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.
Modeling of Friction and Shear in Thermostamping of Composites - Part II
Fabric shear is being considered as the main deformation mode in the thermostamping process of composites. To predict the change of the shear properties of composites during the process, a combined friction and compaction model is developed based on the equilibrium equations of the unit cell of a balanced plainweave glass/polypropylene woven fabric. The advantage of this approach over the earlier models is that it does not require curve fitting of the fabric-level experimental data. In addition, it considers both lateral compaction and friction. Picture frame experimental results are presented to validate this model. A very close correlation is observed between the model predictions and the experimental results. The material stress-strain relationship needed for input to stamping simulations is then calculated using the results of the picture frame model.