Composites Manufacturing最新文献

For the double diaphragm forming process, laminate wrinkling is a major failure mode for both thermoplastic and thermoset composites. In this paper, we compare experimental observations on the wrinkling of aligned fibre thermoset composites with theoretical scaling laws based on ideal kinematics (i.e. constant interfibre spacing and constant thickness). Differences between the ideal predictions and actual results are explained in terms of deviations from ideal kinematics. Differences between thermoplastic and thermoset composites are discussed, and an empirical scaling law for the effect of part size on wrinkling is given.

This paper presents a detailed statistical analysis on geometric imperfections recorded on two series of nominally identical composite cylinders. These defects can be classified in two categories, both due to the particular manufacturing method used: out-of-roundness and change of thickness due to the overlapping of various layers. The statistical analysis is developed for various purposes: to evaluate the common properties of cylinders with different laminations, to build up a characteristic model for the geometric imperfections suitable for probabilistic simulations in buckling analysis and to identify the parameters for quality control processes. The analysis of the change in thickness due to overlapping layers allows evaluation of the stiffening effects of the manufacturing process that, in some cases, could affect the buckling behaviour of composite cylinders. A standard procedure for the characterization and qualification of manufacturing processes for composite shells, with particular attention to the factors that influence their buckling behaviour, is proposed.

Powder impregnation techniques have been developed to increase design and manufacturing flexibility with thermoplastic composites. The effect of pressure, temperature, mould closing rate and time on the consolidation of poly(ether ketone ketone) (PEKK) powder impregnated glass fibre towpregs in compression moulding were studied. A design of experiments approach was used. Isothermal pultrusion experiments using nylon 11 and PEKK powder impregnated glass fibre tows were conducted. These experiments were repeated at different temperatures and pulling speeds. The microstructural changes during coating and consolidation of the powder impregnated tows were studied. Temperature and mould closing rate were observed to be the significant parameters affecting consolidation in compression moulding. Superposition of the pulling force-pulling speed data at different processing temperatures was demonstrated in pultrusion.

The anisotropic rheology of a model composite consisting of a temperature-sensitive viscous liquid matrix reinforced by aligned and virtually inextensible fibres of nylon has been studied experimentally using a custom-built linear oscillator. The composite was characterized dynamically both along and transverse to the fibre direction over a wide frequency range and for different fibre volume concentrations. Towards the limit of zero shear the composite was found to exhibit a yield stress. The temperature dependence of the matrix dynamic viscosity was used to study the dependence of the longitudinal and transverse dynamic viscosities of the composite on the dynamic viscosity of the matrix. The dependence of the composite dynamic viscosities on fibre volume concentration is compared with models of the steady shear dependence for aligned fibre systems that have been reported in the literature.

The Advanced Research Projects Agency initiated a major technology effort to develop and demonstrate cost effective, advanced fabrication methods for marine structures. In situ consolidation of thermoplastic composite structures in concert with automated fibre placement offers the premise to produce affordable, high quality parts. In situ consolidation processing eliminates costs due to hand lay-up, bagging and autoclaving, as well as costs associated with acquiring, operating and maintaining an autoclave. Automated fibre placement with high quality and tight dimensional control offers the ability to make complex parts, to lay materials at any fibre angle and path, to vary bandwidth and to cure using in situ consolidation. This paper will present. process-related issues associated with the thermoplastic, hot gas, in situ consolidation of 61 cm diameter cylindrical demonstration models, NOL rings and test specimens to achieve low manufacturing costs. These process-related issues include process adaptation, throughput, part integration and scalability to larger diameter parts. Optimization of these factors in terms of manufacturing costs and quality (void content, mechanical properties) will enhance the development of the in situ consolidation fibre placement process into an affordable manufacturing technology. Thermoplastic materials investigated included carbon/poly(ether ether ketone) and carbon/poly(phenylene sulfide).

In this paper, the change of fibre orientation that occurs during the moulding flow of a composite material, based on a thermoplastic matrix reinforced by chopped fibres, is investigated. The fibre orientation is described by a distribution function which is divided into two elementary distribution functions, each being concerned with part of the fibres. The model presents the changes of these functions, taking into account the interaction between all fibres and the transverse shear. The model agrees with experimental data obtained in the flow of composites in a plate-type compression moulding process.

Macroscopic capillary pressure and microscopic interparticle forces due to surface tension are examined. A general equation for the capillary pressure during impregnation is derived and subsequently specialized to particular processes. For fibre composites, the capillary pressure can be of the order of ±10⁴ Pa, the sign depending on the contact angle between solid and liquid. Next, the attractive and repulsive forces between particles connected by liquid droplets are analysed by two different model geometries. At contact angles between π/2 and π, an equilibrium particle separation distance is obtained in the absence of applied force. At lower contact angles, spontaneous impregnation can be achieved. The effect of capillary action on impregnation rate may be significant if applied pressures are small (e.g. filament winding) but negligible at applied pressures greater than ∼100 kPa (e.g. compression moulding). The topology and concentration of voids may, however, be greatly influenced by surface energies.