Showing papers on "Composite laminates published in 1980"

Analysis and Performance of Fiber Composites

Abstract: Preface. 1 Introduction. 1.1 Definition. 1.2 Characteristics. 1.3 Classification. 1.4 Particulate Composites. 1.5 Fiber-Reinforced Composites. 1.6 Applications of Fiber Composites. Exercise Problems. References. 2 Fibers, Matrices, and Fabrication of Composites. 2.1 Advanced Fibers. 2.1.1 Glass Fibers. 2.1.2 Carbon and Graphite Fibers. 2.1.3 Aramid Fibers. 2.1.4 Boron Fibers. 2.1.5 Other Fibers. 2.2 Matrix Materials. 2.2.1 Polymers. 2.2.2 Metals. 2.3 Fabrication of Composites. 2.3.1 Fabrication of Thermosetting Resin Matrix Composites. 2.3.2 Fabrication of Thermoplastic-Resin Matrix Composites (Short-Fiber Composites). 2.3.3 Fabrication of Metal Matrix Composites. 2.3.4 Fabrication of Ceramic Matrix Composites. Suggested Reading. 3 Behavior of Unidirectional Composites. 3.1 Introduction. 3.1.1 Nomenclature. 3.1.2 Volume and Weight Fractions. 3.2 Longitudinal Behavior of Unidirectional Composites. 3.2.1 Initial Stiffness. 3.2.2 Load Sharing. 3.2.3 Behavior beyond Initial Deformation. 3.2.4 Failure Mechanism and Strength. 3.2.5 Factors Influencing Longitudinal Strength and Stiffness. 3.3 Transverse Stiffness and Strength. 3.3.1 Constant-Stress Model. 3.3.2 Elasticity Methods of Stiffness Prediction. 3.3.3 Halpin-Tsai Equations for Transverse Modulus. 3.3.4 Transverse Strength. 3.4 Prediction of Shear Modulus. 3.5 Prediction of Poisson's Ratio. 3.6 Failure Modes. 3.6.1 Failure under Longitudinal Tensile Loads. 3.6.2 Failure under Longitudinal Compressive Loads. 3.6.3 Failure under Transverse Tensile Loads. 3.6.4 Failure under Transverse Compressive Loads. 3.6.5 Failure under In-Plane Shear Loads. 3.7 Expansion Coefficients and Transport Properties. 3.7.1 Thermal Expansion Coefficients. 3.7.2 Moisture Expansion Coefficients. 3.7.3 Transport Properties. 3.7.4 Mass Diffusion. 3.8 Typical Unidirectional Fiber Composite Properties. Exercise Problems. References. 4 Short-Fiber Composites. 4.1 Introduction. 4.2 Theories of Stress Transfer. 4.2.1 Approximate Analysis of Stress Transfer. 4.2.2 Stress Distributions from Finite-Element Analysis. 4.2.3 Average Fiber Stress. 4.3 Modulus and Strength of Short-Fiber Composites. 4.3.1 Prediction of Modulus. 4.3.2 Prediction of Strength. 4.3.3 Effect of Matrix Ductility. 4.4 Ribbon-Reinforced Composites. Exercise Problems. References. 5 Analysis of an Orthotropic Lamina. 5.1 Introduction. 5.1.1 Orthotropic Materials. 5.2 Stress-Strain Relations and Engineering Constants. 5.2.1 Stress-Strain Relations for Specially Orthotropic Lamina. 5.2.2 Stress-Strain Relations for Generally Orthotropic Lamina. 5.2.3 Transformation of Engineering Constants. 5.3 Hooke's Law and Stiffness and Compliance Matrices. 5.3.1 General Anisotropic Material. 5.3.2 Specially Orthotropic Material. 5.3.3 Transversely Isotropic Material. 5.3.4 Isotropic Material. 5.3.5 Specially Orthotropic Material under Plane Stress. 5.3.6 Compliance Tensor and Compliance Matrix. 5.3.7 Relations between Engineering Constants and Elements of Stiffness and Compliance Matrices. 5.3.8 Restrictions on Elastic Constants. 5.3.9 Transformation of Stiffness and Compliance Matrices. 5.3.10 Invariant Forms of Stiffness and Compliance Matrices. 5.4 Strengths of an Orthotropic Lamina. 5.4.1 Maximum-Stress Theory. 5.4.2 Maximum-Strain Theory. 5.4.3 Maximum-Work Theory. 5.4.4 Importance of Sign of Shear Stress on Strength of Composites. Exercise Problems. References. 6 Analysis of Laminated Composites. 6.1 Introduction. 6.2 Laminate Strains. 6.3 Variation of Stresses in a Laminate. 6.4 Resultant Forces and Moments: Synthesis of Stiffness Matrix. 6.5 Laminate Description System. 6.6 Construction and Properties of Special Laminates. 6.6.1 Symmetric Laminates. 6.6.2 Unidirectional, Cross-Ply, and Angle-Ply Laminates. 6.6.3 Quasi-isotropic Laminates. 6.7 Determination of Laminae Stresses and Strains. 6.8 Analysis of Laminates after Initial Failure. 6.9 Hygrothermal Stresses in Laminates. 6.9.1 Concepts of Thermal Stresses. 6.9.2 Hygrothermal Stress Calculations. 6.10 Laminate Analysis Through Computers. Exercise Problems. References. 7 Analysis of Laminated Plates and Beams. 7.1 Introduction. 7.2 Governing Equations for Plates. 7.2.1 Equilibrium Equations. 7.2.2 Equilibrium Equations in Terms of Displacements. 7.3 Application of Plate Theory. 7.3.1 Bending. 7.3.2 Buckling. 7.3.3 Free Vibrations. 7.4 Deformations Due to Transverse Shear. 7.4.1 First-Order Shear Deformation Theory. 7.4.2 Higher-Order Shear Deformation Theory. 7.5 Analysis of Laminated Beams. 7.5.1 Governing Equations for Laminated Beams. 7.5.2 Application of Beam Theory. Exercise Problems. References. 8 Advanced Topics in Fiber Composites. 8.1 Interlaminar Stresses and Free-Edge Effects. 8.1.1 Concepts of Interlaminar Stresses. 8.1.2 Determination of Interlaminar Stresses. 8.1.3 Effect of Stacking Sequence on Interlaminar Stresses. 8.1.4 Approximate Solutions for Interlaminar Stresses. 8.1.5 Summary. 8.2 Fracture Mechanics of Fiber Composites. 8.2.1 Introduction. 8.2.2 Fracture Mechanics Concepts and Measures of Fracture Toughness. 8.2.3 Fracture Toughness of Composite Laminates. 8.2.4 Whitney-Nuismer Failure Criteria for Notched Composites. 8.3 Joints for Composite Structures. 8.3.1 Adhesively Bonded Joints. 8.3.2 Mechanically Fastened Joints. 8.3.3 Bonded-Fastened Joints. Exercise Problems. References. 9 Performance of Fiber Composites: Fatigue, Impact, and Environmental Effects. 9.1 Fatigue. 9.1.1 Introduction. 9.1.2 Fatigue Damage. 9.1.3 Factors Influencing Fatigue Behavior of Composites. 9.1.4 Empirical Relations for Fatigue Damage and Fatigue Life. 9.1.5 Fatigue of High-Modulus Fiber-Reinforced Composites. 9.1.6 Fatigue of Short-Fiber Composites. 9.2 Impact. 9.2.1 Introduction and Fracture Process. 9.2.2 Energy-Absorbing Mechanisms and Failure Models. 9.2.3 Effect of Materials and Testing Variables on Impact Properties. 9.2.4 Hybrid Composites and Their Impact Strength. 9.2.5 Damage Due to Low-Velocity Impact. 9.3 Environmental-Interaction Effects. 9.3.1 Fiber Strength. 9.3.2 Matrix Effects. Exercise Problems. References. 10 Experimental Characterization of Composites. 10.1 Introduction. 10.2 Measurement of Physical Properties. 10.2.1 Density. 10.2.2 Constituent Weight and Volume Fractions. 10.2.3 Void Volume Fraction. 10.2.4 Thermal Expansion Coefficients. 10.2.5 Moisture Absorption and Diffusivity. 10.2.6 Moisture Expansion Coefficients. 10.3 Measurement of Mechanical Properties. 10.3.1 Properties in Tension. 10.3.2 Properties in Compression. 10.3.3 In-Place Shear Properties. 10.3.4 Flexural Properties. 10.3.5 Measures of In-Plane Fracture Toughness. 10.3.6 Interlaminar Shear Strength and Fracture Toughness. 10.3.7 Impact Properties. 10.4 Damage Identification Using Nondestructive Evaluation Techniques. 10.4.1 Ultrasonics. 10.4.2 Acoustic Emission. 10.4.3 x-Radiography. 10.4.4 Thermography. 10.4.5 Laser Shearography. 10.5 General Remarks on Characterization. Exercise Problems. References. 11 Emerging Composite Materials. 11.1 Nanocomposites. 11.2 Carbon-Carbon Composites. 11.3 Biocomposites. 11.3.1 Biofibers. 11.3.2 Wood-Plastic Composites (WPCs). 11.3.3 Biopolymers. 11.4 Composites in "Smart" Structures. Suggested Reading. Appendix 1: Matrices and Tensors. Appendix 2: Equations of Theory of Elasticity. Appendix 3: Laminate Orientation Code. Appendix 4: Properties of Fiber Composites. Appendix 5: Computer Programs for Laminate Analysis. Index.

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

Abstract: 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 (±25/90n)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.

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

Abstract: 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.

Static Strength Prediction of Bolted Joint in Composite Material

Abstract: A new method of predicting the strength of double shear single fastener composite joints is presented The analysis method consists of calculating the stress distribution around a fastener hole using a finite exement analysis and predicting the various modes of laminate failure through the use of the average stress criterion as o riginally suggested and through same modifications to the criterion Several joint configurations of a composite bolt bearing specimen have been analyzed using the method discussed above The net tension and bearing and shearout modes of failures have been predicted analytically The analytical predictions have been compared with available experimental data and good correlation has been obtained

Fibrous composites in structural design

Abstract: Comments on the Status of Composite Structures Technology.- Session I Aircraft Structures.- Composite Aircraft Structures.- Composite Wing Substructure Technology on the AV-8B Advanced Aircraft.- Preliminary Design Development AV-8E Forward Fuselage Composite Structure.- Wing Fuselage Critical Component Development Program.- Development of a Preloaded Hybrid Advanced Composite Wing Pivot Fairing.- Design Concepts for Composite Fuselage Structure.- Advanced Composites Integral Structures Meet the Challenge of Future Aircraft Systems.- Session II Missile and Space Applications.- Organic Matrix Composite Application to Missile Structure.- Advanced Composite Satellite Equipment Support Module Study.- Inertial Upper Stage (IUS) Advanced Composite Structure.- Development and Fabrication of Graphite Polyimide Launch Vehicle Structures.- Continuous Filament Advanced Composite Isogrid: A Promising Structural Concept.- Session III Crashworthiness, Impact Damage, Post Buckling and Dynamics Response.- A Crashworthiness Test for Composite Fuselage Structure.- Recent Developments in the Design, Testing and Impact-Damage Tolerance of Stiffened Composite Panels.- Impact Resistance of Graphite and Hybrid Configurations.- Design of Stiffened Composite Panels in the Post-Buckling Range.- Dynamic Stability of Fibrous Composite Cylinders.- Session IV Special Design Considerations.- Effects of Lightning and Nuclear Electromagnetic Pulse on an Advanced Composites Aircraft.- Problems and Options in Advanced Composite Repair.- A Cad Approach to Cost Estimating Composite Aircraft.- Session V Laminate Plate Theories, Edge Effects, Flaw Growth.- Evaluation of a Hybrid-Stress Formulation for Thick Multilayer Laminates 399.- The Effect of Interlaminar Stresses on Composite Flywheel Design.- Interlaminar Stress Gradients and Impact Damage.- An Analytical and Experimental Investigation of Edge Delamination in Composite Laminates.- Serviceability of Composites Surface Damages.- Prediction of Cyclic Growth of Cracks and Debonds in Aluminum Sheets Reinforced with Boron/Epoxy.- Session VI Helicopter Applications.- Application of Advanced Composite Materials to Helicopter Airframe Structures.- Design and Production of Fiberglass Helicopter Rotor Blades.- Preliminary Design of an Advanced Composite Rotor Hub for the UH-60A Black Hawk.- Evaluation of Graphite/Epoxy Shims in a High Capacity Laminate Helicopter Bearing.- Session VII Composite Joints.- Mechanically-Fastened Joints for Advanced Composites - Phenomenological Considerations and Simple Analysis.- On the Structural Behavior of Mechanically Fastened Joints in Composite Structures.- Design of the Spar Wingskin Joint.- Sensitivity of Bonded and Bolted Joints in Composites to Load/Environmental Spectrum Variations.- Composite Fasteners - A Compatible Joining Technique for Fibrous Composites in Structural Design.- Session VIII Other Applications.- Development of an Advanced Composite Hydrofoil Control Flap.- Cost-Effective Application of Advanced Composites to Hydrofoil Structures.- Graphite Composite Box Beams for U.S. Army Mobile Bridging.- Advancements in Composite Material Flywheels.- Can Advanced Composite Materials Really Compete in the Automotive Market?.- Retrofit Hardening of Electronic Shelters with Composite Panels.- Session IX Reliability/Durability Analysis.- Statistical Analysis Methods for Characterizing Composite Materials.- Durability Evaluation of Highly Stressed Wing Box Structure.- Codstran: Composite Durability Structural Analysis.- Session X Environmental Effects.- Environmental Effects on the Mechanical Behavior of AS/3501-6 Material.- The Kinetics of Moisture Diffusion in Three Advanced Composite Epoxy Resin Matrix Material Systems.- Effects of Varying Hygrothermal Environments on Moisture Absorption in Exposy Composites.- Moisture Expansion and Thermal Expansion Coefficients of a Polymer-Matrix Composite Material.- Growth of Four Flaw Types in Graphite/Epoxy Composites Due to Fully Reversed Fatigue.- Effects of Defects on Tension Coupons Undergoing an Accelerated Environmental Spectrum.

An Analytical and Experimental Investigation of Edge Delamination in Composite Laminates

Abstract: Edge delamination in composite laminates undergoing uniaxial extension was investigated both analytically and experimentally. A predictive capability was synthesized by coupling a statistical failure criterion with an approximation for interlaminar normal stress, σz, developed by Pagano and Pipes. The failure criterion is formulated by integrating σz over the volume of material in interlaminar tension while accounting for variation in matrix material strength by using a two-parameter Weibull distribution. Consequently, the predicted occurrence of edge delamination is sensitive not only to stacking sequence but also to laminate thickness and free edge length (volume scaling effect).

An experimental and analytical investigation of the rail shear-test method as applied to composite materials

Abstract: This report presents the results from an experimental and analytical investigation of the stress distributions occurring in a rail shear test. The effects of nonuniform stresses induced by differential thermal expansion, rail flexibility and specimen aspect ratio on measured shear modulus and ultimate strength of composite laminates are shown. A two-dimensional linearly elastic finite-element model was used to analytically determine how various geometric parameters influenced the magnitude and distribution of inplane normal and shear stresses in a tensile-rail-shear specimen. Rail shear tests were conducted at room temperature and 589 K (600°F) on selected graphite-polyimide composite laminates using two titanium rail configurations. The analysis and test methods are discussed, and the results of the effects of the various parameters on shear modulus and ultimate strength are presented.

Effect of High Load on Statistical Fatigue of Unnotched Graphite/Epoxy Laminates*

Abstract: The purpose of this paper is to verify experimentally the theoretical model for predicting the effect of the high load (or proof load) on the fatigue behavior of unnotched composite laminates. It is confirmed by the present test program that a specimen is guaranteed to survive a certain number of load cycles as predicted by the theoretical model, after the specimen has survived a high load or proof load prior to fatigue loading. The correlation between the test results and the theoretical distributions of the fatigue life and the residual strength due to the effect of high load is satisfactory.

Proof test and fatigue of unnotched composite laminates

Abstract: A comprehensive fatigue and residual strength degradation model has been used to predict the effect of proof loads (or high load) on the statistical fatigue behavior of composite laminates. The validity of the theoretical model is confirmed by the experimental test results. The correlation between the test results and the theoretical distributions of the fatigue life and the residual strength for composite specimens with or without the effect of proof loads is shown to be very good.

Effect of Prebuckling Deformations on Buckling of Laminated Composite Circular Cylindrical Shells

Abstract: Laminated composite materials are rapidly replacing metals in contemporary shell applications. Thus, the structural behavior of such shells must be predicted, and, in particular, we must be able to predict the buckling behavior. The orthotropy of the fiber-reinforced laminae and the composition of the laminae lead to coupling stiffnesses between forces and changes in curvature as well as between moments and in-surface strains. These coupling stiffnesses do not occur for isotropic shells except when the boundary supports are not in the middle surface. The effect of prebuckling deformations for isotropic shells is to lower the actual buckling load below the classical buckling load by up to 20% for some loading and boundary conditions. The effect of prebuckling deformations on the axial and lateral pressure buckling loads of circular cylindrical shells made of fiberreinforced composite laminates is found to decrease as the orthotropy increases and as the laminate coupling stiffnesses decrease. Variations in the elastic and geometric parameters are studied for both antisymmetric and unsymmetric laminates. Generally, the effect of prebuckling deformations is far less important for laminated shells than for isotropic shells.-

Application of the Cubic Polynomial Strength Criterion to the Failure Analysis of Composite Materials

Abstract: A comparative failure analysis is presented based on the application of quadratic and cubic forms of the tensor polynomial lamina strength criterion to various composite structural configurations in a plane stress state. Failure loads have been predicted for off-angle laminates under simple loading conditions and for symmetric-balanced laminates subject to varying degrees of biaxial tension, including configurations subject to multimode failures. Some experimental data are also provided to support these calculations. From these results, the necessity of employing a cubic strength criterion to accurately predict the failure of composite laminae is demonstrated.

A new look at numerical analyses of free-edge stresses in composite laminates

Abstract: The edge stress problem for a + or - 45 deg graphite/epoxy laminate was examined. The reliability of the displacement formulated finite element method in analyzing the edge stress problem was investigated. Analyses of two well known elasticity problems, one involving a stress discontinuity and one a singularity, showed that the finite element analysis yields accurate stress distributions everywhere except in two elements closest to the stress discontinuity of singularity. Stress distributions for a + or - 45 deg laminate showed the same behavior near the singularity found in the well known problems with exact solutions. The displacement formulated finite element method appears to be a highly accurate technique for calculating interlaminar stress in composite laminates. The disagreement among the numerical methods was attributed to the unsymmetric stress tensor at the singularity.

Derivation of myocardial fiber stiffness equation based on theory of laminated composite

Abstract: The constitutive equation with stress-dependent coefficients for laminated composite is derived and employed for iterative determination of myocardial fiber's stiffness equation E(f) = Ksigma(f) + C from myocardial strip's stiffness equation E(s) = K(s)sigma(s) + C(s). The strip's stiffness constants K(s) and C(s) are estimated by the least-square curve fitting of the stress-strain data experimentally obtained from uniaxially stretching of strips of left ventricular heart wall excised from seven canine hearts. The values of K and C computed at selected fiber orientations across the thickness of the strip and using three, five, and ten-layer approximations are reported.

Assumed Stress Finite Element Analysis of Through-Cracks in Angle-Ply Laminates

Abstract: In the present paper a new assumed stress finite element method, based on a complementary energy method, is developed for the analysis of cracks in angle-ply laminates. In this procedure, the fully three-dimensional stress state (including transverse normal and shear stresses) is accounted for; the mixed-mode stress and strain singularities, whose intensities vary within each layer near the crack front, are built into the formulation a priori; the interlayer traction reciprocity conditions are satisfied a priori; and the individual cross-sectional rotations of each layer are allowed; thus resulting in a highly efficient and cost-effective computational scheme for practical application to fracture studies of laminates. Results obtained from the present procedure, for the case of an uncracked laminate under bending and for the case of a laminate with a through-thickness crack under far-field tension, their comparison with other available data, and pertinent discussion, are presented. N accurate three-dimensional stress analysis of angle-ply laminates with cracks and/or holes, as opposed to the use of simpler "classical laminates plate theories," is often times mandatory to understand 1) the complicated feature of the often-observed non-self-similar crack growth in sym- metric angle-ply laminates; 2) the subcritical damage in the form of matrix crazing, splitting, and delamination that is observed to precede final failure in a laminate; and 3) to more clearly understand the hole-size effects in a laminate. Quasi-three-dimensional analyses of cracked angle-ply laminates, with the assumption of 1) zero transverse normal stress in the laminate, 2) perfect bonding between lamina, and 3) each laminate being treated as a homogeneous anisotropic medium, were recently reported by Wang et al. ! The procedures in Ref. 1 -do not account, a prior, for the mixed- mode stress and strain singularities near the crack front, and hence involve expensive computations using very fine finite element meshes of conventional, polynomial-based elements. From these very-fine-mesh finite element solutions, even though one may obtain high stress-gradient solutions in the limit, it is often inconvenient to extract the results for mixed- mode stress intensity factors near the crack front. Also, the finite element that is used in Ref. 1 is the multilayer assumed- stress hybrid element originally developed by Mau et al.2 for the analysis of uncracked laminates. In the procedure of Ref. 2, a stress field is assumed independently in each layer and interlayer traction reciprocity conditions are enforced through the method of Lagrange multipliers, which necessarily complicates the formulation and results in expensive com- putations. Also, since the stresses are independently assumed in each layer, the computational procedure in Refs. 1 and 2 become prohibitively expensive for a large number of layers. Finally, it is noted that the effects of transverse normal stress a33 (x3 being the thickness-coordinate of the laminate) are ignored in Refs. 1 and 2.

Failure prediction techniques for compression loaded composite laminates with holes

Abstract: The degree of notch sensitivity of composites in compression and whether their failures can be predicted over a wide range of plate and hole sizes. The notch sensitivity of composites is investigated by comparing actual failure loads of laminates with circular holes, with the extreme failure that would be expected from an ideal notch insensitive material and from an ideal notch sensitive material. The predictability question is addressed by applying the point stress failure criterion to a wide range of plate widths and hole sizes and comparing with available experimental data. The severity of impact is explored by comparing strength reductions resulting from impact with those resulting from comparable size circular holes. Finally, comparison is made of the differences to be expected from the effects of cracks and circular holes on failure strength.

Modeling Nonlinear Deformation of Carbon-Carbon Composite Materials

Abstract: Carbon-carbon composite materials have different nonlinear stress-strain behavior under tension loading than under compression loading. The Jones-Nelson-Morgan nonlinear multimodulus material model is fit to the characteristics of AVCO Mod 3a carbon-carbon, which consists of woven layers of orthogonal fibers in the x-y plane pierced by fibers in the z direction. The model is thus defined by use of measured behavior in the principal material directions. Then, the model is validated by comparison of predicted and measured response under uniaxial off-axis loading in tension and in compression.

Compression fatigue analysis of fiber composites

Abstract: A macromechanics model, based on the delamination propagation between the plies of a composite laminate, has been developed for compression fatigue analysis of fiber composites. The model is based on the assumption that initial defects exist in composites between plies. These defects propagate due to the interlaminar stresses produced by applied fatigue loads. Existing compression fatigue data have been analyzed, using the model, and analytically predicted fatigue life compared with experimental data. Test data have been generated under constant amplitude loading on composite laminates with four different stacking sequences. Good correlations between experimentally observed fatigue data and analytical predictions have been found.

Visualization of impact damage of composite plates by means of the Moire technique

Abstract: The phenomenological aspects of propagation damage due to low velocity impact on heavily loaded graphite-epoxy composite laminates were investigated using high speed photography coupled with the moire fringe technique. High speed moire motion records of the impacted specimens are presented. The results provide information on the time scale and sequence of the failure process. While the generation of the initial damage cannot always be separated temporally from the spreading of the damage, the latter takes place on the average with a speed on the order of 200 m/sec.

Statistical fatigue of unnotched composite laminates

Abstract: A general analytical, phenomenological model is developed for characterizing fatigue damage accumulation in unnotched composite laminates, and experimental results are used to evaluate the model. The fatigue model, which is developed for various types of cyclic loadings, is based on the assumption that the residual strength reduction is a monotonically decreasing function of the fatigue life. Parameters are determined from baseline test data consisting of one set of residual strength data, one set of static strength data, and one set of constant amplitude fatigue life data. Once the parameters are determined, the model is able to predict: (1) the statistical distribution of the fatigue life, and the residual strength under constant and variable amplitude loads, and (2) the effect of periodic proof tests, without any additional experimental data. Experimental test results show that predictions made from the present model agree quite well with the experimental results. The model is also applicable to fatigue of bonded or bolted composite joints.

Boundary-layer hygroscopic stresses in angle-ply composite laminates

Abstract: An investigation on hygroscopically- induced stress fields near g eometric b oundaries of finitewidth, a ngle-ply composite laminates is presented. The study is formulated on the b asis of the t heory of anisotropic hygroelasticity. By the i ntroduction of Lehknitskii's complex stress functions, governing partial differential equations of the sixth-order are established for composite laminates subjected to uniformly distributed moisture absorption. The hygroscopic stress singularity at the edges of a c omposite laminate is determined b y an eigenfunction expansion technique recently developed by the authors. Distributions of stresses and associated strain energy densities in the boundarylayer region of the composite laminate are obtained by using a boundary-colloca tion method. A symmetric, angle-ply (45°/-450/-450/4 50) graphitel epoxy is chosen for illustrative purposes in this study. The solution for the (+45), composite is presented to elucidate t he fundamental behavior of hygroscopic edge stresses in composite laminates. Of particular i nterest are the p resently introduced hygroscopic b oundary-layer stress intensity factors and their implications. Details of these are reported in the paper.