Considerable evidence has recently been presented for anomalous mois ture diffusion in epoxy matrix composites In an earlier paper we noted that there is no a priori reason why moisture should obey simple diffusion theory rather than Kirkwood's (linear) generalization of the Boltzmann transport equation With the intent of keeping the analytical aspects of the moisture diffusion problem as simple as possible, we here present a slightly generalized but linear model which involves sources and sinks of diffusing water molecules With respect to diffusive characteristics, our model is related to the simplest form of neutron transport theory With respect to bound and unbound particles it is similar to the Langmuir theory of ad sorption isotherms, although we here assume bulk absorption in the resin with no implication that surfaces are involved An approximation to our exact solution of the coupled linear differential equations is used to fit mildly anomalous moisture uptake curves for 5208 resin exposed to 

Langmuir-Type Model for Anomalous Moisture Diffusion In Composite Resins:

An analytical investigation was performed to study the damage in laminated composites containing an open hole and subjected to compressive loading. A progressive damage model was developed during t...

Damage Tolerance of Laminated Composites Containing an Open Hole and Subjected to Compressive Loadings: Part I—Analysis

Stress fields in composite laminates

Delamination—a damage mode in composite structures

The behavior of solids can be represented by a variety of continuum theories. For example, Cosserat elasticity allows the points in the continuum to rotate as well as translate, and the continuum supports couple per unit area as well as force per unit area. We examine experimental methods for determining the six Cosserat elastic constants of an isotropic elastic solid, or the six Cosserat relaxation functions of a Cosserat viscoelastic solid. We also consider other generalized continuum theories (including micromorphic elasticity, Cowin's void theory, and nonlocal elasticity). Ways of experimentall y discriminating among various generalized continuum representations are presented. The applicability of Cosserat elasticity to cellular solids and fibrous composite materials is considered as is the application of related generalized continuum theories. I Introduction The classical theory of elasticity is presently used in engineering analyses of deformable objects at small strain. However there are other continuum theories for linear isotropic materials. Some have more freedom, and some have less freedom than classical elasticity. The various continuum theories are all mathematically self consistent. Therefore a discrimination among them is to be made by experiment. It is the purpose of this article to explore the physical consequences of various continuum theories, and how these consequences may be used in the design of experiments to discriminate among the theories. The constitutive equations for several theories are presented, and some of the salient consequences of each theory are stated and discussed. Some of the causal physical mechanisms associated with each theory are briefly discussed. Experimental methods for evaluating materials as generalized continua are presented, with emphasis on Cosserat elasticity. The treatment is restricted to linearly elastic behavior; study of Cosserat plasticity and related issues is presented elsewhere in this volume. A discussion of experimental aspects of generalized continua is considered particularly appropriate in view of the fact that most of the work done thus far in generalized continuum mechanics has been theoretical.

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Experimental methods for study of cosserat elastic solids and other generalized elastic continua

The effect of absorbed moisture on the properties of six neat epoxy resins (3501-5, 3501-6, 3502, 5208, 934, and NMD 2373) and three epoxy composites (AS/3501-5, B/5505, and hybrid B/5505-AS/3501-5) is explored. Excellentagree. ment among the diffusion coefficients, activation energy, and activation entropy is observed for all resins with the exception of NMD 2373. Good correlation exists between neat resins and corresponding composites. The effect of a weekly thermal cycle on the diffusivity and equilibrium moisture content of two resins and three composites is presented. Correlations are developed between preconditioning temperature and relative humidity on the magnitude of the anomalous moisture absorption. In addition, the effect of absorbed water on the glass transition temperature of six neat epoxy resins and two composites using thermomechanical analysis is considered. The water generally behaves as a classical plasticizer of the epoxy resin, lowering the glass transition temperature as predicted from theory. Comparison between neat 3501-5 resin and AS/3501-5 reveals generally good correlation in T g at comparable resin moisture levels, but plasticized hybrid boron-graphite/epoxy laminate exhibits a somewhat lower T g than predicted from theory.

Effect of Moisture on Epoxy Resins and Composites

Strain distributions to failure, tensile and compressive strain-concentration factors, and strength-reduction factors were determined for glass-, boron-, and graphite-epoxy plates with holes loaded in tension. Strain gages, photoelastic coatings and moire techniques were used. Ten variations of layup and stacking sequence were studied. The boron-epoxy composite was found to be the stiffest and strongest of the three. The graphite laminate with the highest stress concentration and the most linear strain response exhibited the highest strength-reduction factor. In all cases, the maximum strain at failure on the hole boundary was higher than the ultimate tensile-coupon strain. In general, it was found that, the higher the stress-concentration factor, the higher the strength-reduction factor. Thus, the [0/90/0/90]s layup with a stress-concentration factor of 4.82 had a strength-reduction factor of 3.18. At the other extreme, the most flexible layup [±45/±45]s with the lowest stress-concentration factor of 2.06 had the lowest strength-reduction factor of 1.10. Stacking sequences associated with the tensile interlaminar normal stress or high interlaminar shear stress near the boundary, resulted in laminates 10 to 20 percent weaker than corresponding alternate stacking sequences. Furthermore, it was found that stacking-sequence variations can alter the mode of failure from catastrophic to noncatastrophic.

Effects of material and stacking sequence on behavior of composite plates with holes.

Experimental and finite-element analyses are presented for the anisotropic states of stress, strain and fracture of a glass-epoxy plate containing a circular hole and subjected to uniaxial tension. Strains were experimentally measured using foil gages, moire and birefringent coating. Stresses are computed in the linear range from the measured strains. While the hole reduces the plate strength by a factor of two, the maximum tensile strain at fracture is greater than the ultimate strain in a plate without a hole. Fracture consists of crack initiation at the hole boundary but off the horizontal axis. Away from the hole, failure is accompanied by considerable delamination. Discontinuous crack propagation is present.

Stress and failure analysis of a glass-epoxy composite plate with a circular hole

The behavior under uniaxial tension of a boron-epoxy composite plate with a circular hole was studied by means of experimental stress analysis and finite-element methods. The specimens were 10 by 26 in boron-epoxy panels of [0/′45/0/90] s layup. They were instrumented with strain gages, photoelastic coatings, and moire grids. The latter were analyzed using moire fringe multiplication techniques. Strain distributions as a function of applied nominal stress were obtained along the axes of symmetry and around the boundary of the hole. Young's modulus and Poisson's ratio were computed from the far-field strains. Tensile and compressive strain concentrations were computed for the linear range of strain response. Experimental results were compared with those obtained by homogeneous anisotropic elasticity and by finite-element analysis. The mode of failure was a combination of interlaminar shearing and tensile cracking starting at the hole boundary off the horizontal axis, at a point where the elastic shear stress and strain reach their maximum and where the tangential normal and shear strains increase rapidly and nonlinearly with load. The average applied stress at failure was 42 200 psi compared with a coupon strength of 94 000 psi. The average ultimate tensile strain at the hole boundary on the horizontal axis was 8 X 10 - 3 in./in. compared with a coupon ultimate strain of 6.8 X 10 - 3 in./in. At higher loads the peak of the tensile strain shifted, resulting in a higher ultimate strain away from the boundary.

Deformation and Failure of Boron-Epoxy Plate with Circular Hole

Stress concentration and strength of uniaxially loaded composite plates containing central circular, elliptical, and square holes are presented Both smooth and rough holes are considered, as are the effects of relative plate width and thickness The plates are subjected to static, cyclic, and controlled strain-rate loading Fracture strength associated with rough holes is slightly lower than with smooth holes, while broad variations in loading history have minimal influence on strength The tensile stress concentration factor increases and strength decreases as the hole-diameter to plate-width ratio increases Strength decreases with decreasing ratio of laminate thickness to hole diameter

J. B. Whiteside

Papers

Effect of Moisture on Epoxy Resins and Composites

Effects of material and stacking sequence on behavior of composite plates with holes.

Stress and failure analysis of a glass-epoxy composite plate with a circular hole

Deformation and Failure of Boron-Epoxy Plate with Circular Hole

Geometric and Loading Effects on Strength of Composite Plates with Cutouts