Composites Engineering最新文献

The aim of this study was to verify that explosive consolidation can be used as a processing route to produce metal-matrix composites, comprising an aluminum-based matrix and boron-carbide reinforcement. Composites containing 30, 50 and 70 volume percent boron carbide in an MB 85 aluminum-alloy matrix were successfully compacted at 10–12 GPa pressure. Transmission electron microscopy was used to examine the resulting microsctructure and interface quality. It is predicted that the high density of dislocations produced during the compaction will lead to accelerated aging in age-hardenable aluminum alloy matrices.

The fundamental frequency of laminated plates with single-stepped lap joints was investigated analytically and experimentally. For the analysis, a two-dimensional finite element code was developed and the fundamental frequencies of the laminated plates were extracted from the calculation of the smallest eigenvalue. In the experiment, external excitation was applied to the plates using an impact hammer. The frequency response functions of the laminated plates were obtained by a fast Fourier transform function built in a dynamic signal analyser from which the fundamental frequencies were obtained. The fundamental frequency decreased as the thickness of the adhesive layer increased. When the stacking sequence of the composite laminates was [θ/-θ]_2s, the fundamental frequencies decreased as θ increased. The analytical predictions were compared with experimental results. The fundamental frequencies of the laminated plates were closer to the results of plates without adhesive joints when the difference between the Young's modulus of the adhesive and the longitudinal modulus of the laminated plates was decreased.

Current advanced manufacturing techniques allow the continuous variation of fiber or inclusion volume fraction in metal matrix composites. With this technology, it is now possible to tailor a composite to the expected loads by using the constituent materials to redistribute the stress and strain states through the material. Lighter and more structurally efficient components will be obtained through this grading process. The focus of this paper is the evaluation of the effects of material property and fiber volume grading on the overall mechanical response of metal matrix composite tubes subjected to mechanical loadings. This is accomplished through the development of a fully elastic-plastic axisymmetric generalized plane strain tube model. This analytical model incorporates a micromechanics algorithm in order to determine the elastic-plastic response of a heterogeneous fiber-reinforced composite cylinder. An arbitrary number of heterogeneous concentric cylinders can be included in the model, each with independent material properties. The inelastic analysis is performed through the method of successive elastic solutions. The optimization algorithm used in conjunction with this solution procedure utilizes the method of feasible directions and accepts any combination of design variables, constraints, and objective functions. As an example of the effectiveness of this grading, it is possible to vary the fiber volume fraction in an SiC/Ti-24Al-11Nb tube in such a way that the effective stress at the critical inner surface of an internally pressurized tube is reduced. For a 50.8 mm (2in) thick tube with an internal radius of 25.4 mm (1 in) and an internal pressure of 206.8 MPa (30 ksi), a uniform 40% fiber volume fraction distribution results in a tube that begins to plastically yield at the inner radius. By grading the fiber volume fraction, the tube now behaves elastically under the same pressure loading, allowing the tube to have a wall thickness of 25.4 mm (1 in) before plastic yielding begins. This grading results in a 60% weight saving.

Based on the Mindlin plate theory, a finite element formulation is presented for the analysis of the free vibration of composite plates with multiple delaminations. The present formulation includes the effect of transverse shear defomations as well as the bending-extension coupling caused by the presence of delaminations, and can be easily used to compute the natural frequencies and mode shapes of delaminated plates with arbitrary boundary conditions. A variety of examples, including composite plates with multiple elliptical delaminations, are presented. The effects of boundary conditions, delamination size, delamination location and the number of delaminations on the natural frequencies and mode shapes of delaminated composite plates are investigated. The presence of local modes in the delaminated segments is also discussed.

An industrial polypropylene polymer is used to demonstrate the possibilities of measurement of texture by X-ray diffraction technique in organic materials. A three-dimensional study has been carried out on an isotactic polypropylene to characterize the influence of continuous and fragmented annealings at 398 K on the crystal orientations. Results are obtained for five planes—(110), (040), (130), (111) and (041)—and presented as pole figures in the direct space. They show a preferential orientation in the untreated sample. This orientation is slightly increased after annealing. Moreover, fragmented or continuous annealings have the same effects on the crystal orientation.

Curved composite right-angle brackets have been shown to warp during manufacture due to the response of the part geometry to the large differences in the in-plane and out-of-plane shrinkage. In addition, it has been suggested that warpage in flat, uniaxial composites can be related to through-thickness volume fraction gradients which yield a significant degree of laminate asymmetry. The combination of this geometry effect and a graded volume fraction is investigated in an effort to explain differences in the amount of warpage in curved right-angle brackets produced on convex tooling versus on concave tooling. A computerized metallographic image analysis technique is used to measure the laminate homogeneity and the volume fraction gradients are determined for composites produced on each tooling geometry. Based on these experimentally measured values of curvature and volume fraction gradient, it is shown that the observed tooling geometry dependent differences in warpage noted in many components can be explained. In addition to explaining the tooling geometry dependent manufacturing warpage, this improved understanding of the mechanisms involved suggests the use of concepts of functionally graded materials to help minimize component distortion.

Since the shape and ingenious construction of biological hard tissues are the result of a continuous process of optimization, their basic characteristics such as microstructures, functions, and modelling systems fascinate the designers of engineering structures. Through the study of functionally graded materials, we hope to develop new superior material/structure concepts by using or modifying the construction of living organisms. The ingenious construction of bamboo was studied herein to help in the understanding of the principles and the design processes found in biological materials which are multi-phased and functionally graded composites. It was found that the ability of a bamboo cell to generate electrical signals when stressed was an apparently similar function to that of the piezoelectric effect in bone which is stressed. It is also suggested in this paper that the electrical properties play an important role in the modelling/remodelling of the skeletal system in biological hard tissues. It is concluded that a bamboo structure is designed to have uniform strength at all positions in both the radial direction on the transverse section and the lengthwise direction, and that bamboo is a self-optimizing graded structure constructed with a cell-based sensing system for external mechanical stimuli.

In this paper, a stress analysis of thick laminated conical tubes is conducted on the basis of a thick-shell theory. The loading applied to the tubes is quite general. It can be surface loads, boundary loads, axisymmetric loads or non-axisymmetric loads. The effect of transverse shear is taken into account by a first-order theory. Governing equations involve ten unknowns: five displacements and five stress resultants. They are solved by a semi-analytical method that is a combination of Fourier series expansion, finite difference scheme and Riccati transfer matrix method. The present theory can also be applied to the analysis of any axisymmetric laminated tube or shell that may approximately be divided into a series of conical shell segments. Since the lamination properties used in the finite difference scheme are defined locally, the present theory allows the axisymmetric tube or shell to be of variable thickness and lay-up. The validity of present analysis is confirmed by experimental results.