R. Byron Pipes

Interlaminar Stresses in Composite Laminates Under Uniform Axial Extension

The response of a finite-width composite laminate under uniform axial strain is treated through the application of classical elasticity theory. Finite-difference solution techniques are employed to obtain solutions for stresses and displacements throughout the region. Results for material properties typical of a high modulus graphite-epoxy composite material system are presented which explain the mechanism of shear transfer within a symmetric laminate. In addition, results of this work are compared to those given in a recent approximate formulation.

Experimental characterization of advanced composite materials

INTRODUCTION Background Laminate Orientation Code Influences of Material Orthotropy on Experimental Characterization Typical Unidirectional Composite Properties References ANALYSIS OF COMPOSITE MATERIALS Constitutive Relations Micromechanics Laminated Plate Theory St. Venants Principle and End Effects in Composites Lamina Strength Analysis Laminate Strength Analysis Fracture Mechanics Concepts Strength of Composite Laminates Containing Holes References PROCESSING OF COMPOSITE LAMINATES Processing of Thermoset Composites Autoclave Processing of Thermoplastic Composites Determination of Volume Fractions of Fibers, Resin and Voids References TEST SPECIMEN PREPARATION, TEST EQUIPMENT, STRAIN AND DEFORMATION MEASUREMENTS Cutting the Composite Laminate Tabbing Materials Tab Bonding Suggested Tab Bonding Procedures Hinge Attachment for DCB and MMB Specimens Specimen Conditioning Strain and Displacement Measurements Testing Machines References LAMINA TENSILE RESPONSE The Need for Lamina Testing Introduction to Tensile Testing Primary Concerns Specimen Configurations and Test Procedures Data Reduction References LAMINA COMPRESSIVE RESPONSE Shear-Loading Test Methods End-Loading Test Methods Combined Loading Compression (CLC) Test Methods Compression Test Procedures Failure Modes General Data Reduction Backing out Unidirectional Lamina Strength from a Test of a Cross-Ply Laminate Summary of Compression Test Methods References LAMINA SHEAR RESPONSE Iosipescu Shear Test Method (ASTM D 5379) Two-Rail Shear Test Method (ASTM D 4255) Three-Rail Shear Test Method (ASTM D 4255) [+-45]ns Tension Shear Test Method (ASTM D 3518) Short Beam Shear Test Method (ASTM D 2344) Summary References LAMINA FLEXURAL RESPONSE Testing Configuration Three-Versus Four-Point Loading Specimen Preparation and Flexural Test Procedures Data Reduction References LAMINA OFF-AXIS TENSILE RESPONSE Deformation and Stress in Unconstrained Specimens Influence of End Constraint Off-Axis Tensile Strength Test Procedure Data Reduction References LAMINA THERMOELASTIC RESPONSE Temperature Gage Sensing System Temperature Compensation Measurement of Thermal Expansion Data Reduction References LAMINATE MECHANICAL RESPONSE Data Reduction for Stiffness Properties Laminate Strength Analysis Test Specimen Preparation Test Procedures Data Reduction Example of a Typical Analysis: Axial Tensile Response of a Laminate References LAMINATE THERMOELASTIC RESPONSE Preparation of Test Specimens and Measurement of Thermal Expansion Data Reduction Analysis of Thermoelastic Response References OPEN-HOLE TENSILE AND COMPRESSIVE STRENGTHS OF LAMINATES Point and Average Stress Criteria Test Specimen Preparation Tensile Test Procedure and Data Reduction Standardized Open-Hole Tension Test Method Standardized Open-Hole Compression Test Methods References CHARACTERIZATION OF DELAMINATION FAILURE Double Cantilever Beam (DCB) Test End-Notched Flexure (ENF) Test Four-Point Bend ENF (4ENF) Test Mixed-Mode Bending (MMB) Test Edge-Cracked Torsion (ECT) Test References Appendix A: Compliance and Stiffness Transformations and Matrix Operations Appendix B: Preparation of Panels and Test Specimens Appendix C: Sample Laboratory Report Appendix D: Unit Conversions Index

The Influence of Stacking Sequence on Laminate Strength

Based upon considerations relating to the nature of the inter laminar stresses in composite laminates, an approach is presented to predict the detailed stacking sequence of specific layer orientations which leads to optimum protection against delamination under uniaxial static and fatigue loadings. In particular, it is argued that the interlaminar normal stress, as well as the interlaminar shear stress, is instrumental in precipitating delamination and subsequent strength degradation. Correlation with existing experimental evidence of the stacking sequence phenomenon is presented.

Some Observations on the Interlaminar Strength of Composite Laminates

Definition of the influence of interlaminar stresses on the failure characteristics of composite laminates may require the development of novel experimental characterization procedures. The so-called free edge problem in laminate elasticity offers a cnovenient mechanism to accomplish this purpose because of the high interlaminar stresses in the neighbourhood of a free boundary. The detailed design of a laminate specimen which can exhibit catastrophic delamination induced by interlaminar tension is presented, along with a preliminary failure hypothesis to characterize this mode of rupture and the associated experimental demonstration of the phenomena.

Notched Strength of Composite Materials

A macroscopic model for predicting the strength of a composite laminate containing a circular notch is introduced. A property, which quantifies the reduction in strength of a given composite material or laminate due to a circular notch, is proposed. The superposition of notched strength data for several important composite material systems and laminate stacking sequences is achieved through development of a notched strength-radius superposition method and the definition of radius shift parameters, ac and am . The superposition allows prediction of the in fluence of laminate stacking sequence upon notched strength. In addition, data for composite material systems or laminates of common stress concen tration factor is shown to superimpose by use of the developed shift parameters.

On Flow through Aligned Fiber Beds and Its Application to Composites Processing

In process modeling of continuous fiber composites, matrix flow through aligned fiber beds is traditionally described by Darcys law, which relates matrix flow rate to matrix pressure gradient, matrix viscosity, and fiber bed permeability. This phenome nological relationship was originally derived for macroscopically isotropic beds of spheri cal or sphere-like particles. Hence it is necessary to establish the conditions under which this relationship may be extended to fiber beds, which are approximately transversely iso tropic. A review of previous work on steady-state Newtonian fluid flow through isotropic porous media is used as a foundation to explore the validity of extending Darcys law to flow through beds of aligned fibers. Theoretical analyses are compared to experimental results obtained from flow through real fiber beds and ideal beds of regularly spaced cylinders. Functional relationships for flow of generalized Newtonian fluids through iso tropic sphere beds are also reviewed, and their extension to aligned fiber beds is suggested as a first order approximation. The findings are discussed in terms of the usefulness and accuracy of the reviewed and suggested flow rate-pressure drop relationships in process modeling of continuous fiber composites.

Interlaminar Fracture of Composite Materials

Interlaminar fracture characteristics of a graphite/epoxy composite material (AS1/3501-6) are investigated under the opening, shearing and mix ed mode conditions. The Arcan test fixture was modified to accomodate the composite specimen containing an embedded interlaminar crack. Both critical stress intensity factor and strain energy release rate data were deter mined and the quadratic interaction for mixed mode behavior was compared to experimental data. Results of over sixty tests showed that the test fixture yielded results which compare favorably with results from contemporary test fixtures. Critical fracture parameters for the opening and shearing modes were shown to differ by a factor of 9.1. Further, the test results showed general agreement with the quadratic interaction relation.

Helical carbon nanotube arrays: mechanical properties

An investigation of the effective mechanical properties of large arrays of carbon nanotubes assembled in helical geometries of circular cross-section is undertaken following two approaches. The first is a traditional textile-mechanics approach wherein the nanotube is treated as a continuous textile fiber, and the second utilized anisotropic elasticity theory to examine the behavior of a layered cylinder with layers consisting of discontinuous carbon nanotubes embedded in a polymeric matrix with the collimation direction in each layer following a helical path prescribed by a linear variation in the tangent of the helix angle with radial position. Stress distribution and effective elastic property results are presented for the two approaches.

On the Off-Axis Strength Test for Anisotropic Materials1

Difficulty in the experimental determination of the interaction com ponent of strength tensor of boron-epoxy composites by off axis tests is discussed. Although off axis data agree well with the prediction of the strength tensor theory, the uniaxial data cannot be used to back calculate the interaction term. The data also showed that responses due to normal and shear stresses can be uncoupled in the linear as well as nonlinear regions.

On the Hygrothermal Response of Laminated Composite Systems

The hygroscopic nature of polymeric systems, which find widespread application as matrices in advanced composite materials, requires that dila tations induced by the absorption of moisture be considered m the stress analysis of composite laminates. Considerable attention has recently been focused upon the reduction m both strength and constitutive properties of fiber-reinforced polymeric composites at elevated temperatures when the composite has been subjected to environments which enhance moisture diffusion. This apparent degradation in elevated temperature properties may be magnified even more by residual stresses induced by both the hygroscopic and thermoelastic characteristics of the unidirectional com posite. A unified treatment of the hygrothermal response of the laminated composite plate element is derived. The analysis develops effective mois ture inplane force resultants and bending resultants, which when coupled with mechanical and thermal loadings, yield laminae stresses resulting from the total hygrothermal and mechanical loading environment. Solutions of the classical diffusion equation are obtained yielding mois ture profiles through the laminate thickness. Typical composite laminates consisting of T300/5208 graphite-epoxy are analyzed. Results reveal both the magnitude and distribution of hygrothermally induced stresses.