Intraply shear characterisation of a fibre reinforced thermoplastic composite

Thermoforming, a commonly used method to manufacture components from fibre reinforced thermoplastic laminates, involves heating the composite to its melt/softening temperature and then applying suitable forces to mould the composite to the required shape. Acquiring complex curvature shapes involves flow phenomena such as interply slip, intraply shear and squeeze flow. To date, the intraply shear mechanism is the least well understood. Approaches have been made to determine both longitudinal and transverse steady intraply shear viscosities of molten unidirectional continuous fibre-reinforced composites in the past but with conflicting results. No truly comprehensive characterisation has been carried out on these materials under a steady shear rate deformation mode. The present study concentrated on experimentally establishing the intraply shear behaviour of carbon fibre reinforced polyetheretherketone (APC-2). Two basic orientations were considered, one where the fibres in the lay-up were parallel to the pull-out direction and the other where the fibres were perpendicular to the pull-out direction. Values of longitudinal and transverse shear viscosities were determined for shear rates between 0.002 and 0.02 s−1. The viscosity was highly shear-rate dependent, and was best modelled using a ‘power-law’. At a rate of 0.01 s−1 the longitudinal viscosity was in the 1 MPa s range, and approximately 2.5 times the transverse value. These values are several orders of magnitude greater than those determined using more traditional means, such as torsional rheometry, but agree with results obtained using a picture frame test method.