Composites Engineering最新文献

The static and dynamic characteristics of NITINOL-reinforced composite plates are influenced primarily by the temperature distribution inside the composite matrix. Such distribution arises from the electrical heating of NITINOL fibers embedded along the neutral plane of these composite plates. When temperatures are developed above the martensite transformation temperature of the NITINOL fiber, the elastic modulus of the fibers increases approximately fourfold and significant phase recovery forces are generated. Such thermal activation of the NITINOL fibers increases the elastic energy of the fibers and enchances the stiffness of the plates, provided that the phase recovery forces are high enough to compensate for the loss of the modulus of elasticity of the composite and counterbalance the generated thermal loads. Understanding the interaction between the thermal, static and dynamic characteristics of the NITINOL-reinforced plates is essential to tailoring the performance of these plates to match changes in the operating conditions. Such an interaction is influenced primarily by the temperature distribution inside the plates during the activation and de-activation of the NITINOL fibers. In this study, a thermal finite element model is developed to determine steady-state and transient temperature distributions inside NITINOL-reinforced composite plates resulting from different activation strategies of the NITINOL fibers. The theoretical predictions are compared with experimental measurements in order to validate the thermal finite element model. The resulting temperature distribution can be used to determine an average modulus of elasticity of the composite. The average temperature rise above ambient can also be used to compute the axial thermal loading on the composite plate. Such predictions are utilized in computing the static and dynamic characteristics of NITINOL-reinforced plates which are presented in Parts II and III of this paper, respectively.

The use of a genetic algorithm for the minimum thickness design of composite laminated plates is explored. A previously developed genetic algorithm for laminate design is thoroughly revised and improved, by incorporating knowledge of the physics of the problem into the genetic algorithm. Constraints are accounted for by combining fixed and progressive penalty functions. Improved selection, mutation, and permutation operators are proposed. The use of an operator called scaling mutation that projects designs toward the feasible domain is investigated. The improvements in the genetic algorithm are shown to reduce the average price of a genetic search by more than 50%.

A set of analytic solutions is presented to predict the dynamic response of simply-supported, doubly curved, cross-ply laminated shells impacted by a solid striker. The solutions are based on a higher-order shear deformation theory (HSDT) which accounts for the parabolic distribution of transverse shear strain through the thickness of a shell and tangential stress-free boundary conditions on the surface of the shell. An analytic impact force function recently proposed by the authors is used to predict the contact force between the striker and the shell and this is incorporated into the solutions. Fundamental frequencies of cross-ply laminated spherical and cylindrical shells are calculated using the present solutions and the results are compared with those published by others. In terms of the characterising coefficients, the solutions can be reduced to versions of the first-order shear deformation theory (FSDT) and the classical shell theory (CST). A comparison of the solutions based on the various theories (HSDT, FSDT and CST) is also made.

In this paper, after a brief discussion of the elementary concepts of fracture mechanics in nonhomogeneous materials, a number of typical problem areas relating to the fracture of functionally gradient materials (FGMs) are identified. The main topics considered are the investigation of the nature of stress singularity near the tip of a crack fully embedded in a nonhomogeneous medium, the general problem of debonding of an FGM coating from a homogeneous substrate, the basic surface crack problem in FGMs and cracking perpendicular to the interfaces, periodic surface cracking and the associated problem of stress and energy relaxation, and the problem of stress concentration at and the initiation and growth of delamination cracks from the stress-free ends of FGM-coated homogeneous substrates under residual or thermal stresses. Each topic is very briefly reviewed, some sample results are presented and comparisons with the corresponding results obtained from homogeneous materials are made.

There is growing speculation that the interphase in polymer composites is often a region of nonuniform material properties. This is significant given the critical role of the interphase in determining overall composite behavior. The present investigation utilizes a micromechanical model based on the three-phase method of cells to examine how spatial variations in the interphase elastic properties are predicted to influence the residual thermal stresses in carbon-fiber-reinforced epoxy. This is the first such study of its kind based on a true three-phase version of the method of cells. A total of sixteen different composite configurations are considered in which the interphase Young's modulus and/or the interphase thermal expansion coefficient may vary as a function of the radial coordinate. The interphases are specified such that their Young's modulus and thermal expansion coefficient may be above or below that of the epoxy matrix. The residual thermal stresses, as well as the effective composite properties, are evaluated as a function of the fiber volume fraction, the interphase thickness and the spatial nonuniformity of the interphase properties. The results indicate that the introduction of interphase property gradients is predicted to primarily influence the state of stress within the interphase. Depending upon how the interphase properties are specified to vary, the residual stresses within the interphase may either be compressive or tensile.

A micromechanical model based on the three-phase method of cells is used to predict the effective properties and thermo-mechanical stresses in carbon fiber/epoxy composites that have been modified by the addition of elastomer. Three composite configurations are considered: (1) carbon fiber/epoxy without elastomer, (2) carbon fiber/epoxy containing an elastomer interphase between the fiber and the matrix and (3) carbon fiber/epoxy with elastomer dispersed in the bulk matrix. The variables investigated include the elastomer volume fraction, the elastomer placement and the fiber volume fraction. Results from the parametric study indicate that the composite response is significantly influenced by all three variables. Positive aspects are identified with respect to each elastomer configuration. In general, a thin, concentric elastomer interphase surrounding each fibril appears more beneficial to the reduction of tensile residual thermal stresses. However, elastomer dispersed in the matrix can also yield favorable stress profiles with relatively less degradation of the composite stiffness. The trends established are predicted to be essentially independent of the fiber volume fraction.

A new method is proposed to analyse the wave field in a laminated composite plate excited by an incident wavelet of low to intermediate frequency. In this method, several techniques are combined to avoid numerical difficulties and obtain a high computational efficiency. To deal with the problem caused by the inhomogeneity of the plate in the thickness direction, the finite element technique is used to divide the plate into strip elements. The Fourier transform is then used in the horizontal direction of the plate. In dealing with the time integration, the Newmark scheme (a direct integral scheme) is employed to obtain the displacement in the Fourier transform domain. Finally, the wave field in the plate is obtained by an inverse Fourier integration. The present method can be used for laminates with many layers, and the integrand for the inverse Fourier transform behaves very well and is easy to evaluate. Examples are presented to demonstrate the efficiency of the present method, and wave fields in laminated composite plates are also investigated.

The detail stress fields caused by fiber pullout or push-in while the fiber-matrix interface is under compressive thermal residual stress and undergoing frictional sliding are studied. The dominant stress fields in the local region at the immediate vicinity of the fiber protrusion point are solved using Muskhelishvili-Kolosov complex potential theory and asymptotic analysis. Parameteric studies determined the effects cast on the local fields by fiber-matrix material property combination, coefficient of friction, and the direction of relative fiber sliding. Reversing fiber sliding from pullout to push-in completely changes the nature of the local field.

Using two specific composite systems as examples (fiber push-in Nicalon/calcium aluminosilicate composite, and fiber pullout in AVCO-SCS-6/borosilicate composite), the germane features of the local fields are independently verified by finite element global analyses.