Colloquia

Summary

When designing sustainable, high-performance, and lightweight components, the use of continuous fiber reinforced thermoplastic composites is getting increasingly popular. Compared to their thermoset counterparts, the key advantage of thermoplastic polymer matrices lies in their ability to be heated, melted, and shaped repeatedly. This characteristic enables automated production processes, assembly by means of welding and recycling without separating fiber and matrix.

During manufacturing of thermoplastic composite parts defects may occur that can lead to a deterioration of the part’s mechanical performance. Instead of going through a wasteful trial and error process to prevent defect formation, virtual simulations can provide a cost-effective alternative. Accurate simulation software takes into account all relevant deformation mechanisms that take place during processing. One of these mechanisms is transverse flow of unidirectionally reinforced plies. Models are being developed to better describe this deformation mechanism but these require material properties that are not readily available, namely the ply’s transverse viscosity. Many attempts of measuring this property have been made but, to date, there is no standardized method or procedure.

The goal of this research is to design a squeeze flow test method that can be used to validate if a squeeze flow plane strain power law model could describe the material behaviour. To achieve this squeeze flow tests have been performed with varying closing speeds and specimen dimensions, and a power-law viscosity model was fitted onto these results. The findings revealed that the transverse viscosity could not be accurately described as a power-law fluid. The flow behaviour index, indicating the rate of change in viscosity, varied widely between the tested closing speeds and specimen dimensions, being low at low shear rates and increased as shear rates increased. Furthermore an elastic component was found as the forces relax to a non-zero plateau that is dependent on the closing rates. Additionally, microscopy images and test recordings provided valuable qualitative insights into specimen behaviour during testing. The observations included significant deconsolidation during the heating phase, a fountain flow velocity profile, and signs of shear banding after compression.

The results of the study provided valuable insights into the viscosity progression of UD composites and some important observations of squeeze flow. These findings, along with the derived testing method, can serve as a starting point for further research in characterizing transverse viscosity through squeeze flow testing.