Navin Jaunky

Optimal Design of General Stiffened Composite Circular Cylinders for Global Buckling with Strength Constraints

A design strategy for optimal design of composite grid-stiffened cylinders subjected to global and local buckling constraints and strength constraints was developed using a discrete optimizer based on a genetic algorithm. An improved smeared stiffener theory was used for the global analysis. Local buckling of skin segments were assessed using a Rayleigh-Ritz method that accounts for material anisotropy. The local buckling of stiffener segments were also assessed. Constraints on the axial membrane strain in the skin and stiffener segments were imposed to include strength criteria in the grid-stiffened cylinder design. Design variables used in this study were the axial and transverse stiffener spacings, stiffener height and thickness, skin laminate stacking sequence and stiffening configuration, where stiffening configuration is a design variable that indicates the combination of axial, transverse and diagonal stiffener in the grid-stiffened cylinder. The design optimization process was adapted to identify the best suited stiffening configurations and stiffener spacings for grid-stiffened composite cylinder with the length and radius of the cylinder, the design in-plane loads and material properties as inputs. The effect of having axial membrane strain constraints in the skin and stiffener segments in the optimization process is also studied for selected stiffening configurations.

An assessment of shell theories for buckling ofcircular cylindrical laminated composite panels loaded inaxial compression

Abstract Buckling loads of circular cylindrical laminated composite panels are obtained usingSanders–Koiter (e.g. Sanders, 1959 ; Koiter, 1959 ) , Love (e.g. Love, 1927 ) and Donnell (e.g. Loo, 1957 ) shell theories with a first-order, shear-deformationapproach and a Rayleigh–Ritz method that accounts for different boundary conditions andmaterial anisotropy. Results obtained using Sanders–Koiter, Love, Donnell shell theories arecompared with those obtained from finite element simulations, where the curved panels aremodeled using nine-node quadrilateral continuum-based shell elements that are independent of anyshell theory. Comparisons with finite element results indicate that Donnells theory could be inerror for some lamination schemes and geometrical parameters.

Formulation of an improved smeared stiffener theory for buckling analysis of grid-stiffened composite panels

An improved smeared stiffener theory for stiffened panels is presented that includes skin-stiffener interaction effects. The neutral surface profile of the skin-stiffener combination is developed analytically using the minimum potential energy principle and statics conditions. The skin-stiffener interaction is accounted for by computing the bending and coupling stiffness due to the stiffener and the skin in the skin-stiffener region about a shift in the neutral axis at the stiffener. Buckling load results for axially stiffened, orthogrid, and general grid-stiffened panels are obtained using the smeared stiffness combined with a Rayleigh-Ritz method and are compared with results from detailed finite element analyses.

Penetration simulation for uncontained engine debris impact on fuselage-like panels using LS-DYNA

Modeling and simulation requirements for uncontained engine debris impact on fuselage skins are proposed and assessed using the tied-nodes-with-failure modeling approach for penetration. A finite element analysis is used to study the penetration of aluminum plates impacted by titanium impactors in order to simulate the effect of such uncontained engine debris impacts on aircraft fuselage-like skin panels. LS-DYNA is used in the simulations to model the impactor, test fixture frame and target barrier plate. The effects of mesh refinement, contact modeling, and impactor initial velocity and orientation are studied using a configuration for which limited test data are available for comparison.

An Irreversible Constitutive Law for Modeling the Delamination Process using Interface Elements

An irreversible constitutive law is postulated for the formulation of interface elements to predict initiation and progression of delamination in composite structures. An exponential function is used for the constitutive law such that it satisfies a multi-axial stress criterion for the onset of delamination, and satisfies a mixed mode fracture criterion for the progression of delamination. A damage parameter is included to prevent the restoration of the previous cohesive state between the interfacial surfaces. To demonstrate the irreversibility capability of the constitutive law, steady-state crack growth is simulated for quasi-static loading-unloading cycle of various fracture test specimens.

Optimal Design of Grid-Stiffened Composite Panels

A design strategy for optimal design of composite grid-stiffened panels subjected to global and local buckling constraints is developed using a discrete optimizer. An improved smeared stiffener theory is used for the global buckling analysis. Local buckling of skin segments is assessed using a Rayleigh-Ritz method that accounts for material anisotropy and transverse shear flexibility. The local buckling of stiffener segments is also assessed. Design variables are the axial and transverse stiffener spacing, stiffener height and thickness, skin laminate, and stiffening configuration, where the stiffening configuration is herein defined as a design variable that indicates the combination of axial, transverse, and diagonal stiffeners in the stiffened panel. The design optimization process is adapted to identify the lightest-weight stiffening configuration and stiffener spacing for grid-stiffened composite panels given the overall panel dimensions, in-plane design loads, material properties, and boundary conditions of the grid-stiffened panel.

Numerical simulations for high-energy impact of thin plates ☆

New approaches to the design of advanced aerospace systems requires an evaluation of extreme loading conditions and assessment of different possible failures scenario. One such scenario involves the high-energy foreign-object impact on relatively thin plates used in fuselage and wing applications. This paper describes a series of LS-DYNA numerical simulations for studying the impact and penetration of thin plates by small fragment impactors. This work supported the development of a gas-actuated penetration device at NASA Langley Research Center for high-energy impact testing of structures. The high-energy impact testing device is used for experimental simulation of uncontained engine failures. Threshold velocities for different combinations of pitch and yaw angles of the impactor were obtained for the impactor-target test configuration in the numerical simulations. Complete penetration of the target plate was also simulated numerically. Finally, a comparison of numerical and experimental results is presented for a complete penetration test of the target by the impactor.

Optimal design of grid-stiffened composite panels using global and local buckling analyses

A design strategy for optimal design of composite grid-stiffened panels subjected to global and local buckling constraints is developed using a discrete optimizer. An improved smeared stiffener theory is used for the global buckling analysis. Local buckling of skin segments is assessed using a Rayleigh-Ritz method that accounts for material anisotropy and transverse shear flexibility. The local buckling of stiffener segments is also assessed. Design variables are the axial and transverse stiffener spacing, stiffener height and thickness, skin laminate, and stiffening configuration. The design optimization process is adapted to identify the lightest-weight stiffening configuration and pattern for grid stiffened composite panels given the overall panel dimensions, design in-plane loads, material properties, and boundary conditions of the grid-stiffened panel.

Buckling analysis of anisotropic variable-curvature panels and shells

Abstract A buckling formulation for anisotropic variable-curvature panels is presented in this paper. The variable-curvature panel is assumed to consist of two or more panels of constant curvature where each panel may have a different curvature. Bezier functions are used as Ritz functions. Displacement ( C 0 ), and slope ( C 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.

PROGRESSIVE FAILURE STUDIES OF COMPOSITE PANELS WITH AND WITHOUT CUTOUTS

Progressive failure analyses results are presented for composite panels with and without a cutout and are subjected to in-plane shear loading and compression loading well into their postbuckling regime. Ply damage modes such as matrix cracking, fiber-matrix shear, and fiber failure are modeled by degrading the material properties. Results from finite element analyses are compared with experimental data. Good agreement between experimental data and numerical results are observed for most structural configurations when initial geometric imperfections are appropriately modeled.