A.S.D. Wang

Some New Results on Edge Effect in Symmetric Composite Laminates

An improved numerical (finite element) procedure is developed to investigate the stress field in symmetrically laminated composites of finite dimensions. Emphasis is placed on assessing the singular behavior of stresses in regions close to ply-interfaces and the exposed free edge. An accurate evaluation of these stresses will help to better understand the failure process of composite laminates. Mechanical load in the form of uniaxial tension and environmental variables such as temperature and humidity exposures are considered in the formulation. Results tor lam inates under uniaxial tension are presented in this paper; thermal and hygroscopic stresses will be reported in a subsequent paper.

Initiation and Growth of Transverse Cracks and Edge Delamination in Composite Laminates Part 1. An Energy Method

This paper is concerned with the basic fracture mechanisms involved in matrix-dominated failures in fibrous composite laminates, Specifically, interlaminar fracture in the form of free-edge ply delamination and intra- laminar fracture in the form of multiple transverse cracks are investigated. In each case, a theory is formulated based on the classical linear fracture mechanics concept of strain energy release rate as a criterion for crack growth. A finite element technique incorporating the virtual crack-closure procedure is developed to generate numerical results. Simultaneously, an experimental study is conducted using a series of graphite epoxy laminates in the form of (±25/90 n ) s , n = 1,2,3. Part 1 of this paper presents the development of the method from the conceptual, physical and numerical considerations, while Part 2 provides for a comparison between the analyti cal and experimental results.

The dependence of transverse cracking and delamination on ply thickness in graphite/epoxy laminates

(25/-25/90 n ) s , n = ½, 1, 2, 3, 4, 6, 8, and (25 2 /-25 2 /90 2 ) s T300/934 graphite/epoxy laminate coupons were tested in tension and examined periodically by di-iodobutane (DIB) enhanced X-radiography following progressively larger increments of applied load. The tensile strain required to initiate transverse cracking in the 90-deg ply was found to depend on the thickness of the 90-deg layer. For n ≤ 3 delamination was initiated at the edge of the coupon. The onset of delamination occurred before or after transverse cracking, depending on the laminate construction. For n ≥ 4 delamination at the 25/90 interface was initiated across the entire width of the coupon by the formation of a transverse crack in the 90-deg ply. In some laminate constructions the location of specimen separation during final fracture was associated with the growth and coalescence of edge delaminations under either incrementally increasing or constant strain cyclic loading. Comparisons of experiments to the fracture sequence predictions by stress and energy analysis are made to assess our ability to account for the ply thickness dependence of fracture in these laminates.

A Stochastic Model for the Growth of Matrix Cracks in Composite Laminates

This paper presents a stochastic simulation model for the growth of multiple matrix cracks in composite laminates subjected to both static and fatigue loads. Working within the premise of ply-elasticity, a new concept of effective flaws is introduced which replaces the conventional constant ply strength criterion. Thus, the model con sists of an application of fracture mechanics and a rational representation of material flaw distributions. Simulation examples are presented on [02/902]s and [0 2/903]s graphite-epoxy laminates which undergo characteristic transverse cracking under uniaxial tension.

Initiation and Growth of Transverse Cracks and Edge Delamination in Composite Laminates Part 2. Experimental Correlation

In part one of this paper, the fracture processes of multiple transverse cracking and free edge delamination in composite laminates have been analyzed by an energy method. Numerical analyses and experimental examination using a series of T300/934 graphite epoxy laminates are pre sented in this part two. While part one is presented in a self-contained form, part two must be regarded as the continuation of part one.

Further Study of Composite Laminates Under Cylindrical Bending

In a series of three papers [1,2,3], the range of applicability of classical laminated plate theory (CPT) in describing the response of composite laminates under static bending has been examined. Briefly, exact solutions within the framework of linear elasticity theory were developed and compared to the respective solutions governed by CPT [4,5,6]. Numerical data calculated based on simple harmonic load distributions have indicated rather wide discrepancy between the two solutions for laminates having low span-to-depth ratios. At high aspect ratios however, the CPT solution is in good agreement with the elasticity solution.

Calculation of Edge Stresses in Multi-Layer Laminates by Sub-Structuring

A simplified method for calculating interlaminar stresses is accom plished by smearing two or more laminas as a quasi-homogeneous lamina. Numerical examples show the accuracy and the limitations of the smearing technique.

Mesomechanics: the microstructure-mechanics connection

Abstract Mesomechanics is a new research thrust to evolve non-continuum mechanics for heterogeneous materials. By fostering a closer collaboration between the materials sciences and the solid mechanics disciplines, this new thrust seeks to apply mechanics principles to the microstructural constituents of multiphase materials, thus placing the microstructure-mechanics relationship on a quantitative basis. This paper describes some of the challenges and opportunities that are brought forth by the study of mesomechanics.

On matrix cracking in fiber reinforced ceramics

Abstract This paper addresses critical stress at the propagation of a fiber-bridged matrix crack of arbitrary length in fiber-reinforced brittle matrix composites. The formulation of the problem follows the approach adopted earlier by Marshall, Cox and Evans, but a new shear-lag model that accounts for the matrix shear deformation above the slipping region is used here to derive the relationship between the crack opening displacement and the crack surface closure traction. The inclusion of the matrix shear deformation above the slipping region significantly affects the calculated crack tip stress intensity factor and the prediction of the critical stress at the propagation of the crack. Illustrative examples are cited using three available composite systems of SiC-borosilicate, C-borosilicate and Nicalon-lithium-aluminosilicate (LAS).

Analysis of bent crack in unidirectional fibre reinforced composites

This paper presents a fracture analysis for a bent crack in an infinite orthotropic plate subjected to a far-field uniform tensile stress. To determine parameters relevant to the mixed-mode fracture conditions at the tip of the bent crack, the problem is formulated in terms of singular integral equations with generalized Cauchy kernels. The resulting system of equations is then solved numberically employing a Gaussian quadrature and the collocation method. Stress intensity factors, k1 and k2, and the strain energy release rates, GI and GII at the tip of the bent crack are obtained for various values of fibres direction θ and L2/L1 ratios. Extensive results for a graphite-epoxy unidirectional composite laminate are presented.