Forming fibre reinforced thermoplastic composite sheets

Abstract

Continuous and long discontinuous fibre reinforced thermoplastic composites are relatively new engineering materials, which possess good structural properties and good chemical resistance. At elevated temperatures they become malleable and are able to be formed into complex components under moderate loads. These two factors make them very desirable raw materials for manufacturing 3-D composite components. However, there are a number of s, as fibre wrinkling, fibre migration, resin squeeze flow, sheet thinning, sheet thickening and gross buckling. Whether any of these defects lead to the rejection of a finished component depends on the severity and the location of a given fault. The best forming operations are those ones that avoid defects completely. In this thesis, a mainly experimental approach is taken to gathering information about the factors leading to process failures, in order to increase the knowledge available to designers who wish to eliminate forming problems. Two key follaing mechanisms have been identified in continuous and long discontinuous fibre reinforced materials. These are intraply shear and interply shear. The first of these two deformation mechanisms has been investigated by applying a large strain analysis technique to components formed from PLYTRON, APC-2 and LDF materials. Without a knowledge of the material properties of these laminates, the in-plane deformations have been quantified. The results indicate that high compressive strain gradients are associated with gross buckling. The likelihood of gross buckling can be reduced if regions of high deformation are removed from a blank before it is formed. Also, the inextensible nature of fibres in these materials has been demonstrated with this technique.Using the kinematic constraints of fibre inextensibility and incompressibility a model of an idealised viscous beam has been developed. This model characterises the interply shear in a Newtonian fluid beam reinforced with a single family of inextensible fibres. The theoretical results from this model are compared with actual V-bend experiments performed on unidirectional laminates under isothermal conditions. A viscous fluid model is shown to be an inadequate approximation to the real behaviour of PLYTRON laminates at temperatures within the material's melting range. Instead a viscoelastic fluid characterisation is more appropriate. But, the beam bending model does provide a good framework for interpreting the experimental results. Furthermore the effects of forming speed, die geometry and strip thickness have been analytically related to the stresses in unidirectional sheets as they are bent out-of-plane. The effect of forming speed and forming temperature on fibre wrinkling and spring-forward are also demonstrated. Lastly, a single integral non-linear viscoelastic fluid model has been presented which describes the mechanical response of bi-directional PLYTRON laminates under isotheimal forming conditions. The results of several stress relaxation tests have been used to determine the three material functions needed to characterise the material. The strain analysis method has also been used to demonstrate the non-homogeneous defolinations in tensile strips as they are progressively stretched. The complex behaviour of CFRTs has been evaluated through extensive experimental work, and some simple models have been developed to describe the behaviour of CFRTs under certain forming conditions. These comprise a basis for further work in this field.