NTRS1998

Behavior of Compression-Loaded Composite Panels with Stringer Terminations and Impact Damage

The results of an analytical and experimental study of graphite-epoxy stiffened panels with impact-damaged stringer terminations are presented. Five stitched graphite-epoxy panels with stiffeners with a gradual reduction in either thickness or height were examined. Panels were analyzed using finite element analysis and tested by loading them in axial compression to a predetermined load. The panels were then subjected to impact damage and loaded to failure. Axial midplane strains, surface strains, interlaminar strains and failure results are discussed.

Jegley, Dawn C.↗
NASA NTRS1990

Nonlinear analysis of compressively loaded linked-plate structures

A method is presented for the geometrically nonlinear analysis of the equilibrium behavior of a general class of compressively loaded, prismatic, linked-plate structures. The semi-analytical method accounts for geometric shape imperfections, and allows the use of laminated composite plate properties. Buckling eigenfunctions are used as displacement shape functions, and the associated second-order displacement fields are incorporated. The nonlinear plate equilibrium equations are expressed in terms of the assumed displacement form, and the Galerkin method is used to obtain a set of algebraic equations which are nonlinear in the modal amplitudes and the load parameter. Buckling eigensolutions for the class of structures under consideration can be obtained from the VIPASA computer code; thus, subject to some additional restrictions in loading and plate anisotropy, structural configurations which can be analyzed with regard to buckling response by VIPASA can be analyzed with regard to geometrically nonlinear response using the new method.

Stoll, Frederick↗
NASA NTRS1990

Importance of anisotropy on design of compression-loaded composite corrugated panels

An investigation is conducted of the importance of anisotropic terms in the design of composite corrugated panels, for a range of axial compressive load intensities. The two panel configurations treated were panels with tailored laminates and panels with a continuous laminate; both are of interest to aircraft designers and prone to anisotropic effects which are of as-yet undetermined extent. The importance of the anisotropic terms is measured by the difference between the design load and the buckling load obtained from the ultimate structural analysis.

Gurdal, Zafer↗
NASA NTRS1987

Failure modes for compression loaded angle-ply plates with holes

A combined theoretical-experimental investigation of failure in notched, graphite-epoxy, angle-ply laminates subjected to far-field compression loading indicates that failure generally initiates on the hole boundary and propagates along a line parallel to the fiber orientation of the laminate. The strength of notched laminates with specimen width-to-hole diameter ratios of 5 and 10 are compared to the strength of unnotched laminates. The experimental results are complemented by a three-dimensional finite element stress analysis that includes interlaminar stresses around holes in (+/- theta)s laminates. The finite element predictions indicate that failure is initiated by shear stresses at the hole boundary.

Burns, S. W.↗
NASA NTRS2001

High-Fidelity Nonlinear Analysis of Compression-Loaded Composite Shells

The results of an experimental and analytical study of the effects of initial imperfections on the buckling and postbuckling response of unstiffened thin-walled compression-loaded graphite-epoxy cylindrical shells are presented. The shells considered in the study have four different shell-wall laminates and two different shell-radius-to-thickness ratios. The shell-wall laminates include two different orthotropic laminates and two different quasi-isotropic laminates. The shell-radius-to-thickness ratios include shell-radius-to-thickness ratios equal to 100 and 200. The results identify the effects of traditional and nontraditional initial imperfections on the nonlinear response characteristics and buckling loads of the shells. The traditional imperfections include the geometric shell-wall mid-surface imperfections that are commonly discussed in the literature on thin shell buckling. The nontraditional imperfections include 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 traditional and nontraditional imperfections on the nonlinear response characteristics and buckling loads of the shells is described. The analysis procedure includes a nonlinear static analysis that predicts the stable response characteristics of the shells, and a nonlinear transient analysis that predicts the unstable response characteristics. The results of a local shell-wall stress analysis used to estimate failure stresses are also described.

Hilburger, Mark W.↗
NASA NTRS2004

Effects of Elastic Edge Restraints and Initial Prestress on the Buckling Response of Compression-Loaded Composite Panels

A parametric study of the effects of test-fixture-induced initial prestress and elastic edge restraints on the prebuckling and buckling responses of a compression-loaded, quasi-isotropic curved panel is presented. The numerical results were obtained by using a geometrically nonlinear finite element analysis code with high-fidelity models. The results presented show that a wide range of prebuckling and buckling behavior can be obtained by varying parameters that represent circumferential loaded-edge restraint and rotational unloaded-edge restraint provided by a test fixture and that represent the mismatch in specimen and test-fixture radii of curvature. For a certain range of parameters, the panels exhibit substantial nonlinear prebuckling deformations that yield buckling loads nearly twice the corresponding buckling load predicted by a traditional linear bifurcation buckling analysis for shallow curved panels. In contrast, the results show another range of parameters exist for which the nonlinear prebuckling deformations either do not exist or are relatively benign, and the panels exhibit buckling loads that are nearly equal to the corresponding linear bifurcation buckling load. Overall, the results should also be of particular interest to scientists, engineers, and designers involved in simulating flight-hardware boundary conditions in structural verification and certification tests, involved in validating structural analysis tools, and interested in tailoring buckling performance.

Hilburger, Mark W.↗