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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Buckling and Damage Resistance of Transversely-Loaded Composite Shells

Experimental and numerical work was conducted to better understand composite shell response to transverse loadings which simulate damage-causing impact events. The quasi-static, centered, transverse loading response of laminated graphite/epoxy shells in a [+/-45(sub n)/O(sub n)](sub s) layup having geometric characteristics of a commercial fuselage are studied. The singly-curved composite shell structures are hinged along the straight circumferential edges and are either free or simply supported along the curved axial edges. Key components of the shell response are response instabilities due to limit-point and/or bifurcation buckling. Experimentally, deflection-controlled shell response is characterized via load-deflection data, deformation-shape evolutions, and the resulting damage state. Finite element models are used to study the kinematically nonlinear shell response, including bifurcation, limit-points, and postbuckling. A novel technique is developed for evaluating bifurcation from nonlinear prebuckling states utilizing asymmetric spatial discretization to introduce numerical perturbations. Advantages of the asymmetric meshing technique (AMT) over traditional techniques include efficiency, robustness, ease of application, and solution of the actual (not modified) problems. The AMT is validated by comparison to traditional numerical analysis of a benchmark problem and verified by comparison to experimental data. Applying the technique, bifurcation in a benchmark shell-buckling problem is correctly identified. Excellent agreement between the numerical and experimental results are obtained for a number of composite shells although predictive capability decreases for stiffer (thicker) specimens which is attributed to compliance of the test fixture. Restraining the axial edge (simple support) has the effect of creating a more complex response which involves unstable bifurcation, limit-point buckling, and dynamic collapse. Such shells were noted to bifurcate into asymmetric deformation modes but were undamaged during testing. Shells in this study which were damaged were not observed to bifurcate. Thus, a direct link between bifurcation and atypical damage could not be established although the mechanism (bifurcation) was identified. Recommendations for further work in these related areas are provided and include extensions of the AMT to other shell geometries and structural problems.

Wardle, Brian L.↗
NASA NTRS1972

Buckling of circular cylindrical shells under axial compression.

The infinitely long cylindrical shell under axial compression with axisymmetric sinusoidal imperfections is considered. The bifurcation problem is formulated exactly, and a complete family of buckling modes is identified. Koiter's (1963) upper bound pertains to the critical stress associated with one restricted set of modes, and the critical stress for a set of long wavelength modes reproduces some numerical results obtained by Almroth (1966). The initial postbuckling analysis is also formulated exactly. An exact analytical solution is obtained for the limiting case of modes with infinitely long wavelengths, and numerical analysis is used to solve the equations for the other cases. An appendix gives the details of the analysis.

Budiansky, B.↗
NASA NTRS1970

Flutter, vibration, and buckling of truncated orthotropic conical shells with generalized elastic edge restraint

A theoretical investigation has been made of the flutter, vibration, and buckling of truncated conical shells with generalized elastic edge restraint. The shell analysis is of the classical Donnell type, in-plane inertias and structural damping are neglected, and the aerodynamic loading is represented by the inviscid two-dimensional quasi-steady approximation. An approximate solution is obtained by the generalized Galerkin method. The accuracy and limitations of the analysis are illustrated by comparing numerical results for buckling an vibration with results of other investigations for various boundary conditions, applied loads, and shell geometries and stiffness. Sufficient numerical results are presented to permit the determination of the flutter condition for simply supported isotropic conical shells for a wide range of cone angle, length-radius ration, and radius-thickness ratio. Results are also presented to indicate some effects of variations in edge restraint, applied loads, and ring or stringer stiffening.

Flutter↗
NASA NTRSDate not supplied

Buckling Behavior of Conical-Cylindrical Shells and Design Considerations for Launch-Vehicle Applications

Traditionally, launch vehicles are constructed with a series of buckling-prone thin-walled cylindrical and conical shells, in which the buckling behavior of these shells has been well studied and buckling design guidance exists. Conical-cylindrical shell geometry is now being utilized for launch-vehicle stage adapters and payload adapters due to advances in manufacturing and numerical techniques, but there is no available buckling design guidance for this nontraditional combined geometry. In order to provide design recommendations, the buckling behavior and imperfection sensitivity of conical-cylindrical shells and how it differs from the conical and cylindrical components needs to be better understood. From this premise, it is possible to investigate whether or not the buckling knockdown factor guidelines for conical and cylindrical shells outlined in NASA SP-8019 and NASA SP-8007, respectively, are still applicable. The results in this paper will show that the current recommendations are not appropriate in some cases. In addition, it was observed that the large rotations and displacements near the transition between the cone and cylinder can have a larger effect on the buckling load than the presence of radial imperfections for conical-cylindrical shells, which is different than conical and cylindrical shells. More interesting is the fact that design modifications to increase the buckling capability of a conical-cylindrical shell such as adding reinforcement, which may add mass, will make the shell more sensitive to imperfections. The increased imperfection sensitivity may negate the increase in buckling capability that was thought to be achievable. In the end, it may be more beneficial to design a conical-cylindrical shell in which the buckling behavior is dominated by the more predictable geometric nonlinearity, which may lead to an overall lower buckling load, but a lower knockdown factor may be possible since it will not be as sensitive to the less-known radial imperfections.

Buckling↗
NASA NTRS1993

Buckling analysis of laminated thin shells in a hot environment

Results are presented of parametric studies to assess the effects of various parameters on the buckling behavior of angle-ply, laminated thin shells in a hot environment. These results were obtained by using a three-dimensional finite element analysis. An angle-ply, laminated thin shell with fiber orientation of (theta/-theta)(sub 2) was subjected to compressive mechanical loads. The laminated thin shell had a cylindrical geometry. The laminate contained T300 graphite fibers embedded in an intermediate-modulus, high-strength (IMHS) matrix. The fiber volume fraction was 55 percent and the moisture content was 2 percent. The residual stresses induced into the laminate structure during the curing were taken into account. Parametric studies were performed to examine the effect on the critical buckling load of the following parameters: cylinder length and thickness, internal hydrostatic pressure, different ply thicknesses, different temperature profiles through the thickness of the structure, and different lay up configurations and fiber volume fractions. In conjunction with these parameters the ply orientation was varied from 0 deg to 90 deg. Seven ply angles were examined: 0 deg, 15 deg, 30 deg, 45 deg, 60 deg, 75 deg, and 90 deg. The results show that the ply angle theta and the laminate thickness had significant effects on the critical buckling load. The fiber volume fraction, the fiber orientations, and the internal hydrostatic pressure had important effects on the critical buckling load. The cylinder length had a moderate influence on the buckling load. The thin shell with (theta/-theta)(sub 2) or (theta/-theta)(sub s) angle-ply laminate had better buckling-load performance than the thin shell with (theta)(sub 4) angle-ply laminate. The temperature profiles through the laminate thickness and various laminates with the different ply thicknesses has insignificant effects on the buckling behavior of the thin shells.

Gotsis, Pascal K.↗
NASA NTRS1998

Buckling and Stable Tearing Responses of Unstiffened Aluminum Shells with Long Cracks

The results of an analytical and experimental study of the nonlinear response of thin, unstiffened, aluminum cylindrical shells with a long longitudinal crack are presented. The shells are analyzed with a nonlinear shell analysis code that accurately accounts for global and local structural response phenomena. Results are presented for internal pressure and for axial compression loads. The effect of initial crack length on the initiation of stable tearing and unstable crack growth in typical shells subjected to internal pressure loads is predicted using geo- metrically nonlinear elastic-plastic finite element analyses. The results of these analyses and of the experiments indicate that the pressure required to initiate stable tearing and unstable tearing in a shell subjected to internal pressure loads decreases as the crack length increases. The effects of crack length on the prebuckling, buckling and postbuckling responses of typical shells subjected to axial compression are also described. For this loading condition, the crack length is held constant. The results of the analyses illustrate the influence of crack length on shell buckling instabilities. The experimental and analytical results correlate well.

Starnes, James H., Jr.↗
NASA NTRS1998

Buckling Analysis of Anisotropic Curved Panels and Shells with Variable Curvature

A buckling formulation for anisotropic curved panels with variable curvature is presented in this paper. The variable curvature panel is assumed to consists of two or more panels of constant but different curvatures. Bezier functions are used as Ritz functions Displacement (C(sup 0)), and slope (C(sup 1)) continuities between segments are imposed by manipulation of the Bezier control points. A first-order shear-deformation theory is used in the buckling formulation. Results obtained from the present formulation are compared with those from finite element simulations and are found to be in good agreement.

Jaunky, Navin↗
NASA NTRS1962

BUCKLING OF A SPHERE OF EXTREMELY HIGH RADIUS-THICKNESS RATIO

Proposed passive communications satellites are ultrathin-walled spherical shells which may experience buckling from external pressure due to solar radiation or atmospheric drag. Large buckling deformations of such a satellite are undesirable because they seriously reduce the satellite’s performance as a reflector. At present, shell buckling technology is lacking in several respects with regard to design of spherical shells subjected to external pressure. Large discrepancies exist between theoretical and experimental buckling pressure; therefore, the designer must be guided primarily by experimental data. However, very little experimental data on complete spheres are available; the primary research effort has been concerned with shallow spherical caps. All available experimental data are for radius-thickness ratios several orders of magnitude smaller than that for the proposed satellites. In order to provide a basis for estimating the buckling strength of these satellites, therefore, buckling experiments were conducted on a complete spherical shell having a radius-thickness ratio of about 85,000. This value is much higher than that of any spherical cap or complete sphere used in previously reported experiments.

SHELL STABILITY↗
NASA NTRS1998

Stable Tearing and Buckling Responses of Unstiffened Aluminum Shells with Long Cracks

The results of an analytical and experimental study of the nonlinear response of thin, unstiffened, aluminum cylindrical shells with a long longitudinal crack are presented. The shells are analyzed with a nonlinear shell analysis code that accurately accounts for global and local structural response phenomena. Results are presented for internal pressure and for axial compression loads. The effect of initial crack length on the initiation of stable crack growth and unstable crack growth in typical shells subjected to internal pressure loads is predicted using geometrically nonlinear elastic-plastic finite element analyses and the crack-tip-opening angle (CTOA) fracture criterion. The results of these analyses and of the experiments indicate that the pressure required to initiate stable crack growth and unstable crack growth in a shell subjected to internal pressure loads decreases as the initial crack length increases. The effects of crack length on the prebuckling, buckling and postbuckling responses of typical shells subjected to axial compression loads are also described. For this loading condition, the crack length was not allowed to increase as the load was increased. The results of the analyses and of the experiments indicate that the initial buckling load and collapse load for a shell subjected to axial compression loads decrease as the initial crack length increases. Initial buckling causes general instability or collapse of a shell for shorter initial crack lengths. Initial buckling is a stable local response mode for longer initial crack lengths. This stable local buckling response is followed by a stable postbuckling response, which is followed by general or overall instability of the shell.

Starnes, James H., Jr.↗
NASA NTRS1999

Stable Tearing and Buckling Responses of Unstiffened Aluminum Shells with Long Cracks

The results of an analytical and experimental study of the nonlinear response of thin, unstiffened, aluminum cylindrical shells with a long longitudinal crack are presented. The shells are analyzed with a nonlinear shell analysis code that accurately accounts for global and local structural response phenomena. Results are presented for internal pressure and for axial compression loads. The effect of initial crack length on the initiation of stable crack growth and unstable crack growth in typical shells subjected to internal pressure loads is predicted using geometrically nonlinear elastic-plastic finite element analyses and the crack-tip-opening angle (CTOA) fracture criterion. The results of these analyses and of the experiments indicate that the pressure required to initiate stable crack growth and unstable crack growth in a shell subjected to internal pressure loads decreases as the initial crack length increases. The effects of crack length on the prebuckling, buckling and postbuckling responses of typical shells subjected to axial compression loads are also described. For this loading condition, the crack length was not allowed to increase as the load was increased. The results of the analyses and of the experiments indicate that the initial buckling load and collapse load for a shell subjected to axial compression loads decrease as the initial crack length increases. Initial buckling causes general instability or collapse of a shell for shorter initial crack lengths. Initial buckling is a stable local response mode for longer initial crack lengths. This stable local buckling response is followed by a stable postbuckling response, which is followed by general or overall instability of the shell.

Starnes, James H., Jr.↗
NASA NTRS1972

Effect of a slot on the buckling load of a cylindrical shell with a circular cutout.

The results are discussed of a study of the effect of slots on the buckling of cylinders with circular holes of systematically varied circularity. The results obtained suggest that the substantial increase in the buckling loads of cylindrical shells resulting from increases in slot length is due to a relief of the bending stress field.

Starnes, J. H., Jr.↗
NASA NTRS1992

Postbuckling response of stiffened composite cylindrical shells

The local buckling and the associated postbuckling response of stiffened composite shells are studied using an asymptotic approach. Both the classical (Kirchhoff) theory (CPT) and a first-order shear deformation theory (SDT) are employed in the study. SDT analysis was carried out using p-version type finite strips, whereas CPT analysis employed h-version based strips. Convergence studies confirm the superiority of the p-version approach. A panel width-to-thickness ratio of 50 was chosen to investigate the influence of shear deformation for stiffened and isolated panels fabricated out of isotropic and composite layered materials. The shear deformation effects were significant for composite stiffened shell panels with CPT giving a noticeably stiffer response. In comparison with isotropic shell panels, orthotropic layered panels buckle at lower nondimensional buckling loads but are appreciably less sensitive to imperfection.

Sridharan, Srinivasan↗