Showing papers on "Composite laminates published in 1991"

A Progressive Failure Model for Composite Laminates Containing Openings

Abstract: A progressive failure model is developed for laminated composites containing stress concentrations subjected to in-plane loading. The fundamental approach is to model a damaged lamina using a damaged ply constitutive relation in a simplified manner. The environmental effects including the thermal residual stresses and hygroscopic stresses arc taken into consideration. Parametric studies show that load increment only has little effect on the ultimate strength. When the number of elements for the finite element mesh increases to a certain value, the predicted ultimate strength approaches a stable value. The predictions for the ultimate strength, stress-strain behavior and the damage progression agree reasonably well with the experimental result.

Plate wave acoustic emission

Abstract: Plate theory is more easily applied to the analysis of composite laminates than exact three‐dimensional elasticity theory. Under conditions such that plate theory is applicable, it is suggested that plate waves are useful for understanding acoustic emission (AE) phenomena. To test this idea, pencil leads were broken on aluminum plates and composite plates, and the resulting waves were detected with a broadband ultrasonic transducer. Both the fundamental extensional and flexural modes were observed. Their characteristics are described and the implications for AE source location are discussed as well. Several transducers, commonly used for acoustic emission measurements, are compared with regard to their ability to reproduce the characteristic shapes of plate waves. Their different responses show why similar test specimens and test conditions can yield disparate results.

Fatigue of composite materials

Abstract: 1. Introduction (K.L. Reifsnider). 2. Damage and Damage Mechanics (K.L. Reifsnider). The fatigue effect in composite materials. The fatigue process. The mechanics of damage development. The mechanics of strength reduction. 3. Damage Characterization (R. Talreja). Damage in composite materials. Internal variable characterization of damage. Vectorial representation of composite damage. A continuum damage theory. Damage evaluation. 4. Fatigue Behaviour of Composite Laminates (W.W. Stinchcomb, C.E. Bakis). Loads, damage, and response - an example. The damage process. Mechanics of stress redistribution. Consequences of cyclic loading: strength, stiffness, and life. 5. Delamination of Composite Materials (T.K. O'Brien). Perspective. Mechanics of delamination. Delamination growth. Characterization of delamination resistance. Conclusions and future directions. 6. Fatigue of Metal Matrix Composites (W.S. Johnson). Matrix-dominated fatigue damage. Fiber-dominated fatigue damage. Self-similar fatigue damage growth. 7. Fatigue Behaviour of Short Fibre Composite Materials (J.F. Mandell). Fatigue lifeline trends. Aligned short fiber composites. Chopped strand composites. Injection molded composites. Summary and recommendations. 8. Viscoelastic Behaviour of Laminated Composite Materials (D.A. Dillard). Linear viscoelastic analysis. Non-linear viscoelasticity. Environmental and time effects. Viscoelastic design philosophy. 9. Moisture in Composites: Sorption and Damage (Y. Weitsman). Elementary considerations. Viscoelastic behaviour. Stress effects on diffusion. Moisture-induced damage. Chemical and physical effects and other observations. 10. Life Prediction for Resin-Matrix Composite Materials (G.P. Sendeckyj). Constant amplitude fatigue theories. Cumulative damage theories. Statistical interpretation of fatigue experiments. Discussion and recommendations. 11. Statistical Considerations (R. Talreja). Fatigue reliability: basic concepts. Fatigue reliability of composites. Probabilistic models for composite strength. Conclusions. References.

Experimental and theoretical analysis of composite I-beams with elastic couplings

Abstract: This paper presents a theoretical-cum-experimental study on the static structural response of composite I-beams with elastic couplings. A Vlasov type linear theory is developed to analyze composite open section beams made out of general composite laminates, where the transverse shear deformation of the beam cross-section is included. In order to validate this analysis, graphite-epoxy and kevlar-epoxy symmetric I-beams were fabricated using an autoclave molding technique. The beams were tested under tip bending and torsional loads, and their structural response in terms of bending slope and twist measured with a laser optical system. Good correlation between theoretical and experimental results is achieved. A 630 percent increase in the torsional stiffness due to constrained warping is noticed for graphite-epoxy beams with slenderness ratio of 30. Also extension-twist coupling 'B16' of flanges of these I-beams increases the bending-torsion coupling stiffness of

Multiple Transverse Cracking and Stiffness Reduction in Cross-Ply Laminates

Abstract: The stiffness reduction as a result of multiple transverse cracking in cross-ply laminates and the crack density dependence on the applied tensile stress are analyzed by linear elastic fracture mechanics. The stress field distribution is obtained by the principle of minimum complementary energy. Two models are suggested which describe the non-uniform stress distribution in the thickness direction of the 0° layer. They contain the variational approach presented by Hashin as a particular case. Elastic ply properties and the Mode I critical strain energy release rate GIc for transverse cracking are the experimental data needed. Model predictions are compared with experimental data for glass fiber/epoxy, AS4/3502, and AS/3501-06 carbon fiber/epoxy cross-ply laminates. The predictions from the suggested models describe both the constraint effect and the crack saturation phenomenon.

Effects of Stacking Sequence on Impact Damage Resistance and Residual Strength for Quasi-Isotropic Laminates

Abstract: Residual strength of an impacted composite laminate is dependent on details of the damage state. Stacking sequence was varied to judge its effect on damage caused by low-velocity impact. This was done for quasi-isotropic layups of a toughened composite material. Experimental observations on changes in the impact damage state and postimpact compressive performance were presented for seven different laminate stacking sequences. The applicability and limitations of analysis compared to experimental results were also discussed. Postimpact compressive behavior was found to be a strong function of the laminate stacking sequence. This relationship was found to depend on thickness, stacking sequence, size, and location of sublaminates that comprise the impact damage state. The postimpact strength for specimens with a relatively symmetric distribution of damage through the laminate thickness was accurately predicted by models that accounted for sublaminate stability and in-plane stress redistribution. An asymmetric distribution of damage in some laminate stacking sequences tended to alter specimen stability. Geometrically nonlinear finite element analysis was used to predict this behavior.

A Plasticity-Based Constitutive Model for Fibre-Reinforced Composite Laminates

Abstract: A continuum mechanics approach is adopted herein to develop a constitutive model for the nonlinear behaviour of laminated composites up to and including ultimate failure. The proposed model for single layers of fibre-reinforced composites (FRC) is derived within the framework of rate-independent theory of orthotropic plasticity. Both unidirectional and bidirectional (e.g., woven) FRC layers are modelled. The individual layer constitutive equations are superimposed using classical lamination theory to yield the global laminate response. The model's accuracy is illustrated by comparing the results of numerical simulations with experimental data available in the literature.

Application of Neutron Diffraction to Measure Residual Strains in Various Engineering Composite Materials

Abstract: An experimental neutron diffraction technique was used to measure residual strains that developed in multiphase composite materials during postfabrication cooling. The reinforcement geometries that were studied include unidirectional fibers, randomly oriented single crystal whiskers, and equiaxed particles. Both metal and ceramic matrices and reinforcements were considered

Fatigue Behavior of Composite Laminates

Abstract: Fatigue behavior of composite materials and structures is a complex phenomenon consisting of cyclic load-induced events and processes over time which determine long-term performance. The events and processes, generically called damage, combine in such a way as to alter the state of the material and change the response of the composite to the extent that it may fail to satisfy its intended service requirements. This chapter begins with an illustration of some of the distinctive features of fatigue behavior of composites, followed by a description of the fatigue damage process. The mechanics of stress redistribution due to damage are presented in the context of experimental evidence. Finally, fatigue behavior of several composite material systems is discussed in terms of strength, stiffness, and life of notched and unnotched composite laminates under various loading modes.

Prediction of the Compressive Strength of Thick-Section Advanced Composite Laminates:

Abstract: An analytical study, with the twin objectives of (1) development of a simple, yet comprehensive approach for prediction of the compressive strength of thick-section advanced composite (graphitc/cpoxy) cross-ply laminates and (2) explanation of unacceptably low strength of the (90N°/0°)k cylindrical specimens tested at David Taylor Research Center (DTRC) under hydrostatic compression, has been undertaken. A simple closed-form expression for the kink mode compressive strength has been derived, under the simplifying assumptions of small deformation, material linearity, local flatness and piecewise linear distribution of the surface-parallel displacements through the thickness, using the minimum potential energy approach. Initial fiber misalignment (maximum tangent angle), ultimate fiber shear strain and the two transverse shear moduli are found to be the key parameters, which limit the compressive strength of the aforementioned thick-section composite laminates. An analysis pertaining to the elastic plane st...

Damage Prediction in Cross-Plied Curved Composite Laminates

Abstract: Analytical and experimental work is detailed which is required to predict delamination onset and growth in a curved cross plied composite laminate subjected to static and fatigue loads. The composite used was AS4/3501/6, graphite/epoxy. Analytically, a closed form stress analysis and 2-D and 3-D finite element analyses were conducted to determine the stress distribution in an undamaged curved laminate. The finite element analysis was also used to determine values of strain energy release rate at a delamination emanating from a matrix crack in a 90 deg ply. Experimentally, transverse tensile strength and fatigue life were determined from flat 90 deg coupons. The interlaminar tensile strength and fatigue life were determined from double cantilevered beam specimens. Cross plied curved laminates were tested statically and in fatigue to give a comparison to the analytical predictions. A comparison of the fracture mechanics life prediction technique and the strength based prediction technique is given.

Optimal Design for Two-Dimensional Structures Made of Composite Materials

Abstract: The paper deals with the problem of finding the optimal orientation of orthotropic properties for an elastic body, subjected to a plane state of stress, in order to maximize the stiffness of the body itself

Axisymmetric transient thermal stress analysis of a multilayered composite hollow cylinder

Abstract: This article is concerned with axisymmetric, transient, thermal stress analysis of a hollow cylinder composed of multilayered composite laminates with temperature changes in the radial and axial directions due to axisymmetric heating from the outer and/or the inner surfaces. For analysis, we apply the methods of Fourier cosine transform and Laplace transform to the temperature field and the thermo-eiastic potential function and apply Love's displacement function to the thermo-etastic field. We then obtain the exact solutions for the temperature and thermal stress distributions in a transient state. Moreover, we apply the theoretical developments proposed in our present article into the analysis of a hollow cylinder with nonhomogeneous material properties such as a functionally gradient material.

Buckling of anisotropic composite plates under stress gradient

Abstract: In composite structures characterized by lightweight, thin‐walled members, the linear buckling load is one of the most important design considerations. Plate structures (bridge decks and ship hulls) are often subjected to differential compression due to nonuniform bending during their service life. This paper presents a general formulation for the buckling of rectangular, anisotropic, symmetric, angle‐ply composite laminates under linearly varying, uniaxial compressive force using the energy method in conjunction with orthogonal polynomial sequences, generated by a Gram‐Schmidt process. Orthogonal polynomials provide a simpler and efficient tool for handling complex combinations of simple and clamped boundaries. The present study highlights the unusual insensitivity of the buckling load of anisotropic laminates to fiber orientation under in‐plane tension‐compression‐type loading. Such behavior is not known to exist under idealized loading of constant, uniaxial compression, and orthotropic material behavio...

Issues in diaphragm forming of continuous fiber reinforced thermoplastic composites

Abstract: A literature survey is presented on sheet-forming methods and mechanisms for continuous fiber reinforced thermoplastics. The diaphragm forming process is shown to be one of the more promising fabrication routes for complex-curvature structures. The primary deformation mechanisms involved in the sheet-forming processes are identified and discussed. Earlier approaches to develop mathematical models either have been kinematically based or have treated only one of the primary deformation mechanisms. A promising constitutive model for the highly anisotropic behavior of the composite at forming temperature is examined. The composite sheet is assumed to behave as a transversely isotropic Newtonian fluid that is both incompressible and inextensible in the fiber direction. The second section of the paper treats the experimental development of the polymeric diaphragm forming process for thermoplastic composites. The viscoelastic properties of the diaphragm material are characterized by dynamic mechanical analysis. The rate sensitivity of the phenomenon of shear-buckling during forming of certain cross-ply and quasi-isotropic composite laminates is investigated, using a shallow female mold. Finally, the interface condition between the diaphragm and the composite during forming is examined.

Analysis of Test Specimens for Interlaminar Mode II Fracture Toughness, Part 1. Elastic Laminates:

Abstract: Results from approximate and exact stress analyses are reported for elastic stress and displacement fields in two interlaminar mode II fracture toughness specimens. End-notched flexure tests for laminates and unidirectional composites are studied in detail. Based on approximate closed form solutions for energy release rates procedures are suggested for fitting compliance versus delamination length data. Such procedures should be followed when through the thickness shear moduli of the layers are small as compared to axial moduli.

Strain-energy release rates and the fatigue growth of matrix cracks in model arrays in composite laminates

Abstract: Crack growth in the transverse plies of cross-ply composite laminates has been investigated both experimentally and theoretically. Expressions for the strain energy release rate associated with the growth of cracks in model arrays have been obtained using both the compliance approach and the energy method. Measurements of compliance change with crack length were obtained using glass-epoxy laminates and compared with various predictions. Correlations between the crack growth rate and the strain energy release rate range indicate that a Paris law is applicable.

Dual polymer bonding of thermoplastic composite structures

Abstract: This paper describes a process that facilitates fusion bonding of thermoplastic composite components without the need for complex fixtures and without disrupting the fiber alignment in the component laminates. The dual polymer bonding process, Thermabond, requires that an interlayer polymer be fused to the surface of each laminate prior to bonding. The characteristics of the interlayer polymer allow for joining of the components at a temperature below the softening/melting point of the reinforced polymer in the composite laminates. This leads to significant processing advantages without significant loss in mechanical performance. Discussions of resin compatibility, the effect of process conditions on mechanical performance, and the application of the APC-2/PEI Thermabond system to various structural components are included.

The Adiabatic Thermoelastic Effect in Laminated Fiber Composites

Abstract: The influence of material inhomogeneity and anisotropy on the reversible, adiabatic thermoelastic effect in laminated, continuous-fiber composites is investigated analytically and experimentally. The plane-stress solution for strains in a uniform laminate is combined with a simple micromechanics description of a fiber-reinforced lamina to approximate the nonhomogeneous strains in the fibers and matrix. The equations of anisotropic, linear thermoelasticity are then used to evaluate the temperature change in each of the microconstituents during an adiabatic deformation. The average temperature change of the surface plies of several carbon/epoxy laminates are computed and compared with experimental data obtained via differential infrared thermography. Results indicate that material parameters such as the volume fraction and thermoelastic properties of the microconstituent materials, the orientations of the laminae within the laminate, and the orientation of the lamina on the surface of observation affect the...

Non-linear dynamics of laminated plates with a higher-order theory and C0 finite elements

Abstract: Impact responses of composite laminates are investigated with C 0 finite elements. A nine-node isoparametric quadrilateral element based on a higher-order theory and the von Karman large deflection assumptions is developed. An experimentally established contact law which accounts for the permanent indentation is incorporated into the finite element program to evaluate the impact force. In the time integration, the explicit central difference technique is used in conjunction with the special mass matrix diagonalization scheme. Numerical results, including the contact force histories, deflections and strains in the plate, are presented.