Numerical modelling of fibre-reinforced thermoplastic sheet forming
Continuous Fibre Reinforced Thermoplastics (CFRTPs) combine high strength, stiffness, impact and chemical resistance with possibilities for efficient part production by various thermoforming processes. Sheet forming of molten CFRTP laminates has generated much interest, but problems of buckling, wrinkling and predicting fibre distribution have meant a deeper understanding of these processes is needed.
The first part of this study looks at the problem of gross buckling in homogeneous “trellis” flows of bidirectional laminates. Modelling the molten composite as a Newtonian fluid reinforced by inextensible fibres, linear stability analysis is used to determine the growth rate of small out-of-plane imperfections. Buckling is predicted when fibre tensions are negative, indicating that laminates must be kept in tension during forming to reduce such defects.
A new approach to Grid Strain Analysis is presented, which uses surface fitting to determine the deformations occurring in sheet forming. The new method improves analysis of smooth, inhomogeneous deformations, and allows greater flexibility in viewing the results. The technique has been used to visualise deformations in a blister fairing made from cross-ply PLYTRON laminates. Arrow diagrams produced from the part demonstrate the tendency of bidirectional composites to deform by trellis flow, while transverse flow results from the action of the diaphragms used to form the part.
The significance of inter-ply slip in CFRTP sheet forming provided the impetus to develop a finite element model for molten laminates, which treats each ply as a separate continuum. Contact between plies is modelled, with slip given a viscous response. Ply deformations are governed by a highly anisotropic elastic law, to handle the stiff fibres and as a first step towards a viscoelastic model of major intra-ply deformation modes.
The finite element model parameters were adjusted to fit the part shape and load response of unidirectional PLYTRON laminates in bending. However, a perfect fit is unobtainable due to local transverse flow occurring at the bend in the real laminate. Nevertheless, the bending of the remainder of the ply is well described by the elastic model, using a fibre direction stiffness 25000 times that in the transverse direction. With the present model, a somewhat less anisotropic set of parameters gives the best overall fit and has been applied in several thermoforming simulations.
As observed in experiments, matched-die bending simulations display ply buckling at high forming speeds. Hemispherical dome forming simulations exhibit out-of-plane buckling and near-inextensible fibre behaviour, with trellis-like deformation predominant in cross-ply laminates. In simulated double-diaphragm forming of bends and hemispherical domes, tension superimposed on the laminate from the stretching diaphragms is shown to eliminate buckling. However, high forming pressures and excessive transverse flow are a problem with current, stiff diaphragms.
Final discussions look at improving contact modelling, reducing model sizes by adopting thin shell assumptions, and improving the ply model.
Advisor:Associate Professor Debes Bhattacharyya; Professor Ian Collins
School:The University of Auckland / Te Whare Wananga o Tamaki Makaurau
School Location:New Zealand
Source Type:Master's Thesis
Keywords:fields of research 290000 engineering and technology 290500 mechanical industrial 290501
Date of Publication:01/01/1997