Composites Manufacturing最新文献

The cure and consolidation of thermoset composite materials is a complex physical process involving combined heat transfer, resin curing, fibre bed compaction and fluid flow. Finite difference simulations for either the heat transfer part of this process or for the resin flow part can be found in the literature. The present paper develops and implements the finite element form of the equations governing composites processing and successfully combines resin flow and heat transfer modelling through the temperature-dependent viscosity. Specifically, this paper (i) sets up the finite element form of the Fourier heat conduction equation and the Gutowski/Davé resin flow equation through the Ritz formulation, (ii) overviews the computer implementation of these equations and (iii) presents selected output results to demonstrate the engineering value of the simulation program. An autoclave process is analysed as a test case. The results illustrate that the developed computer code (FCURE) combines existing mathematical models with the versatility of the finite element method into a simulation tool with engineering potential.

In order to develop preforms for large complex parts capable of meeting the demands of global and local loading conditions, resin transfer moulded parts are typically broken down into multiple elements. A separate preform is developed for each of these sub-sections, which are then assembled in the mould cavity and combined by the moulding process. While the decomposition of a structure into sub-sections eases the burden on the preforming operation(s) to form the desired shape with the appropriate microstructure, it also raises the issue of joining these multi-element preform sections. In previous composites applications, the presence of a joint was traditionally considered only in terms of its influence on the performance of the part, since the fibre and resin were already combined into pre-impregnated tape. However, resin transfer moulding (RTM) is a two-step process: fibre preforming followed by resin impregnation and cure. Since the resin must flow through the preform and wet-out the individual fibre bundles, the inclusion of a preform joint in the design of an RTM part's microstructure must be considered in terms of processing (resin flow and wet-out) as well as performance. In this paper we examine the influence of preform joint on resin flow through both computational and experimental approaches. The implications of such joints on the design and processing of RTM parts is discussed.

When forming continuous fibre-reinforced thermoplastic (CFRT) sheets into three-dimensional components, interply shearing may be necessary in order to accommodate the out-of-plane bending because the fibres severely constrain the deformation along the fibre directions within their planes. Furthermore, thermoforming takes place at elevated temperatures so that the molten matrix polymer becomes fluid. These two factors are of prime importance in analysing any forming process with thermoplastic composite materials. This paper examines the process of forming unidirectional Plytron® (a glass fibre-reinforced polypropylene composite, originally developed by ICI, UK) sheets into V-bends at a constant elevated temperature, and compares the experimental results with those predicted by an analytical model for plane strain bending of an incompressible Newtonian fluid reinforced with a single family of inextensible fibres. The shape of a strip as it is formed, the effects of temperature and forming speed on the forming loads are also investigated. A major conclusion from this study is that Plytron sheets demonstrate a viscoelastic response when formed within their melting range and the degree of elasticity is increased by reducing the temperature, which, in turn, can reduce the fibre instability. The theoretical model provides useful results for evaluating the effective longitudinal viscosity of the composite sheet, the effects of forming speed and punch geometry on the bending stresses and also highlights the limitations of a Newtonian fluid model in comparison with the actual material response.

During filament winding of thick cylinders, fibre wrinkling often occurs which severely decreases compressive strength. To eliminate fibre wrinkling, appropriate processing conditions must be found. Fibre migration and stress relaxation due to resin flow are generally considered the most important factors affecting fibre buckling. Therefore, the effect of stress relaxation on fibre wrinkling during the filament winding process was investigated. To study the stress development during filament winding of thick cylinders, experiments were carried out using graphite/epoxy prepreg tows as well as dry graphite fibre. Cylinders of approximately 12 mm thickness were hoop wound on a 50.8 mm diameter aluminium mandrel. Winding tensions ranged from 13 to 34 N and winding speed was constant. A foil-type pressure sensor was applied on the mandrel to monitor the interface pressure throughout winding and storage of the cylinder. Significant stress relaxation was found to occur during winding with prepreg tow. Mandrel pressure increased over the winding of the first eight layers or so. However, between the winding of one layer and the next, mandrel pressure dropped quickly. Also, it began to decrease after reaching a maximum value. A stress relaxation analysis was carried out to determine the stress in the cylinders during winding. Several parameters were not known a priori and had to be inferred from the data. Stress distributions following winding were calculated for each case. The radial stress in prepreg wound cylinders was found to relax nearly to zero in the inner part of the tubes. Compressive circumferential stresses occurred throughout each of the cylinders. However, they reached greater magnitudes in the dry wound cylinders due to very low radial moduli. No fibre wrinkling was evident in any of the wound cylinders.

The development of industrial-scale manufacturing techniques for thermoplastic composites requires new processing technologies. The filament winding of thermoplastic preforms, such as powder-impregnated or intermingled yarns, would be more economical than winding of thermosets, if similar fibre placement speeds could be achieved for similar costs of equipment and preforms. The present experimental work compares two different processing techniques based on hot air and short wave infra-red spot heating, and discusses the influence of preheating the preforms and mandrel. The research work is based on powder-impregnated poly(ether imide) preforms, which are processed to hoop wound tube specimens. These preforms were commercialized under the trade name FIT (Fibres Impregnated with Thermoplastic) in the early 1980s. Characterization of the laminate quality is based on micrographs. The results show the importance of continuous heating at low heating rate. Discontinuous temperature peaks above the matrix melting temperature lead to its thermal degradation and therefore to poor laminate quality.

One way of overcoming the shortcomings of carbon fibres is to coat them with SiC. In this work, carbon fibres were coated with a polycarbosilane solution and then pyrolysed continuously at high temperature to obtain the SiC coating. Effects of the coating process on the structure of the coating and the properties of the coated fibres were studied in detail. The results show that the solution coating process is a simple and applicable technique. The uniform and continuous coating improved the oxidation resistance and strength of the carbon fibres, as well as their wettability with aluminium. The coating also controlled the harmful reaction at the interface of aluminium matrix composites and improved composite strength.