Characterisation and modelling friction at the tool-ply interface for thermoplastic woven composites
Press forming of thermoplastic textile composites is a 
fast and efficient method of production. Friction 
occurring between the composite material and pressure 
blank-holder during forming imparts tensile stresses in 
the sheet which help counteract compressive stresses 
generated by material deformation; stresses that can 
result in unwanted wrinkling of the forming sheet. In 
order to model the press forming process and optimise 
boundary conditions, this friction behaviour must be 
characterised and modelled. Two different test methods 
are employed to characterise the friction and two 
complementary modelling strategies, an empirical and a 
predictive approach, are presented.
Press forming a 0/90 cross-ply advanced thermoplastic composite using the double-dome benchmark geometry
A pre-consolidated thermoplastic advanced composite cross-ply sheet comprised of two uniaxial plies orientated at 0/90° has been thermoformed using tooling based on the double-dome bench-mark geometry. Mitigation of wrinkling was achieved using springs to apply tension to the forming sheet rather than using a friction-based blank-holder. The shear angle across the surface of the formed geometry has been measured and compared with data collected previously from experiments on woven engineering fabrics. The shear behaviour of the material has been characterised as a function of rate and temperature using the picture frame shear test technique. Multi-scale modelling predictions of the material’s shear behaviour have been incorporated in finite element forming predictions; the latter are compared against the experimental results.
Effects of Fabric Structure on Frictional Properties
When the yarn sett of a woven fabric is systematically increased, there is also a systematic increase in the frictional resistance to motion, and it is greater for weft-over-weft motion than for warp-over-warp. The greater warp crimp producing a “knuckle effect” could be the explanation, although the diminishing spaces between the weft threads could also be a factor. In another case involving a series of pile-type woven fabrics, an increase in the height of fiber tufts produces an increase in the resistance to motion and also the amplitude of the stick-slip motion. A good linear relation exists between the number of peaks and number of cords. The size of cords and the degree of spacing between them may enhance interlocking and intermeshing of fiber pile.