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At least 451 records · Page 25

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NASA NTRS1987

Optimal cure cycle design of a resin-fiber composite laminate

A unified computed aided design method was studied for the cure cycle design that incorporates an optimal design technique with the analytical model of a composite cure process. The preliminary results of using this proposed method for optimal cure cycle design are reported and discussed. The cure process of interest is the compression molding of a polyester which is described by a diffusion reaction system. The finite element method is employed to convert the initial boundary value problem into a set of first order differential equations which are solved simultaneously by the DE program. The equations for thermal design sensitivities are derived by using the direct differentiation method and are solved by the DE program. A recursive quadratic programming algorithm with an active set strategy called a linearization method is used to optimally design the cure cycle, subjected to the given design performance requirements. The difficulty of casting the cure cycle design process into a proper mathematical form is recognized. Various optimal design problems are formulated to address theses aspects. The optimal solutions of these formulations are compared and discussed.

Hou, Jean W.↗

NASA NTRS1988

Fracture analysis of local delaminations in laminated composites

A shear deformation model was developed to analyze local delaminations growing from transverse cracks in 90 degree plies located around the mid plane of symmetric laminates. The predictions of the model agree reasonably with experimental data from T300/934 graphite epoxy laminates. The predicted behavior is such that, in combination with an edge delamination model, the critical loads can be predicted accurately in the range of n from .5 to 8.

Sriram, P.↗

NASA NTRS1989

Fracture analysis of local delaminations in laminated composites

Armanios, E. A.↗

NASA NTRS1990

Stationary and nonstationary random vibrations of laminated composite plates via a higher order theory

Higher order shear deformation theory is utilized to study he weakly stationary and nonstationary random vibrations of cross-ply laminated plates. Normal mode method, in conjunction with the biorthogonality condition, for the nonsymmetric differential equations is applied. Detailed derivation is given for the governing equations, biorthogonality condition, the generalized mass and the generalized forces. Results are listed for a plate which is simply supported at all the edges, and subjected to a point load which is either timewise stationary or nonstationary random process.

Elishakoff, Isaac↗

NASA NTRS1981

Bifurcation and post-buckling analysis of laminated composite plates via reduced basis technique

A reduced basis technique and a problem-adaptive computational algorithm are presented for the bifurcation and post-buckling analysis of laminated anisotropic plates. The computational algorithm can be conveniently divided into three distinct stages. The first stage is that of determining the bifurcation point. The plate is discretized by using displacement finite element (or finite difference) models. The special symmetries exhibited by the response of the anisotropic plate are used to reduce the size of the analysis region. The vector of unknown nodal parameters is expressed as a linear combination of a small number of basis vectors, and a Rayleigh-Ritz technique is used to approximate the finite element equations by a small system of algebraic equations. The reduced equations are used to determine the bifurcation point and the associated eigenmode of the panel. In the second stage of the bifurcation buckling mode is used to obtain a nonlinear solution in the vicinity of the bifurcation point and new (updated) sets of basis vectors and reduced equations are generated. In the third stage the reduced equations are used to trace the post-buckling paths. The effectiveness of the proposed technique for predicting the bifurcation and post-buckling behavior of plates is demonstrated by means of numerical examples for plates loaded by means of prescribed edge displacements.

Noor, A. K.↗

NASA NTRS1988

A model for predicting damage dependent damping in laminated composites

A constitutive model has been developed for predicting the dynamic properties of composite beams which accounts for the influence of microstructural damage. The model assumes the damage state to be fixed. The model can account for both the damping increase and stiffness loss due to microstructural damage. Damage dependent material constants obtained from a baseline data set can be used to predict damping changes for several composite layups. The validity of the model has been verified by comparing the experimental results to model predictions for crossply laminates with a variety of stacking sequences.

Kalyanasundaram, S.↗

NASA NTRS1989

Internal state variable approach for predicting stiffness reductions in fibrous laminated composites with matrix cracks

A mathematical model utilizing the internal state variable concept is proposed for predicting the upper bound of the reduced axial stiffnesses in cross-ply laminates with matrix cracks. The axial crack opening displacement is explicitly expressed in terms of the observable axial strain and the undamaged material properties. A crack parameter representing the effect of matrix cracks on the observable axial Young's modulus is calculated for glass/epoxy and graphite/epoxy material systems. The results show that the matrix crack opening displacement and the effective Young's modulus depend not on the crack length, but on its ratio to the crack spacing.

Lee, Jong-Won↗

NASA NTRS1991

Fracture analysis of transverse crack-tip and free-edge delamination in laminated composites

A shear deformation model including hygrothermal effects is developed for the analysis of local delaminations originating from transverse cracks in 90-deg plies located in and around the laminate midplane. A sublaminate approach is used and the model is applied to (+/- 25/90n)s T300/934 graphite/epoxy laminates for n values between 0.5 and 8, along with previously developed edge-delamination shear-deformation models. Critical loads and delamination modes are identified and compared with experimental results. Hygrothermal effects are included in all the models to make the comparisons realistic.

Armanios, Erian A.↗

NASA NTRS1993

Analysis of the anisotropic viscoplastic-damage response of composite laminates - Continuum basis and computational algorithms

The mathematical structure underlying the rate equations of a recently-developed constitutive model for the coupled viscoplastic-damage response of anisotropic composites is critically examined. In this regard, a number of tensor projection operators have been identified, and their properties were exploited to enable the development of a general computational framework for their numerical implementation using the Euler fully-implicit integration method. In particular, this facilitated (i) the derivation of explicit expressions of the (consistent) material tangent stiffnesses that are valid for both three-dimensional as well as subspace (e.g. plane stress) formulations, (ii) the implications of the symmetry or unsymmetry properties of these tangent operators from a thermodynamic standpoint, and (iii) the development of an effective time-step control strategy to ensure accuracy and convergence of the solution. In addition, the special limiting case of inviscid elastoplasticity is treated. The results of several numerical simulations are given to demonstrate the effectiveness of the schemes developed.

Saleeb, A. F.↗

NASA NTRS1981

Boundary layer thermal stresses in angle-ply composite laminates, part 1

Thermal boundary-layer stresses (near free edges) and displacements were determined by a an eigenfunction expansion technique and the establishment of an appropriate particular solution. Current solutions in the region away from the singular domain (free edge) are found to be excellent agreement with existing approximate numerical results. As the edge is approached, the singular term controls the near field behavior of the boundary layer. Results are presented for cases of various angle-ply graphite/epoxy laminates with (theta/-theta/theta/theta) configurations. These results show high interlaminar (through-the-thickness) stresses. Thermal boundary-layer thicknesses of different composite systems are determined by examining the strain energy density distribution in composites. It is shown that the boundary-layer thickness depends on the degree of anisotropy of each individual lamina, thermomechanical properties of each ply, and the relative thickness of adjacent layers. The interlaminar thermal stresses are compressive with increasing temperature. The corresponding residual stresses are tensile and may enhance interply delaminations.

Wang, S. S.↗

NASA NTRS1987

Influence of large deflection and transverse shear on random response of rectangular symmetric composite laminates to acoustic loads

Nonlinear equations of motion of symmetrically laminated anisotropic plates are derived accounting for von Karman strains. The effect of transverse shear is included in the formulation and the rotatory inertia effect is ignored. Using a single-mode Galerkin procedure the nonlinear modal equation is obtained. Direct equivalent linearization is employed. The response of acoustic excitation on moderately thick composite panels is studied. Further, the effects of transverse shear on large deflection vibration of laminates under random excitation are studied. Mean-square deflection and mean-square inplane stresses are obtained for some symmetric graphite-epoxy laminates. Using equilibrium equations and the continuity requirements, the mean-square transverse shear stresses are calculated. The results obtained will be useful in the sonic fatigue design of composite aircraft panels. The analysis is presented in detail for simply supported plate. The analogous equations for a clamped case are given in the appendix.

Mei, Chuh↗

NASA NTRS1987

Effects of large deflection and transverse shear on response of rectangular symmetric composite laminates subjected to acoustic excitation

Nonlinear equations of motion of symmetrically laminated anisotropic plates are derived accounting for von Karman strains. The effect of transverse shear is included in the formulation. Using a single-mode Galerkin procedure, the nonlinear modal equation is obtained. The direct equivalent linearization method is employed for solution of this equation. The response to acoustic excitation of moderately thick composite panels is studied. Further, the effects of transverse shear on large deflection vibration of laminates under random excitation are studied. Mean-square deflections and mean-square inplane stresses are obtained for symmetric graphite-epoxy laminates. Using equilibrium equations and the continuity requirements, the mean-square transverse shear stresses are calculated. The results obtained will be useful in the sonic fatigue design of composite aircraft panels. The analysis is presented in detail for simply supported plates. The analogous equations for a clamped case are also obtained.

Mei, Chuh↗

NASA NTRS1984

The nonlinear viscoelastic response of resin matrix composite laminates

Possible treatments of the nonlinear viscoelastic behavior of materials are reviewed. A thermodynamic based approach, developed by Schapery, is discussed and used to interpret the nonlinear viscoelastic response of a graphite epoxy laminate, T300/934. Test data to verify the analysis for Fiberite 934 neat resin as well as transverse and shear properties of the unidirectional T300/934 composited are presented. Long time creep characteristics as a function of stress level and temperature are generated. Favorable comparisons between the traditional, graphical, and the current analytical approaches are shown. A free energy based rupture criterion is proposed as a way to estimate the life that remains in a structure at any time.

Hiel, C.↗

NASA NTRS1993

Compression strength failure mechanisms in unidirectional composite laminates containing a hole

Experiments on graphite-epoxy laminated plates containing unloaded small holes show that these laminates are notch insensitive. That is, the uniaxial strength of these laminates with small holes exceeds the strength predicted by a point stress criterion using the stress concentration factor for the in-plane stress field. Laminates containing large holes exhibit notch sensitive behavior and consequently their strength is reasonably well predicted by the stress concentration effect. This hole size effect is manifested both in tension and in compression. Apparently, some mechanism must cause in-plane stress relief for laminates containing small holes. The purpose of this research was to study the influence of geometric nonlinearity on the micromechanical response of a filamentary composite material in the presence of a strain gradient caused by a discontinuity such as a hole. A mathematical model was developed at the micromechanical level to investigate this geometrically nonlinear effect.

Johnson, Eric R.↗

NASA NTRS1989

A shear deformable theory of laminated composite shallow shell-type panels and their response analysis. II - Static response

In the second part of this paper, by using the static counterparts of the governing equations derived in Librescu (1989), the static response of shallow composite shell-type panels subjected to a sinusoidal transverse load is investigated. The numerical applications, encompassing a large number of boundary conditions and various lamination schemes, allow one to obtain some conclusions which are formulated in the paper.

Khdeir, A. A.↗

NASA NTRS1989

Micromechanics of compression failures in open hole composite laminates

The local softening in the damage zone that precedes compression failures in laminates with holes was modeled using a Dugdale-type approach, and the calculated load to damage-size relationships were compared to the actual data obtained in two composite systems, AS4/PEEK and T2C145/F155, loaded to compression. The comparison indicates that the Dugdale model, which works well for metals, did not accurately predict the load-damage zone size relationship in these specimens. Possible reasons for the poor agreement are considered to be the fact that the constitutive relationship used in the Dugdale model is not an accurate description of material response in the crush zone, and the fact that the crush zone size is measured on the surface of the specimen, while this size is not an accurate indication of the actual crush zone size through the laminate.

Guynn, E. Gail↗

NASA NTRS1993

Toughened High-Temperature Thermoset Composites

Laminated composites of carbon fibers and either PMR-15 or LARC(TM)RP-46 polyimides toughened selectively by incorporating thin layers of Matrimid(TM) 5218 thermoplastic polyimide at interfaces between plies to form gradient semi-interpenetrating microstuctures. Toughening of composites accompanied by acceptably small decreases in strength and stiffness. Thick laminates of toughened composites fabricated easily at 500 psi. Materials useful in components of aircraft engines, where exposed to high temperatures for short times.

Johnston, Norman J.↗

NASA NTRS1989

Effects of transverse shear on large deflection random response of symmetric composite laminates with mixed boundary conditions

Nonlinear equations of motion of symmetrically laminated anisotropic plates are derived considering von Karman strains and transverse shear effects. Using a single-mode Galerkin procedure, the nonlinear modal equation is obtained. The direct equivalent linearization method is employed for solution of this equation. The effects of transverse shear on large deflection vibration of laminated plates with mixed boundary support conditions under random excitation are studied. Mean-square deflections and mean-square inplane strains are obtained for symmetric graphite-epoxy laminates. Using equilibrium equations and the continuity requirements, the mean-square transverse shear stresses are calculated. The results obtained will be useful in the sonic fatigue design of composite aircraft panels. The analysis is presented in detail for plates with two opposing edges simply supported and the remaining edges clamped.

Prasad, C. B.↗