Showing papers on "Composite laminates published in 1992"

Process-Induced Stress and Deformation in Thick-Section Thermoset Composite Laminates:

Abstract: A study of process-induced stress and deformation in thick-section thermosetting composite laminates is presented. A methodology is proposed for predict ing the evolution of residual stress develop...

Orthotropic Thermal Conductivity of Plain-Weave Fabric Composites Using a Homogenization Technique

Abstract: This article presents a new application of two-scale asymptotic homogenization schemes to predict the orthotropic thermal conductivity of plain-weave fabric reinforced composite laminates. A unit-cell, enclosing the characteristic periodic repeat pattern in the fabric weave, is isolated and modeled. A new three-dimensional series-parallel thermal resistance network is developed to solve a steady-state heat transfer boundary value problem (BVP) for this unit-cell. Laminate effective orthotropic thermal conductivities are obtained analytically and numerically as functions of (1) thermal conductivity of the constituent materials, (2) fiber volume fraction, and (3) weave style. The analytically predicted thermal conductivity values are compared with numerical finite element predictions, with existing models in the literature and with experimentally obtained values. 27 refs.

Non-Fickian Moisture Diffusion in Polymeric Composites

Abstract: This article concerns the effects of non-Fickian water diffusion in fiber-reinforced polymeric composites. The departure from classical diffusion is attributed to the time-dependent response of the polymer, akin to viscoelastic mechanical response.The purpose of the present work is to propose a methodology to reduce non-Fickian moisture weight gain data in a manner which enables the evaluation of the diffusion coefficient and moisture profiles across the thickness of composite laminates. The evaluation of those moisture profiles is essential to the computation of residual stresses within composite laminates.It is demonstrated that in some circumstances the non-Fickian moisture profiles result in residual hygro-thermal stresses which differ by about 25% from predictions based upon classical diffusion.

Stacking sequence optimization of simply supported laminates with stability and strain constraints

Abstract: An integer programming formulation for the design of symmetric and balanced rectangular composite laminates with simply supported boundary conditions subject to buckling and strain constraints is presented. The design variables that define the stacking sequence of the laminate are ply-identity zero-one integers. The buckling constraint is linear in terms of the ply-identity design variables, but strains are nonlinear functions of these variables. A linear approximation is developed for the strain constraints so that the problem can be solved by sequential linearization using the branch and bound algorithm. Examples of graphite-epox y plates under biaxial compression are presented. Optimum stacking sequences obtained using the linear approximation are compared with global optimum designs obtained using a genetic search procedure.

A Variational Approach to Three-Dimensional Elasticity Solutions of Laminated Composite Plates

Abstract: The displacements in a laminated composite are represented as products of two sets of unknown functions, one of which is only a function of the thickness coordinate and the other is a function of the in-plane coordinates (i.e., separation of variables approach), and the minimization of the total potential energy is reduced to a sequence of iterative linear problems. Analytical solutions are developed for cross-ply and angle-ply laminated composite rectangular plates. The solution for simply-supported cross-ply plates under sinusoidal transverse load reduces to that of Pagano. Numerical results for stresses and is placements for antisymmetric angle-ply laminates are presented. The three-dimensional elasticity solutions developed are important because they can be used to study the behavior of composite laminates, in addition to serving as reference for approximate solutions by numerical methods and two-dimensional theories.

A Parametric Study of Variables That Affect Fiber Microbuckling Initiation in Composite Laminates: Part 2 — Experiments

Abstract: The effects of the stacking sequence (orientation of plies adjacent to the 0-deg plies), free surfaces, fiber/matrix interfacial bond strength, initial fiber waviness, resin-rich regions, and nonlinear shear constitutive behavior of the resin on the initiation of fiber microbuckling in thermoplastic composites were investigated using nonlinear geometric and nonlinear 2D finite-element analyses. Results show that reductions in the resin shear tangent modulus, large amplitudes of the initial fiber waviness, and debonds each cause increases in the localized matrix shear strains; these increases lead in turn to premature initiation of fiber microbuckling. The numerical results are compared to experimental data obtained using three thermoplastic composite material systems: (1) commercial APC-2, (2) QUADRAX Unidirectional Interlaced Tape, and AU4U/PEEK.

Influence of Ply Waviness on the Stiffness and Strength Reduction on Composite Laminates

Abstract: An analytic model, based on two-dimensional laminated plate theory, is developed to investigate the influence of ply waviness on the stiffness and strength reduc tion of [90/0/90] laminate constructions. The model predicts:

Structural behavior of two-cell composite rotor blades with elastic couplings

Abstract: This paper presents an analytical-cum-experimental study of the structural response of composite rotor blades with elastic couplings. Vlasov theory is expanded to analyze two-cell composite rotor blades made out of general composite laminates including the transverse shear deformation of the crosssection. Variation of shear stiffness along the contour of the section is included in the warping function . In order to validate this analysis, two-cell graphite-epoxy composite blades with extension-torsion coupling were fabricated using matched-die molding technique. These blades were tested under tip bending and torsional loads, and their structural response in terms of bending slope and twist was measured with a laser optical system. Good correlation between theory and experiment is achieved. Axial force induced twist rate of the order of 0.2 degree per inch length can be realized in extensiontorsion coupled blades with a hygrothermally stable [20/-70]2s layup for potential applications in the design of tilt rotors.

Thermoviscoelastic analysis of residual stresses and warpage in composite laminates

Abstract: Linear thermoviscoelastic lamination theory was used to determine resid ual stresses and warpage in multidirectional woven-glass/epoxy laminates. The single "unidirectional" layer of the material was characterized by means of an accelerated proce dure. The material was assumed to be thermorheologically simple and the time-tempera ture superposition principle (TTSP) was used to obtain "master curves" for the viscoelastic properties.A numerical procedure was developed for determination of residual stresses and warp age, taking into consideration the irreversible polymerization shrinkage and boundary conditions during curing. Warpage was measured experimentally for [06/906] and [011/30] laminates using the projection moire method and results were in good agreement with the analytical prediction.

Composite laminate translaminar reinforcement apparatus and method

Abstract: A technique for the translaminar reinforcement in the z-axis direction of composite laminates utilizing an apparatus and method for heating and softening the composite laminates by ultrasonic energy, penetrating the composite laminate, moving the composite laminate fibers aside, inserting a reinforcing fiber into the composite laminate and allowing the composite laminate and reinforcement fiber to cool and bond.

Composite laminates with spatially varying fiber orientations - 'Variable stiffness panel concept'

Abstract: A solution has been obtained to the plane elasticity problem for a symmetrically laminated composite panel with spatially varying fiber orientations. Since variation of the fiber angles along the length of a composite laminate results in stiffness properties that change as a function of location, the laminates are called variable stiffness panels. An analysis of the stiffness variation and its effect on the elastic response of the panel is presented here. A numerical solution has been obtained using an iterative collocation technique. Corresponding closed-form solutions are given for three different sets of boundary conditions. Two of the cases considered have exact solutions and thus serve to validate the numerical model.

Effects of Interply Damping Layers on the Dynamic Characteristics of Composite Plates

Abstract: Integrated damping mechanics for composite plates with constrained interlaminar layers of polymer damping materials are developed. Discrete layer damping mechanics are presented for composite laminates with damping layers, in connection with a semianalytical method for predicting the modal damping in simply supported specialty composite plates. Correlations between predicted and measured response in graphite/epox y plates illustrate the accuracy of the method. Additional application cases for graphite/epoxy plates of various laminations demonstrate the potential for higher damping than geometrically equivalent aluminum plates. The effects of aspect ratio, damping layer thickness, and fiber volume ratio on static and dynamic characteristics of the composite plate are also investigated. AMPING is a significant dynamic parameter for vibration and sound control, dynamic stability, positioning accu- racy, fatigue endurance, and impact resistance. Many current structural applications (e.g., large space structures, engine blades, and high-speed machinery) require light weight and high dynamic performance. Therefore, candidate sources of passive damping should add minimal parasitic weight and be compatible with the structural configuration. Two potential damping sources satisfying the previous re- quirements are the constrained damping layer approach and the damping capacity of composites. Constrained damping layers in isotropic metallic structures have been widely applied and investigated.1 They provide high damping, but tend to increase the structural weight and offer limited means for damping tailoring. The inherent damping capacity of compos- ite materials also seems promising. Although the damping of composite structures is not very high, it is significantly higher than that for most common metallic structures. Moreover, composites are the materials of preference in many cases, since they readily provide high specific stiffness and strength. More importantly, research on the damping mechanics of composite laminates2'4 and structures5'6 has shown that composite damp- ing is anisotropic, highly tailorable, and depends on an array of micromechanic al, laminate, and structural parameters. It has been further demonstrated that optimal tailoring may significantly improve the damped dynamic performance of composite structures.7 It seems likely that the combination of both approaches (i.e., composite structures with interlaminar damping layers) will offer the advantages of high damping, damping tailoring, good mechanical properties, and low weight addition. In addi- tion, the interlaminar damping concept is highly compatible with the laminated configuration of composite structures and their fabrication techniques. In contrast to isotropic materials, the variations in anisotropy and elastic properties of each

Compression Response of Thick Layer Composite Laminates with Through-the-Thickness Reinforcement

Abstract: Compression and compression-after-impact (CAI) tests were conducted on seven different AS4-3501-6 (0/90) 0.64-cm thick composite laminates. Four of the seven laminates had through-the-thickness (TTT) reinforcement fibers. Two TTT reinforcement methods, stitching and integral weaving, and two reinforcement fibers, Kevlar and carbon, were used. The remaining three laminates were made without TTT reinforcements and were tested to establish a baseline for comparison with the laminates having TTT reinforcement. Six of the seven laminates consisted of nine thick layers whereas the seventh material was composed of 46 thin plies. The use of thick-layer material has the potential for reducing structural part cost because of the reduced part count (layers of material). The compression strengths of the TTT reinforced laminates were approximately one half those of the materials without TTT reinforcements. However, the CAI strengths of the TTT reinforced materials were approximately twice those of materials without TTT reinforcements. The improvement in CAI strength is due to an increase in interlaminar strength produced by the TTT reinforcement. Stitched laminates had slightly higher compression and CAI strengths than the integrally woven laminates.

Strength scaling in fiber composites

Abstract: A research program was initiated to study and isolate the factors responsible for scale effects in the tensile strength of graphite/epoxy composite laminates. Four layups were chosen with appropriate stacking sequences so as to highlight individual and interacting failure modes. Four scale sizes were selected for investigation including full scale size, 3/4, 2/4, and 1/4, with n = to 4, 3, 2, and 1, respectively. The full scale specimen sizes was 32 piles thick as compared to 24, 16, and 8 piles for the 3/4, 2/4, and 1/4 specimen sizes respectively. Results were obtained in the form of tensile strength, stress-strain curves and damage development. Problems associated with strength degradation with increasing specimen sizes are isolated and discussed. Inconsistencies associated with strain measurements were also identified. Enhanced x ray radiography was employed for damage evaluation, following step loading. It was shown that fiber dominated layups were less sensitive to scaling effects compared to the matrix dominated layups.

Performance of graphite/bismaleimide laminates with embedded optical fibers. I. Uniaxial tension

Abstract: Optical fibers should not compromise structural integrity when embedded in smart composite structures for strain sensing. In particular, the tensile mechanical properties are of interest due to their use as basic design data for most applications. This study quantified the effects of the orientation and location of embedded optical fibers on the uniaxial tensile performance of graphite/bismaleimide (Gr/BMI) laminates. To investigate these relations, experimental strength and stiffness data were obtained and compared for eight different test configurations, each comprising five to eleven specimens fabricated with acrylate-coated 250 mu m diameter optical fibers embedded symmetrically or asymmetrically with respect to the laminate midplane, and parallel or perpendicular to the applied uniaxial loading and/or adjacent reinforcing fiber directions. All specimens were manufactured from G40-600/5245C Gr/BMI prepreg with a (03/902/0)s stacking sequence. Optical fibers were found to only modestly reduce the tensile properties of composite laminates (up to 10%) when embedded parallel or perpendicular to the applied uniaxial loading and/or adjacent reinforcing fiber directions. The largest reductions occurred in composite laminates with optical fibers embedded perpendicular to the loading direction and the adjacent graphite fibers. Thus, in order to decrease the extent of structural degradation, optical fibers should be embedded parallel to both the loading direction and adjacent reinforcing fibers.

Multiobjective shape and material optimization of composite structures including damping

Abstract: A multiobjective optimal design methodology is developed for lightweight, low-cost composite structures of improved dynamic performance. The design objectives may include minimization of damped resonance amplitudes (or maximization of modal damping), weight, and material cost. The design vector includes micromechanics, laminate, and structural shape parameters. Constraints are imposed on static displacements, static and dynamic ply stresses, dynamic amplitudes, and natural frequencies. The effects of composite damping tailoring on the dynamics of the composite structure are incorporated. Applications on a cantilever composite beam and plate illustrate that only the proposed multiobjective formulation, as opposed to single objective functions, may simultaneously improve the objectives. The significance of composite damping in the design of advanced composite structures is also demonstrated, and the results indicate that the minimum-weight design or design methods based on undamped dynamics may fail to improve the dynamic performance near resonances. Nomenclature A = area [C],[c] = global and modal damping matrices, respectively E = normal modulus F(z) = objective functions / = frequency fd = damped frequency G = shear modulus G(z) = inequality constraints h = thickness [/£],[/:] = global and modal stiffness matrices, respectively k = volume ratio [M],[m] = global and modal mass matrices, respectively p^p = global and modal excitation force, respectively q — modal vector 5 = strength t = time U = dynamic amplitude u = displacement vector

Non-linear finite strip analysis of rectangular laminates under end shortening, using classical plate theory

Abstract: The finite strip method is developed for predicting the geometrically non-linear response of rectangular composite laminates with simply supported ends when subjected to uniform end shortening in their plane. At the loaded ends lateral in-plane expansion may be allowed freely or may be prevented completely in different versions of the approach. The permitted laminate material properties are quite general, encompassing anisotropy and full coupling between in-plane and out-of-plane behaviour. The analysis is based on the use of the classical plate theory and the non-linearity is introduced in the strain-displacement equations in the manner of the von Karman assumptions. Three different types of finite strip are presented with either linear, quadratic or cubic interpolation of the membrane displacement components across a strip: in each case the bending displacement component varies cubically across a strip. Results are presented for isotropic plates and for unsymmetric cross-ply, angle-ply and arbitrary laminates.

Nonlinear flutter of two-dimensional simply supported symmetric composite laminated plates

Abstract: The nonlinear flutter behavior of a two-dimensional simply supported symmetric composite laminated plate at high supersonic Mach number has been investigated. Yon Karman's large deflection plate theory and quasisteady aerodynamic theory have been employed. Galerkin's method has been used to reduce the governing equations to a system of nonlinear ordinary differential equations in time, which are then solved by a direct numerical integration method. Nonlinear flutter results are presented with the effects of aerodynamic damping, in-plane force, static pressure differential, and anisotropic properties. Results show that the anisotropic properties such as fiber orientation and elastic modulus ratio have significant effects on the behavior of both limit cycle oscillation and chaotic motion.