Showing papers on "Composite laminates published in 1978"

Fatigue and Residual Strength Degradation for Graphite/Epoxy Composites Under Tension-Compression Cyclic Loadings:

Abstract: A fatigue residual strength degradation model has been generalized to account for the effect of tension-compression fatigue loading. The model is verified experimentally by use of graphite/epoxy unnotched composite laminates. It is shown that the correlation between the experimental results and the theoretical predictions on both the residual strength degradation and fatigue life is very reasonable.

Interlaminar Stresses in Symmetric Composite Laminates

Abstract: Interlaminar stresses and displacements of each layer satisfying geo metrical boundary conditions are represented in series of Legendre poly nomials The extended Galerkin procedure is used for each layer, and the final solutions are obtained by requiring the satisfaction of continuity conditions at each interface and the stress boundary conditions at exterior planes A computer program has been prepared, and numerical results are presented and compared with existing data

Swelling of Composite Laminates

Abstract: : The swelling behavior of a graphite/epoxy (AS/3501-5) is shown to be described by a model which is based on the assumption that the swelling is negligible until the moisture concentration reaches a threshhold value and then increases linearly thereafter. The model also provides a means of measuring the volume fraction of voids. The analytical results, which take into account the curing stresses, are compared with the experimental data for the transverse and thickness strains of symmetric laminates as well as for the warpage of unsymmetric laminates. The initial absorption on virgin specimens facilitates the subsequent diffusion and the residual stresses appear to be responsible for the absorption being slower than the desorption during the early stages of moisture conditioning. Warpage of unsymmetric laminates is discussed in light of the cracks found in constituent plies. Finally derived is a temperature-relative humidity relationship which renders the graphite/epoxy composite laminates free of residual stresses. (Author)

Investigation of Angle-Ply Delamination Specimen For Interlaminar Strength Test

Abstract: The use of fiber composite laminates in many structural applications requires that the designer be provided with accurate information about interlaminar strength. In particular, a knowledge of inte...

Mechanics of failure of composite materials

Abstract: Composite materials are both inhomogeneous and anisotropic. Both of these characteristics affect the internal stress distributions since inhomogeneity involves variations in both strength and stiffness. The fracture mechanics of nonuniform materials are considered, taking into account the effect of nonuniformity on stress distributions near the crack tip, predicted yield zones in nonuniform and uniform materials, and the fracture of a center-notched unidirectional specimen. The mechanics of failure of laminated materials is discussed. It is found that the development of damage in a laminate with increasing load and, possibly, increasing numbers of cycles of loading is peculiar to the laminate in question, i.e., the material system, the stacking sequence, and the geometry. Approaches for monitoring damage development are also described.

Stress Field Induced by Transient Moisture Sorption in Finite-Width Composite Laminates

Abstract: To describe the moisture sorption process in a long, finite-width com posite laminate, the two-dimensional diffusion equation is solved using a finite element method. The solution which describes the time-varying moisture concentration profile in the solid, is then incorporated in the calculation of the elastic stress field induced by moisture swelling. Time- varying stress concentrations are found present near the free edge region. The moisture sorption history is shown to be important in the evaluation of the laminate's residual stress field.This is the concluding paper in a series of three by the authors, which study the hygrothermal and mechanical aspects of free-edge effects in composite laminates by means of elastic finite element analysis.

Electromagnetic Shielding by Advanced Composite Materials.

Abstract: : The transmission of electromagnetic waves through planar sheets and cylindrical shells of advanced composite laminates is considered in both the frequency and time domains. Attention is concentrated on the frequency range characteristic of the nuclear electromagnetic pulse. The composite laminates are modeled for the purposes of this study by isotropic dielectric or conducting materials. A new 'boundary connection operator' is developed to describe the connection between the tangential electric and magnetic fields on either side of a general multilayer shield. An equivalent sheet impedance operator is developed to describe the bonded wire-mesh screen which is often incorporated in boron-epoxy composite laminates to improve their shielding effectiveness. These analyses are used to study the transmission of EMP signals through planar composite sheets and into cylindrical composite shells in the frequency and time domains. Both graphite-epoxy and 'screened' boron-epoxy laminates are considered. Numerical results are presented to illustrate the analytical formulas which are derived. It is found that the shielding effect of a screened boron-epoxy composite laminate is essentially that of the screen itself. The laminate only has an effect on the equivalent sheet impedance of the screen for parallel-polarized fields, and this effect is relatively minor over the EMP frequency spectrum.

ASOP-3: A Program for Optimum Structural Design to Satisfy Strength and Deflection Constraints

Abstract: Optimally criteria are applied in the iterative resizing of the elements of a finite-element model to achieve minimum weight. In addition to materials that can be considered to be homogeneous for analysis purposes, composite laminates with layups of considerable generality can be treated. Strength resizing of composite elements is done by treating the laminate as a unit, permitting the application of criteria consistent with current design practice. In addition, the program nominally treats a single generalized deflection constraint (linear combination of nodal deflections), but multiple constraints can be accommodated in many cases of practical interest by multiple submissions of the program. Results are presented for two representative lifting-surface structures, subject to both strength and twist constraints.

Prediction of moisture and temperature changes in composites during atmospheric exposure

Abstract: The effects of variations in diffusion coefficients, surface properties of the composite, panel tilt, ground reflection, and geographical location on the moisture concentration profiles and average moisture content of composite laminates were studied analytically. A heat balance which included heat input due to direct and sky diffuse solar radiation, ground reflection, and heat loss due to reradiation and convection was used to determine the temperature of composites during atmospheric exposure. The equilibrium moisture content was assumed proportional to the relative humidity of the air in the boundary layer of the composite. Condensation on the surface was neglected. Histograms of composite temperatures were determined and compared with those for the ambient environment.

Preliminary Investigation of Crack Arrest in Composite Laminates Containing Buffer Strips.

Abstract: The mechanical properties of some hybrid buffer strip laminates and the crack arrest potential of laminates containing buffer strips were determined. The hybrid laminates consisted of graphite with either S-glass, E-glass, or Kevlar. Unnotched tensile coupons and center-cracked fracture coupons were tested. Elastic properties, complete stress/strain curves, and critical stress intensity values are given. The measured elastic properties compare well with those calculated by classical lamination theory for laminates with linear stress/strain behavior. The glass hybrids had more delamination and higher fracture toughness than the all-graphite or the Kevlar hybrid.

Evaluation and expansion of an analytical model for fatigue of notched composite laminates

Abstract: The analytical and experimental study performed to expand the existing static and fatigue failure analysis is described. The analytical effort extended the analysis to include interlaminar effects, while the experimental effort developed methods to obtain basic experimental data required as input to the analysis. The static failure analysis for notched laminates was modified to include interlaminar effects near the notch. Three dimensional elastic and two dimensional elasticplastic finite element analysis were performed for some notched laminates.

Residual stresses in angleplied laminates and their effects on laminate behavior

Abstract: Evidence of the presence of lamination residual stresses in angleplied laminates were transply cracks and warpage of unsymmetric laminates which occur prior to application of any mechanical load. Lamination residual strains were measured using the embedded strain gage technique. These strains result from the temperature differences between cure and room temperature and vary linearly within this temperature range. Lamination residual stresses were usually present in angleplied fiber composites laminates; they were also present in unidirectional hybrids and superhybrids. For specific applications, the magnitudes of lamination residual stresses were determined and evaluated relative to the anticipated applied stresses. Particular attention was given to cyclic thermal loadings in applications where the thermal cycling takes place over a wide temperature range.

Moisture Diffusion Analysis of Composite Strength Degradation

Abstract: Moisture diffusion analysis (MDA) has been developed as a new non-destructive evaluation methodology to monitor the strength degradation of graphite-epoxy composite materials. Studies of composite strength degradation due to high moisture identify reversible strength loss due to current moisture content and irreversible strength loss related to prior moisture exposure with microstructure damage. MDA measures current moisture content, directional diffusion coefficients, and moisture concentration profiles. MDA measurement utilizes an electrolytic cell to concurrently record cumulative .oisture content and moisture effusion rate. Extension of the diffusion analysis of Shen and Springer isolates directional diffusion coefficients and indicates a high sensitivity of MDA to moisture degradation of the fibermatrix interface. Extension of the methodology to analyze non-Fickian diffusion shows that MDA can be applied to locate regions of composite strength degradation related to hydrothenaal cycling effects. Studies which correlate MDA with ultrasonic response, and interlaminar shear strength of composite laminates subjected to localized moisture damage, show that MDA can be applied to map and locate low strength regions of a composite structure.

Environmental fatigue crack propagation in metal/composite laminates

Abstract: Flight simulation fatigue crack propagation tests with gust spectrum loading were carried out on 2024-T3/carbon-epoxy, 7475-T76l/carbon-epoxy and Ti-6Al-4V/carbon-epoxy laminates with nominal weight savings of -30% as compared to equivalent 2024-T3 panels. The performance of the 2024-T3/carbon-epoxy laminates was much superior to those of the other material combinations. Overall, there was a fairly strong effect of changing from an environment of normal air to an air plus water spray environment. Probable reasons for this strong effect are discussed.

Certification of Composite Aircraft Structures Under Impact, Fatigue and Environmental Conditions. Part 3. Environmental Effects on Compressive Strength.

Abstract: : The compressive behavior of a graphite/epoxy (AS-3501-6) composite system is studied here experimentally and analytically. First, a test method is developed using a flat-end specimen. Tests are then performed for six (6) different composite laminates. In each case, the variation of the test temperature ranges from -12 C to 121 C while the variation of the moisture content in the specimens ranges from dry to saturated-wet. Two cases of dry/wet/dry conditioned specimens are also studied. An analytical model which is based on the concept of microbuckling is developed to predict the compression strength of unidirectional laminates. The analytical results compare well with experiment. The rule-of-mixture method is shown to predict the compressive strength of laminates of complex lay-ups. (Author)

Effect of High Load on the Fatigue Behavior of Unnotched Graphite/Epoxy Laminates.

Abstract: : The purpose of this research program is to verify experimentally the theoretical prediction for the effect of the high load on the fatigue behavior of unnotched composite laminates. A three-parameter fatigue and residual strength degradation model has been used to derive the number of load cycles a specimen can survive and the statistical distributions of the fatigue life and the residual strength after the specimen has survived an initial high load or periodic high loads. One set of the static ultimate strength data and one set of the fatigue scan data were generated to determine the three-parameter values appearing in the theoretical model.

Reproducibility of structural strength and stiffness for graphite-epoxy aircraft spoilers

Abstract: Structural strength reproducibility of graphite epoxy composite spoilers for the Boeing 737 aircraft was evaluated by statically loading fifteen spoilers to failure at conditions simulating aerodynamic loads Spoiler strength and stiffness data were statistically modeled using a two parameter Weibull distribution function Shape parameter values calculated for the composite spoiler strength and stiffness were within the range of corresponding shape parameter values calculated for material property data of composite laminates This agreement showed that reproducibility of full scale component structural properties was within the reproducibility range of data from material property tests

Composite Materials for Cryogenic Structures

Abstract: The attractiveness of composite laminates for cryogenic applications is illustrated in Fig. 1 (a–d) in terms of critical property ratios or figures of merit, which are based on NBS data. A low ratio of thermal conductivity to modulus, λ/E, or to strength, λ/σ, reduces refrigeration costs whenever the structural components must be located in a temperature gradient. High ratios of modulus or strength to density (E/ρ, σ/ρ) reduce inertial loads in rotating machinery. By these criteria, uniaxial boron-epoxy or glass-epoxy composites can have 1.5 to 2 orders of magnitude advantage over annealed AISI 304 stainless steel in the λ/σ ratio, while boron-epoxy has about an order-of-magnitude advantage in the λ/E ratio. A modest gain of 20% is expected in the E/ρ ratio when substituting uniaxial glass-epoxy for stainless steel; but improvements of 200 to 400% are predicted when the higher-modulus boron or graphite fibers are used. Substitution of either glass, graphite, or boron-reinforced composites for stainless steel can increase the σ/ρ ratio by an order of magnitude. These data are valid at the most favorable orientation of each composite and for welded stainless steel.

Stress intensity factors of composite orthotropic plates containing periodic buffer strips

Abstract: The fracture problem of laminated plates which consist of bonded orthotropic layers is studied. The fields equations for an elastic orthotropic body are transformed to give the displacement and stress expressions for each layer or strip. The unknown functions in these expressions are found by satisfying the remaining boundary and continuity conditions. A system of singular integral equations is obtained from the mixed boundary conditions. The singular behavior around the crack tip and at the bimaterial interface is studied. The stress intensity factors are computed for various material combinations and various crack geometries. The results are discussed and are compared with those for isotropic materials.

Wave propagation in transversely impacted composite laminates.

Abstract: An experimental study was conducted to determine the wave-propagation characteristics, transient strains and residual properties of unidirectional and angle-ply boron/epoxy and graphite/epoxy laminates impacted with silicon-rubber projectiles at velocities up to 250 m/sec. Results include the following: (1) the predominant wave is the flexural wave propagating at different velocities in different directions; (2) peak strains and strain rates in the transverse to the (outer) fiber direction are much higher than those in the direction of the fibers; (3) strain rates up to 640/sec were measured; and (4) unidirectional laminates under impact showed appreciable modulus and strength degradation in the direction transverse to fibers.

Finite element stress analysis of idealized composite damage zones

Abstract: A quasi three dimensional finite element stress analysis of idealized damage zones in composite laminates is presented. The damage zones consist of a long centered groove or cutout extending one or two layers in depth from both top and bottom surfaces of a thin composite laminate. Elastic results are presented for compressive loading of four and eight layer laminates. It is shown that a boundary layer exists near the cutout edge similar to that previously shown to exist along free edges. The cutout is shown to produce significant interlaminar stresses in the interior of the laminate away from free cutout edges. The interlaminar stresses are also shown to contribute to failure which is defined using the Tsai-Wu failure criteria.