Showing papers on "Composite laminates published in 1990"

Progressive Transverse Cracking of Crossply Composite Laminates

Abstract: A simplified shear lag analysis using a progressive damage scheme is pro posed for crossply composite laminates under uniaxial tensile loading. Closed form solu tions for stress distributions, tran...

Strength of Tensile Loaded Graphite/Epoxy Laminates Containing Cracks, Open and Filled Holes

Abstract: Most methods currently used for predicting tensile strength of composite laminates containing holes and cracks adopt a characteristic distance approach such as the Point Stress Criterion (PSC) or the Average Stress Criterion (ASC). These and similar ap proaches are attractive to designers since they are simple to apply. The limitation of such approaches, however, is that the characteristic distance is not a physical parameter but is an empirically determined constant, dependent on the geometry of the specimen. The Damage Zone Criterion (DZC) presented in this paper is based on fundamental physical principles. A damage zone is assumed to be present in the maximum stress region of the laminate when the tensile stress reaches the tensile strength of the unnotched laminate. Taking into account the stress redistribution followed by a damage development, a closed form analytical expression of the strength is derived from the equilibrium conditions of the specimen. The strength of laminates with holes and cracks...

A Stiffness Degradation Model for Graphite/Epoxy Laminates

Abstract: Stiffness reduction in composite laminates is an important measure of fa tigue damage. The determination of fatigue damage and the prediction of fatigue life can be made through the development of a stiffness degradation model. This paper proposes a stiffness degradation model that can be used to predict the statistical distribution of the residual stiffness of composite laminates subjected to fatigue cycling. Based on the pro posed model, two analytical methods are presented, which are capable of predicting the stiffness degradation of a particular composite specimen under cyclic loading. One method is based on the linear regression analysis and the other on the Bayesian approach. Experi ments have been performed on graphite/epoxy [90, +45, —45,0], laminates to generate sta tistically significant data for evaluating the proposed analytical models and for verifying the predicted results. It is shown that theoretical predictions for the stiffness degradation of an individual specimen and for the statistica...

Towards a Damage Tolerance Philosophy for Composite Materials and Structures

Abstract: A damage-threshold/fail-safe approach is proposed to ensure that composite structures are both sufficiently durable for economy of operation, as well as adequately fail-safe or damage tolerant for flight safety. Matrix cracks are assumed to exist throughout the off-axis plies. Delamination onset is predicted using a strain energy release rate characterization. Delamination growth is accounted for in one of three ways: either analytically, using delamination growth laws in conjunction with strain energy release rate analyses incorporating delamination resistance curves; experimentally, using measured stiffness loss; or conservatively, assuming delamination onset corresponds to catastrophic delamination growth. Fail-safety is assessed by accounting for the accumulation of delaminations through the thickness. A tension fatigue life prediction for composite laminates is presented as a case study to illustrate how this approach may be implemented. Suggestions are made for applying the damage-threshold/fail-safe approach to compression fatigue, tension/compression fatigue, and compression strength following low velocity impact.

An accurate determination of stresses in thick laminates using a generalized plate theory

Abstract: Analytical solutions for displacements and stresses in composite laminates are developed using the laminate plate theory of Reddy. The theory accounts for a desired degree of approximation of the displacements through the laminate thickness, allowing for piecewise approximation of the inplane deformation through individual laminae. The solutions are compared with the 3-D elasticity solutions for the simply supported case and excellent agreement is found. Analytical solutions are also presented for other boundary conditions. The results indicate that the generalized shear deformation plate theory predicts accurate stress distributions in thick composite laminates.

Mode II interlaminar fracture of the center notch flexural specimen under impact loading

Abstract: The Mode II interlaminar fracture of unidirectional graphite/epoxy (AS4/2220-3) and graphite/polyetheretherketone (APC-2) composite laminates subjected to impact loading is investigated using the Center Notch Flexural (CNF) test specimen. Experimentally, instrumented impact testing of the CNF specimen enables the impact force history and the absorbed energy during delamination propagation to be estimated. Scanning electron microscopic examination is conducted to assess the influence of impact loading on fracture morphology in the crack initiation and propagation regions of the frac ture surface. Data reduction for evaluating the Mode II interlaminar initiation toughness is based upon beam theory that includes kinetic energy effects and dynamic finite element analysis of the specimen in conjunction with virtual crack closure techniques. The initia tion toughness under impact loading was approximately 20 and 28% lower than the static values for AS4/2220-3 and APC-2, respectively. Data reduction for evaluati...

On the bearing strength of bolted graphite/epoxy laminates

Abstract: One of the basic failure modes of bolted composite laminates is bearing fail ure. This mode of failure occurs in the material immediately adjacent to the contact points of bolt and laminate, and is caused primarily by compressive stresses acting on the hole boundary. The results of an experimental program which measured the bearing strengths of two different types of graphite/epoxy specimens are presented. The influence on the bearing strength of several important parameters including lateral constraint and ply orien tation are shown. Strains were measured in the vicinity of the loaded hole to quantify the extent of bearing failure. A new approach to predicting bearing failure is outlined. The Delamination Buckling Model (DBM) assumes that ply buckling following delaminations is a major feature of bearing failure. Quantitatively good agreement was found between ex periment and model results.

A failure analysis of water jet drilling in composite laminates

Abstract: Delamination is a major concern in the manufacturing processes of composite materials. It reduces not only the structural integrity of the laminate but also the long-term reliability of the assembly. Water jet drilling, in spite of its advantages of no tool wear and thermal damage, often creates delamination composite laminate at bottom. The current paper presents an analytical approach to study the delamination during drilling by water jet piercing. The analysis uses fracture mechanics with plate theory to describe the mechanism of delamination. This model predicts an optimal water jet pressure for no delamination as a function of hole depth and material parameters (opening-mode delamination fracture toughness and modulus of elasticity). Good agreement is achieved with data obtained from water jet drilling of graphite epoxy laminate. The predicted optimal water jet pressure can be applied in a control scheme for maximizing the productivity of water jet drilling of composite laminates.

Determination of Interlaminar Stresses in Composite Laminates under Combined Loads

Abstract: A method to determine the complete state of stress at free edges of com posite laminates under combined loads (uniaxial tension or compression, moment, and out-of-plane shear) is presented. Based on the principle of minimum complementary en ergy, two coupled ordinary differential equations are derived with use of calculus of varia tions. The stresses are determined in closed form and are in good agreement with other solutions published in the literature for uniaxial tension and applied moment. This solu tion is applicable to general laminates with straight free edges and is computationally very efficient.

Nonlinear analysis of composite laminates using a generalized laminated plate theory

Abstract: A plate-bending finite element based on the theory of Reddy is developed. Stress relaxation is shown to be an important factor in the design of composite plates.

Lamb Wave Propagation in Anisotropic Laminates

Abstract: The wave propagation in arbitrary anisotropic laminates is investigated on the basis of an exact theory. The dispersion relations of Lamb waves are determined for graphite/epoxy symmetric angle-ply laminates and hybrid composite ones which consist of carbon/epoxy and glass/epoxy layers. The dispersion and anisotropy of phase velocities for fundamental modes are discussed in detail. The energy distributions in the thickness direction of laminates are calculated for each kind of Lamb wave. A hybrid composite laminate is found to have better capability in absorbing impact energy by analyzing the strain energy distribution during the wave propagation. The results of the strain energy distribution are useful in determining the arrangement and the fiber orientation of the layers of hybrid composite laminates.

Mechanics of damping for fiber composite laminates including hygrothermal effects

Abstract: An integrated mechanics theory was developed for the modeling of composite damping from the micromechanics to the laminate level. Simplified, design oriented equations based on hysteretic damping are presented for on-axis plies, off-axis plies, and laminates including the effect of temperature, moisture, and interply hysteretic damping. The temperature rise within vibrating composite laminates resulting from strain energy dissipation is also modeled, and their coupled hygro-thermo-mechanical response is predicted. The method correlates well with reported damping measurements. Application examples illustrate the effect of various ply, laminate, and hygro-thermal parameters on the overall damping performance of composite laminates.

Thermal and Mechanical behavior of Metal Matrix and Ceramic Matrix Composites

Abstract: The present conference discusses local stresses in metal-matrix composites (MMCs) subjected to thermal and mechanical loads, the computational simulation of high-temperature MMCs' cyclic behavior, an analysis of a ceramic-matrix composite (CMC) flexure specimen, and a plasticity analysis of fibrous composite laminates under thermomechanical loads. Also discussed are a comparison of methods for determining the fiber-matrix interface frictional stresses of CMCs, the monotonic and cyclic behavior of an SiC/calcium aluminosilicate CMC, the mechanical and thermal properties of an SiC particle-reinforced Al alloy MMC, the temperature-dependent tensile and shear response of a graphite-reinforced 6061 Al-alloy MMC, the fiber/matrix interface bonding strength of MMCs, and fatigue crack growth in an Al2O3 short fiber-reinforced Al-2Mg matrix MMC.

Tensile Strength of Stitching Joint in Woven Glass Fabrics

Abstract: Stitching has been found to be able to improve the interlaminar strength of composite laminates. Its application as composite joining has been also explored. This study examined the tensile strength of some stitched composite beams made of vomen glass fabric and epoxy matrix. The effects of stitching parameters on joining strength were evaluated by both experimental technique and finite element method

Tensile strength of stitching joint in women glass fabrics

Abstract: Stitching has been found to be able to improve the interlaminar strength of composite laminates. Its application as composite joining has been also explored. This study examined the tensile strength of some stitched composite beams made of vomen glass fabric and epoxy matrix. The effects of stitching parameters on joining strength were evaluated by both experimental technique and finite element method

Ultrasonic Reflection and Guided Waves in Fluid-Coupled Composite Laminates

Abstract: In this article, we review the present authors' own approach to elastic wave modeling and experimental measurements in fibrous composites. The materials and structural problem addressed here concerns the propagation of guided elastic leaky waves in continuous-fiber composite plates. The guided wavevector may be oriented along the fibers or in an arbitrary azimuthal direction. These plates can be structured as single-layer or multilayer media, where each successive layer contains fibers in different directions. Further, the multiaxial plates are loaded by a fluid or by different fluids on each boundary. Each of these cases has been investigated both experimentally and theoretically. It is found that some reasonable approximations lead to significant simplifications in treating these complicated structures and yet preserve the accuracy needed to make useful predictions of realistic sound wave behavior. Comparisons of the results of model calculations and experimental measurements of ultrasonic reflection show very good agreement over a wide range of experimental parameters and types of composites. Suggestions are offered at the end of the article for extensions of the modeling to account for non-ideal behavior of the materials and the chosen means of interrogation.

Analysis of Thick Section Composite Laminates Using Effective Moduli

Abstract: Three-dimensional effective moduli are used in the analysis of thick lami nates consisting of a large number of repeating identical sublaminates. Local stresses and strains in the laminae are recovered from the global average stresses and strains. A global- local method is developed in which the effective modulus theory is used to model the thick laminate except for the local region where accurate stresses are desired. Evaluative ex amples involving delamination cracks and free edge stresses are presented.

Failure Analysis of Composite Laminates with Free Edge

Abstract: A failure theory which combines classical lamination theory with free-edge interlaminar stress was employed to predict the strength of laminates with free edges. Laminates studied were [±θ]2s angle-ply laminates, [±θ/90]s laminates, and fiber-dominated π/4 laminates. The present theory was capable of predicting failure loads for [±θ]2s and [±θ/90]s laminates as well as delamination on-set loads for π/4 laminates.

Laminate Behavior for SiC Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites

Abstract: The room temperature mechanical properties of SiC fiber reinforced reaction-bonded silicon nitride matrix composite laminates (SiC/RBSN) have been measured. The laminates contained approx 30 volume fraction of aligned 142-micron diameter SiC fiber in a porous RBSN matrix. Three types of laminate studied were unidirectional: (1) (0) sub 8, (2) (10) sub 8, and (3) (45) sub 8, and (90) sub 8; cross plied laminates (0 sub 2/90 sub 2); and angle plied laminates: (+45 sub 2/-45 sub 2). Each laminate contained eight fiber plies. Results of the unidirectionally reinforced composites tested at various angles to the reinforcement direction indicate large anisotropy in in-plane properties. In addition, strength properties of these composites along the fiber direction were independent of specimen gage length and were unaffected by notches normal to the fiber direction. Splitting parallel to the fiber at the notch tip appears to be the dominant crack blunting mechanism responsible for notch insensitive behavior of these composites. In-plane properties of the composites can be improved by 2-D laminate construction. Mechanical property results for (0 sub 2/90 sub 2)sub s and (+45/-45 sub 2) sub s laminates showed that their matrix failure strains were similar to that for (0) sub 8 laminates, but their primary elastic moduli, matrix cracking strengths, and ultimate composite strengths were lower. The elastic properties of unidirectional, cross-ply, and angle-ply composites can be predicted from modified constitutive equations and laminate theory. Further improvements in laminate properties may be achieved by reducing the matrix porosity and by optimizing the bond strength between the SiC fiber and RBSN matrix.

Degradation of graphite/bismaleimide laminates with multiple embedded fiber-optic sensors

Abstract: This paper summarizes the reduction in mechanical properties of graphite/bismaleimide (Gr/BMI) laminates due to the presence of multiple embedded fiber-optic sensors. Previous work has shown that small quantities of optical fibers embedded parallel to the loading direction have only a negligible effect on the tension behavior of laminated structures. This work establishes the significance of larger quantities of embedded optical fibers on the tensile and compressive behavior of composite laminates. Experimental strength and modulus data from six test groups of seven tension and seven compression specimens are compared to a control group. All specimens were fabricated from G40-600/5245C Gr/BMI pre-preg tape to form a [°3/9o2/°]s stacking sequence. The quantity of optical fibers embedded parallel to the loading direction was varied in the different test groups. Uniaxial compression testing using an IITRI fixture and uniaxial tension testing were performed in accordance with ASTM Standards. Catastrophic failure was induced by fiber fracture. Results indicate that large quantities of embedded optical fibers reduce the static tensile strength up to 4% and stiffness up to 9%, while reducing the static compressive strength up to 24% and stiffness up to 20%. It is concluded that large quantities of embedded optical fibers result in significant degradation of the compressive strength of laminated composite structures, but do not significantly affect the tensile behavior.

Determination of Chemical Cure Shrinkage in Composite Laminates

Abstract: Residual stresses in composites (microscopic and macroscopic) result from shrinkage of the matrix during cure. It is usual to distinguish between the chemical and thermal components of shrinkage. The former depends on the polymerization reaction, whereas the latter is caused by the difference between the thermal expansion coefficients of the reinforcement and the matrix. A method was developed for determination of the chemical cure shrinkage. A prepreg layer is applied on an identical layer of the same material that has already been cured and postcured. The assembled laminate is then cured, and the resulting warpage is recorded by means of the shadow moire method. The resulting warpage, consisting of elliptical equal deflection contours, is related to the chemical shrinkage.

Torsional Response of Rectangular Composite Laminates

Abstract: A general closed-form relationship was derived between torque and angle of twist in terms of the geometric parameters and shear properties of the laminae. In the case of unidirectional laminates, relations can be expressed involving the three principal shear moduli, G 12 , G 13 , and G 23 . An experimental method was also developed for determining these moduli by measuring surface and edge strains with strain gages

Optimal Use of Adhesive Layers in Reducing Impact Damage in Composite Laminates

Abstract: Impact experiments on a group of cross-plied graphite/epoxy laminates with and without adhesive layers were conducted. Impact-damage modes were examined by sectioning the impacted specimen and examining the sections with an optimal microscope. The effectiveness of using adhesive layers to suppress impact-induced delamination was investigated. It was found that the presence of adhesive layers could toughen the interface between two laminae and also reduce matrix cracking. A guide to the optical use of adhesive layers for improving impact resistance properties was proposed.

2-D to 3-D global/local finite element analysis of cross-ply composite laminates

Abstract: An example of two-dimensional to three-dimensional global/local finite element analysis of a laminated composite plate with a hole is presented. The 'zoom' technique of global/local analysis is used, where displacements of the global/local interface from the two-dimensional global model are applied to the edges of the three-dimensional local model. Three different hole diameters, one, three, and six inches, are considered in order to compare the effect of hole size on the three-dimensional stress state around the hole. In addition, three different stacking sequences are analyzed for the six inch hole case in order to study the effect of stacking sequence. The existence of a 'critical' hole size, where the interlaminar stresses are maximum, is indicated. Dispersion of plies at the same angle, as opposed to clustering, is found to reduce the magnitude of some interlaminar stress components and increase others.