Picture-frame rheological shear testing for composites sheet-forming processes

摘要

Composites sheet forming processes have been developed in both thermosetting and thermolastic matrix materials in order to automate manufacturing methods. Forming of double-curvature shapes requries that fabrics or unidirectional laminates shear in the plane of the sheet, a deformation mode known as intraply shear. Finite element process simulations of composite sheet-forming have been developed for which the major viscosities of the prepregs at forming temperature are requried as inputs. The objective of this work is to directly measure the intraply shear viscosities of high volume-fraction impregnated polymer melts by using a specially-developed picture-frame apparatus which is loaded in a test machine at the composite process temperature. The material is pinned to the picture frame in a manner which leaves it free to rotate, yielding a homogeneous large-deformation. Measuremdent of the load history at various test displacement rates allows, the material to be characterized using various model responses. Materials tested in this work include two types of glass-fabric reinforced Nylon-12 (VESTOPREG), a carbon-fabric reinforced PEI (CETEX) and a unifdirectional carbon-fibre PEEK (APC-2). These materials are characterised as either anisotropic viscous or viscoelastic fluids, with intraply shear viscosities typically of the order of MPa, sec, many orders of magnitude greater than the equivalent matrix viscosities. Conclusions as to the critical buckling angles and loads of the materials in shear are also made from the apparatus.