Composites Engineering最新文献

This paper presents a formulation of the stability problem for a rectangular composite plate reinforced by two types of fibers, one of them being both stiffer and more expensive than the other. An obvious design solution based on cost containment is to concentrate stiffer and more expensive fibers in the area of the plate where they can provide a maximum benefit to its stability. In the present paper, the stiffer fibers replace a certain fraction of “ordinary” fibers in the layers of the plate oriented along the load direction. Moreover, a distribution of the volume fraction of these fibers across the width of the corresponding layers is nonuniform (piece-wise distribution).

The goal is to maximize the buckling load subject to the constraint on the total cross-sectional area of the stiffer fibers. The solution can be obtained exactly by integrating the equation of equilibrium for each plate region where the stiffnesses are constant and satisfying the continuity and boundary conditions. Another approach, which is employed in this paper, is based on the Galerkin procedure. Numerical examples illustrate a possibility of a significant enhancement of the buckling load using functionally graded hybrid composite plates.

A parametric study is carried out to investigate various factors that influence the transverse properties of unidirectional SiC/Ti. Interface debonding is the beginning of material failure. It is important because a composite with debonded interfaces is no longer integral and it will be weakened. An interface failure criterion is developed. Selected models are analysed to investigate possible effects on interface failure initiation. The main considerations are the interfacial residual stresses and the stress concentration factors for the applied transverse load. Residual stresses are beneficial for transverse loading because they tend to delay interface debonding. The residual stress distribution is discussed. Increasing the interface strength improves the transverse properties. Decreasing fibre volume fraction by keeping the same fibre spacing and increasing the ply thickness yields higher stress and strain to interface failure. When the volume fraction is kept constant, the closer the fibres are placed, the higher the interface failure stress and strain. With the same model geometry, the interface debonds slightly earlier with rectangular fibre packing than with staggered packing. In most cases, the matrix material is elastic before interface failure initiates. The non-linear stress-strain behaviour is mainly due to interface debonding.

An elastic-plastic contact law that includes the effect of permanent deformation in the contact zone is used to study the transient response of a composite beam subject to impact. The governing differential equations are normalized to yield four nondimensional parameters which completely describe the impact event. A modal analysis procedure is used to spatially discretize the normalized boundary value problem to obtain a set of ordinary differential equations which are integrated numerically. Simulation results show that the local permanent deformations significantly affect the impact response. This indicates the importance of using an adequate contact law for an accurate prediction of the impact forces and strains during impact of composite structures.

An experimental study was carried out on woven fabric/epoxy composites focusing on their mechanical properties under uniaxial tensile, flexural, compressive, short beam shear and Mode I interlaminar fracture loading conditions. Composites with fiber glass and Kevlar 49 woven fabrics and with different fabric constructions and microfiber additives were investigated. The interlaminar fracture behavior was characterized using double cantilever beam (DCB) test and the fracture mechanism was analyzed by means of scanning electron microscopy (SEM). The mechanical properties were evaluated in uniaxial tension, three-point flexure, compression and short beam shear. The effects of the reinforcing fabric structure and microfiber additives to the matrix on the mechanical behavior and failure mechanisms of the composites were analyzed.

The bending behavior of a sandwich beam with a “soft” core and unsymmetrical laminated composite skins has been analytically investigated. The effects of the extension-bending coupling caused by the unsymmetrical layups on the bending behavior, are presented. The skins may be made of different materials, different layups, i.e. symmetrical or unsymmetrical, different geometries and may have different boundary conditions (even at the same section). The “soft” core is considered compressible and a change in its height is allowed. The analysis uses variational principles and models the core as a two-dimensional elastic medium, and the skins as one-dimensional composite laminated beams, or a composite panel with cylindrical bending. The analysis is general, rigorous, includes high-order effects and uses closed-form solutions for any type of sandwich construction, for any type of loading and for any combination of boundary conditions. The solutions include the stress and displacement fields along the beam and through the height of the core. Local effects in the vicinity of concentrated loads that consist of dimples and high bending moments in the skins and abrupt changes in the peeling stresses, are studied.

Fracture mechanics analyses of composite skin-stiffener debond configurations using shell elements are presented. Two types of debond configurations are studied: a flange skin strip debond configuration and a skin-stiffener debond configuration. The flange-skin strip configuration examines debond growth perpendicular to the stiffener while the skin-stiffener configuration examines debond growth parallel to the stiffener. Four-node and 9-node shell elements are used to model both debond configurations. The stiffener flange and skin are modeled as two different layers of elements whose translational degrees-of-freedom, in the bonded portion, of the corresponding flange and skin nodes are constrained to be identical. Strain energy release rate formulae are presented for both 4-node and 9-node element models based on the virtual crack closure technique (VCCT). In addition, average values of the strain energy release rates are calculated using a gradient method. The VCCT formulae and the gradient method are used to compute the strain energy release rates (G-values) for both debond configurations. The G-values predicted by these methods are compared with those predicted by plane-strain and 3D finite element analyses. Excellent correlation is obtained among all the analysis results, thus helping to validate the VCCT formulae derived for the 4- and 9-node shell elements.

The propagation of harmonic waves in a laminated composite consisting of an arbitrary number of layered anisotropic plates is studied. In the context of the generalized theory of thermoelasticity, each layer is allowed to have a monoclinic degree of symmetry in the thermoelastic sense. Waves are allowed to propagate along any angle from the normal to the plates, as well as along any azimuthal angle. The problem is treated analytically using a combination of linear orthogonal transformations and the transfer matrix method to reach the characteristic equation of the composite. Numerical illustrations are given in the form of dispersion and attenuation curves of different representative layering.

In this paper, a three-phase (fiber-interphase-matrix unit cell) micro-mechanics model (Gardner et al. (1993), Comp. Sci. Technol. 46, 307–318) is used to study the influence of fiber-matrix interphase properties on the overall effective elastic constants. Then, the behavior of measurable ultrasonic parameters such as velocities of the three ultrasonic bulk wave modes, and their reflection factor (as a function of wave orientation and frequency) is discussed as a function of interphase properties. Theoretical results are compared with experimental studies on both glass-epoxy and graphite-epoxy composites.