Chapter 9 Bending of continuous fibre-reinforced thermoplastic sheets

Abstract

Abstract This chapter is primarily concerned with the rheological behaviour of continuous fibre-reinforced thermoplastic (CFRT) materials in shear. The analysis presented in this chapter centres around a novel piece of testing equipment which establishes veebending as a means of determining both the longitudinal and transverse shear viscosities of such materials. In the analysis presented here, no distinction is drawn between the constituents of the composite. Instead an idealised continuum model subject to the kinematic constraints of incompressibility and fibre inextensibility has been adopted. The first part of the chapter deals, with various concepts related to the deformation and related flow mechanisms which predominate in CFRT material. This is followed by the development of an idealised material model for an incompressible viscous fluid reinforced with a single family of inextensible fibres. The analytical model leads to a strainghtforward interpretation of the effects of forming speed and geometry on the bending, stresses expected in a real sheet during forming. An experimental programme is then outlined which details the forming rates and temperatures over which the vee-bending experiments are conducted. The results of these tests are discussed in two parts. Firstly, the quality of the samples is assessed with regards to fibre instability and the spring-back/forward phenomenon. The second part of the discussion centres around the interpretation of the material's longitudinal shear behaviour. A further modification to the bending mechanism is then introduced which allows the tests to be carried out at constant shear rates. This is then followed by the development of a method for predicting both the longitudinal and transverse shear viscosities of CFRTs. The experimentally obtained viscosity ratios from these final tests are compared to a number of alternative models which relate the longitudinal and transverse viscosities to the fibre volume fraction and the viscosity of the matrix.