Continuous Fibre Thermoplastic Composites Research Articles

Stamping of continuous fiber thermoplastic composites

Despite demonstrated success in low volume aerospace and defense applications, structural composites remain at the periphery of high volume industries such as construction, automotive, and consumer goods because of long cycle times. Stamping provides a means of making composite sheet products at rates ten to a hundred times faster than any existing continuous fiber processes. However, to make composites stamping a viable process, one must understand how the combination of fabric architecture, tool design, and process conditions interact to produce a part free of wrinkling and tearing. In this paper, the effect of temperature, stamping rate, and boundary constraints on the material deformation is presented. The focus of this study is a co-mingled glass/polypropylene fabric, in the form of a layer of unidirectional yarns held together by stitches. The results show that temperature variations have the greatest effect on deformation. In addition, a finite element model of parallel strips with linear constraints was shown to successfully simulate the sliding deformation or draw-in of the stitched unidirectional material.

Folding of continuous fibre thermoplastic composites

Thermoplastic composites with continuous fibres are attractive construction materials because of their good specific mechanical properties and their possibility to be processed very rapidly. An extremely fast processing method is folding of sheet material. Folding can be performed by local heating of a sheet along a line. Heating is done to a temperature, well above the softening or melting point of the thermoplastic polymer. Subsequent folding along the heated line requires very low forces. The folded geometry becomes permanent after cooling below the softening point of the polymer. Unfortunately, folding causes microbuckling of the reinforcing fibres at the compression side of the fold causing a severe reduction in the local strength. The present report describes a method for the folding of thermoplastic composites, which controls the direction of fibre micro-buckling. The method results in a smaller strength reduction. The folding equipment is designed in such a way that microbuckling of the fibres occurs in a direction in the local plane of the sheet rather than being perpendicular to that plane. The result is a folded sheet with a reduction of about half of the original strength, as compared to a strength of only 15 the original strength, which is typical for folds produced with more conventional means. In other words, the remaining strength is more then doubled using the new folding technology.

Die-Less Forming of Large and Variable-Radii of Curvature in Continuous-Fiber Thermoplastic-Matrix Composite Materials

Die-less forming is a method of forming complex shapes in continuousfiber thermoplastic-matrix laminates under only software control, i.e., without fixed dies. The "continuous" die-less forming concept for thermoplastic composite laminates has been extended to encompass (a) constant-curvature parts having curvature larger than the forming rollers, and (b) variable-curvature parts. Shape accuracies of 0.016" have been attained. Prior work with the same process indicates that aircraft quality (< 6 dB ultrasonic loss) can also be maintained throughout the operations. The key to good shape accuracy and quality is maintenance of a substantial (e.g., 25"C) temperature gradient from the main part of the forming zone to the tangent point where the material exits this zone. Manipulation of advective heat transport by proper selection of the forming velocity was effective in maintaining this temperature gradient. Key internal behavior which governs the shape accuracy includes adequate interply sliding within the forming zone and avoidance of plastic bending outside of it. This work contributes to methods for forming continuous-fiber thermoplastic composites to final net shapes without dies.

Surface friction effects related to pressforming of continuous fibre thermoplastic composites

Abstract In the pressforming of thermoplastic composite sheet, the heated laminate is rapidly formed into a mould. The moulding force is transmitted using either matched metal dies or by a rubber pad/metal mould combination. Friction must occur between the metal or rubber mould surface and the heated composite as the laminate moves across the tool surface until it is fully formed. This paper describes work carried out to characterize and measure these frictional forces. Composites such as unidirectional carbon fibre-reinforced poly(ether ether ketone) and glass fibre fabric-reinforced PA-12 have been tested, with rubber and tool steel as the mould materials. Two methods of testing were used, one comprising two fixed heated platens, between which the surfaces to be tested were placed while the composite was sheared from between the surfaces. Pulling-out force was achieved using a variable velocity shearing rig and by using dead-weight loading. A heated friction sled was also built which allowed various samples of metal and rubber material to be dragged across a heated composite sheet. The effects of varying surface temperature, normal pressure, surface fibre orientation and mould release agent were investigated. An adhesive bond was found to occur if the surfaces were left in contact during heating. By varying the shearing velocity, the friction between the composite and tool surface was found to be hydrodynamic in nature, i.e. velocity-dependent, at forming temperature.

A local theory of heating in cross‐ply carbon fiber thermoplastic composites by magnetic induction

AbstractFor joining and repair of continuous fiber thermoplastic composites, induction heating has been viewed a strong candidate. Induction heating employs an applied alternating magnetic field, which induces a rotational emf in a grid of conductive carbon fibers, which are then used to carry resulting currents. In continuous carbon fiber crossply composites the available paths for “eddy current” loops are along the network of conductive carbon fibers. For this to occur, an electrical transfer must take place between crossing fibers in adjacent plies. Tests involving variable thicknesses of interply neat film layers have been performed to provide insight into the mechanisms taking place. These tests indicate that the primary mechanism for heating in such laminates is dielectric losses in the polymeric region between fibers in adjacent planes that form the conductive loop. Therefore, heating is not uniform in such composites despite a uniform magnetic flux. Heating patterns were viewed using liquid crystal materials and E‐type thermocouples. Several factors leading to nonhomogeneous thermal distributions have been considered, including current density effects, internal emf cancellation, and rotational field effects. Global and local considerations are addressed, a localized model is proposed, and the corresponding theory is developed qualifying the early results. Additional testing has supported the theory.

Anisotropic rheology of continuous fibre thermoplastic composites

The anisotropic rheology of continuous fibre thermoplastic composites has been resolved into along fibre and transverse components using a balanced pair of off-axis specimens in a commercial rotational rheometer with a parallel disc geometry. Existing theory has been used to demonstrate a small but consistent difference between the components of shear modulus and viscosity obtained from dynamic viscoelastic measurements. Transverse viscosity and yield stress are higher. The use of maximum shear rate (product of maximum shear strain and angular frequency), used here with apparent Maxwell viscosity, eliminates the strain dependence of the rates and correlates with steady shear viscosity and shear rate data. Differences in yield stress and viscosity are associated with the fibre-polymer system and the fibre volume fractions.

Dynamic mechanical analysis of thermoplastic composites and resins

AbstractThe anisotropy of continuous fiber thermoplastic composites limits the number of geometries that can be employed effectively in dynamic mechanical analysis (DMA). Therefore, a Mechanical Energy Resolver, developed for use with elastomers, was modified with a bending fixture using a cantilever beam sample geometry and a high temperature oven. The bending fixture allows determination of composite transition temperatures. Based upon the simple geometry, one can also determine elastic moduli. This test scheme can be coupled with simple shear DMA on melts to determine neat resin properties, including solidification temperature. Results are presented for several semicrystalline and armorphous thermoplastic/graphite fiber laminates and neat resins.

A characterization of shear flow in continuous fibre thermoplastic laminates

The viscoelastic response of some continuous fibre thermoplastic composites has been investigated dynamically. The contribution of the number and direction of laminate ply layers and fibre type, together with the geometry, have been explored in relation to the matrix polymer. An interpretation in terms of an apparent Maxwell viscosity is in agreement with steady shear flow viscosity, and approximates to a large strain dynamic viscosity. A yield stress of the order of 1 kPa is implied by the Maxwell interpretation together with a high stress viscosity more than 10 times the viscosity of the matrix polymer. The use of model laminates leads to the prediction of both interply and intraply polymer flow in continuous fibre thermoplastic composites.