Composites Manufacturing最新文献

When a rectangular sample of aligned, continuous fibre-reinforced composite is subjected to normal pressure, it has been observed that resin is squeezed out parallel to the fibres and the fibres flow transversely. The fibres deform so that the sample becomes barrel-shaped. A three-dimensional code has been developed to simulate this flow. The material is modelled as a transversely isotropic continuum in which the fibre direction is specified at each point by a vector a. The system of coupled equations is solved using a finite difference technique. The transverse and longitudinal viscosities are assumed to be functions of the fibre volume fraction which increases as the resin is forced to percolate parallel to the fibres. The stress equations of motion are discretized using central differences for a fixed orientation and the discretized equations are solved using a pseudo-time technique. The converged solution is then used to determine the change in fibre direction at each point of the continuum. The process is repeated in real time using the new fibre orientation. In the momentum equations, the viscous terms are treated explicitly and the pressure gradient implicitly. A projection method is used to ensure that the mass is conserved at each time step. The results are in broad agreement with the experimental observations and demonstrate the success of the continuum model to predict flow behaviour.

The paper investigates the suitability of the idealized fibre-reinforced fluid (IFRF) model for the thermoforming of fabric-reinforced thermoplastic sheets, and a strategy is proposed for determining the materials parameters required to characterize the sheet theological behaviour. The IFRF theory for a viscous fluid with two inextensible directions is developed for modelling fabric sheets and specific forms of the constitutive equation are derived. Some simple flows are analysed and it is shown that in throughthickness shear flows, as for example in a torsion rheometer experiment, the fabric angle ϖ remains constant, whereas in in-plane flows ϖ is a function of the strain rate. Trellis deformations are investigated by considering the in-plane stretching flow of a fabric with fibres inclined to the load direction. The torsion rheometer test is analysed for a fabric pre-deformed to a fabric angle ϖ. In this case, tests on rectangular specimens with different aspect ratios and fabric angles are proposed which enable the three viscosities in the model to be determined.

An important issue in sheet forming of continuous fibre-reinforced thermoplastic composites is the tendency of the laminates to buckle out of plane under rapid forming conditions. This paper outlines a method used to predict the stress patterns responsible for buckling, and presents experimental results that correspond with the predictions. A finite element formulation for ideal fibre-reinforced Newtonian fluids, featuring the twin kinematic constraints of material incompressibility and fibre inextensibility, is used. A mixed penalty finite element approach is adopted, with independent interpolation of tension and velocity solution fields. An analysis model consistent with an assumption of plane stress is used. For multi-ply lay-ups, each ply is analysed individually, and average stress predictions for the laminate are obtained on this basis. A detailed comparison between numerical stress predictions and experimental buckling patterns is presented for central indentation of circular unidirectional, cross-ply and quasi-isotropic preforms. Parameters influencing the magnitude and location of peak tangential stresses include tangential fibre lengths and diaphragm/composite viscosity ratios. The effect of sheet width and shape on the instability patterns is investigated for quasi-isotropic laminates of different shapes, using both numerical and experimental techniques.

When designing filament-wound parts, use of an integrated strategy is recommended to take advantage of the benefits of composites despite limitations of the filament winding process. This paper describes a computer-integrated methodology for the design of filament-wound parts which includes: (1) initial part design using a computer-aided design system; (2) preliminary finite element analysis to determine ideal fibre orientations; (3) fibre path generation, including non-geodesics, to obtain feasible fibre paths; (4) choice of final lay-up sequence; and (5) composite finite element analysis to adapt the final lay-up until strength and stiffness requirements are met. The proposed methodology, embodied in the computer code CAWAR, is illustrated by application to a conical filament-wound part.

In order to optimize the diaphragm forming process, forming and heating studies have been carried out for continuous fibre-reinforced poly(ether imide) laminates and polyimide diaphragms. Different tooling concepts have been evaluated with respect to the reduction of manufacturing time. Based on these experimental results, a new machine has been designed and built for the automated manufacturing of advanced thermoplastic composite parts.

Diaphragm forming offers several advantages over other forming techniques in the manufacture of advanced thermoplastic composites. The technique can be used in the forming of parts with complex curvature and can produce excellent surface finish. This work investigates the effect of buckling in both single- and double-curvature moulds, while forming carbon fibre-reinforced poly(ether ether ketone) (APC-2). A control system was set up to provide linear displacement of parts at rates of 1–100 mm min⁻¹. Buckling was established for both cross-ply and quasi-isotropic lay-ups in a double-curvature elliptical dish mould. Forming rate experiments were also carried out on a single-curvature 90° mould, with no buckling occurring at forming rates up to 100 mm min⁻¹ The conditions likely to cause buckling were calculated for the 90° female mould, using both the tensile properties of the diaphragm material and interply shear data for APC-2 laminates. An investigation was also made into the spring-forward effect on 90° parts formed using male and female tools for both APC-2 and carbon fibre-reinforced poly(ether imide) materials. The parts made from the male tool using these materials had a larger spring-forward effect in each case. The influence of part thickness was investigated and found to reduce the spring forward which occurred. The effect of mould radius of curvature was also investigated and found to be negligible. The effect on part quality when varying the consolidation pressure was investigated for [0°/90°]_2S and [0°/±45°/90°]_S APC-2 lay-ups in the male 90° mould. The parts were ultrasonically C-scanned to assess their quality; interlaminar shear tests were also carried out to validate the ultrasonic tests. It was found that a consolidation pressure in excess of 200 kPa was required to fully consolidate these parts.