James M. Whitney

The Effect of Transverse Shear Deformation on the Bending of Laminated Plates

A bending theory which includes transverse shear deformations is presented for laminated plates. Closed form solutions are obtained for bending deflections, flexural vibration frequencies, and buckling loads of simply-supported rectangular plates of special construction. Results show that shear deformation can significantly effect gross plate re sponse for highly anisotropic laminates. Comparison to exact elasticity solutions show excellent agreement for gross behavior of laminates.

A Double Cantilever Beam Test for Characterizing Mode I Delamination of Composite Materials

The double cantilever beam test is examined as a candidate for measuring in terlaminar fracture resistance as related to normal stress induced delamina tion. A number of approaches to data reduction schemes used in conjunction with this test method for determining critical strain energy release rate are discussed. Experimental data on unidirectional tape and bidirectional cloth graphite fiber reinforced polymeric matrix composites are compared to assess the potential of the double cantilever beam test as a materials screening tool. Matrix materials of varying inherent toughness were chosen for comparative purposes. Center notch data for 90-degree unidirectional graphite-epoxy composites were also obtained as a basis for test method comparison.

Stress Analysis of Thick Laminated Composite and Sandwich Plates

Because of the relatively soft interlaminar shear modulus in high per formance composites, laminated plate theory based on the Kirchhoff hypothesis becomes inaccurate for determining gross plate response and internal stresses of thick composites and sandwich type laminates. In this paper a procedure is developed for accurately calculating the mechanical behavior of a thick laminated composite or sandwich plate of arbitrary stacking sequence. The procedure is an extension of an existing laminated plate theory which includes the effect of transverse shear deformation, and is easily computerized for design application with a minimum of running time. Accuracy of the approach is ascertained by comparing solutions from the modified plate theory to exact elasticity theory.

Approximate Stresses in an Orthotropic Plate Containing a Circular Hole

An approximate solution in the form of a polynomial is presented for the normal stress distribution adjacent to a circular hole in an infinite orthotropic plate. Comparison of the approximate solution to the exact solution, obtained from theory of elasticity, shows good agreement for a wide range of orthotropic stress concentration factors. The approximate solution is useful in assessing hole size effect on the notched strength of laminated orthotropic composite plates. The simplicity of the approximate solution gives the design engineer a useful tool for optimizing laminate construction in the presence of a circular hole.

The Laminate Analogy for 2 and 3 Dimensional Composite Materials

A previously developed laminate analogy for predicting the elastic stiffness and thermal expansion properties of a randomly oriented short fiber composite is extended to include a large class of fiber reinforced composites possessing very complex geometry. In particular, the stiffness and thermal expansion coefficients of short fiber com posites having biased filaments, misaligned filaments, and variable fiber aspect ratios are determined from a laminate analogy. In addition, the laminate analogy is extended to 2 and 3 dimensional woven fabric composites. Theoretical results for the short fiber com posites show excellent agreement with experimental results, while theoretical results for the woven fabric composites are shown to be qualitatively correct.

On short-beam shear tests for composite materials

Interlaminar beam tests in the form of three-point and four-point flexure are examined both experimentally and analytically. Experimental data are obtained on unidirectional composites. Photomicrographs of actual failure modes and results of a stress analysis based on classical theory of elasticity are utilized to supplement the experimental data. Complex failure modes in the presence of extremely high combined stress gradients are observed and cast serious doubts on the usefulness of interlaminar-beam experiments for characterizing the delamination resistance of composite materials. Further difficulties are encountered with ductile-matrix-resin composites.

Analysis of the Rail Shear Test-Applications and Limitations

A detailed theoretical and experimental analysis of the rail shear test is presented. A Fourier series solution is obtained for the stresses in an idealized rail shear specimen. The theoretical results are quali tatively verified on fiber reinforced rubber composites. Rail shear data on the modulus and strength of current high-modulus rein forced composites is compared to predicted values obtained from lamination theory. Results show that, despite definite limitations, the rail shear test can be useful in the determination of the in-plane shear properties of a laminated composite.

Elastic Moduli of Unidirectional Composites with Anisotropic Filaments

Principal elastic moduli of unidirectional composites with aniso tropic filaments are predicted from available elasticity solutions of multiple-inclusion problems. Measured data obtained from composites made of graphite filaments and epoxy resins agree well with the predicted values. Filament anisotropy is shown to have significant effects on the composite elastic properties.

Moisture Effects in Epoxy Matrix Composites

Abstract : The effect of absorbed moisture on the mechanical properties of a neat epoxy resin and derived graphite/epoxy composites is investigated. Glass transition temperature and modulus as a function of moisture content and temperature is determined for both the neat resin and composite. In addition, elastic modulus for the neat resin, and strength and elastic moduli for unidirectional composites and quasi-isotropic laminates are determined as a function of absorbed moisture and temperature. A method for predicting moisture content and through-the-thickness profile for a laminate exposed to a constant humidity and temperature is discussed. Particular attention is also given to the details of environmental testing. Data indicates that absorbed moisture and temperature effects on the neat resin translate directly to matrix dominated properties of the composite and can lead to a change in failure mode, while filament dominated properties show very little environmental sensitivity. Results also indicate that resin glass transition temperature is a function of processing as well absorbed moisture.

The Relationship between Tensile Strength and Flexure Strength in Fiber Reinforced Composites.

Tensile data on unidirectional composites generated from a flexure test usually yield a higher strength than observed from a standard tensile coupon. According to a statistical-strength theory based on a Weibull distribution, the presence of a stress gradient in the flexure-test results in an apparent increase in tensile strength as compared to the tensile test under uniform stress. In the present paper, this concept is explored by utilizing data from unidirectional graphite-epoxy composites to compare with theoretical results generated from a two-parameter Weibull distribution. A larger variation in tensile strength is observed from tensile-coupon data than from flexure data. Such differences are not in accordance with strength theories based on a uniform flaw distribution and raise questions concerning variability of the test methods, as well as sources of material variability.