Forming simulations for unidirectional thermoplastic composites: Improving in-plane shear characterization and modeling

Fiber reinforced thermoplastic polymer composites are highly effective for lightweight construction and cost-efficient manufacturing. Their ability to repeatedly soften and solidify allows fast and automated forming of parts using a press. However, defect-free production becomes challenging for complex parts with doubly curved geometries, variable thickness layups, and structural loading requirements. Composite forming simulations using the finite element method can predict the manufacturing process, including the formation of potential defects, early in the design stage to reduce the risks for added costs and delays. This research aims to enhance the prediction of deformations and wrinkling defects by improving the material descriptions in the simulations.

Press forming experiments were conducted, with detailed observations of local deformations and wrinkles, and used to evaluate the predictive quality of the simulation models. A potential for improved predictions was identified and linked to deficiencies in the material models for in-plane shear, bending, and ply-ply friction, resulting in a strategy for successive improvements for material characterization and modeling.

The research continued on the characterization of the in-plane shear behavior for unidirectional thermoplastic composites under forming conditions. The analysis of the existing characterization methodology revealed anisotropic and inhomogeneous deformations that rendered the results unreliable. A novel bias extension experiment was developed based on cross-ply specimens that allowed control and monitoring of the local deformation at various deformation rates and temperatures. Additionally, a constitutive relation was proposed based on the transversely isotropic “Ideal Fibre Reinforced Fluid” description, including rate- and temperature-dependent viscosities and viscoelastic start-up behavior, to accurately describe the bias extension results for forming simulations.

The newly acquired models for in-plane shear were validated using updated simulation predictions and showed an improved correlation for the local deformation but were insufficient to fully resolve the deficient wrinkling predictions. Recommendations for changes to the material models for the bending and ply-ply friction were explored and shown to be promising for further improving the correlation on wrinkling defects. The enhanced description of the in-plane behavior combined with the additional knowledge gained throughout this research provides a strong basis to further improve the prediction of deformations and wrinkling defects for unidirectional thermoplastic composite materials using forming simulations.