UTFacultiesETEventsPhD Defence Dennis Brands | Forming Simulations for Unidirectional Thermoplastic Composites - Improving in-plane shear characterization and modeling

PhD Defence Dennis Brands | Forming Simulations for Unidirectional Thermoplastic Composites - Improving in-plane shear characterization and modeling

Forming Simulations for Unidirectional Thermoplastic Composites - Improving in-plane shear characterization and modeling

The PhD defence of Dennis Brands will take place in the Waaier Building of the University of Twente and can be followed by a live stream.
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Dennis Brands is a PhD student in the Department of Production Technology. (Co)Promotors are prof.dr.ir. R. Akkerman and dr.ir. W.J.B. Grouve from the Faculty of Engineering Technology.

Fiber-reinforced thermoplastic polymer composites are highly effective for lightweight construction due to their high stiffness and strength properties relative to the density. Additionally, the ability to be repeatedly softened and solidified offers significant advantages in manufacturing, allowing welding, recycling, and forming of the material in high-rate automated processes. This research explores press forming, where a heated composite blank is rapidly shaped and solidified for cost-efficient mass production of small to medium-sized parts. Although press forming is an established process, ensuring defect-free production is increasingly challenging due to the demand for more complex parts with doubly curved geometries, variable thickness layups, and structural loading requirements.

Composites forming simulations using the finite element method can predict the manufacturing process outcome, including any potential defects, and can be applied during the design of parts and processes to reduce the risks associated with costs and delays from rework. Current simulations already capture global deformations and major defects effectively, but the challenge remains to predict the smaller wrinkles that are relevant for unidirectional thermoplastic composites in the detailed design stage. This research aims to enhance the prediction of deformation and wrinkling defects by improving the material models in forming simulations, particularly the in-plane shear behavior under forming conditions.

Press forming experiments were performed using two different unidirectional thermoplastic composite materials, including detailed observations of the local deformations and wrinkles, which allowed for an in-depth validation of the simulation models. Discrepancies were identified in initial simulation predictions and related to the material models for in-plane shear, bending, and ply-ply friction. Finally, a strategy for the successive enhancement of material characterization and modeling was proposed.

Focus was on improving the characterization of the in-plane shear behavior of unidirectional thermoplastic composites under forming conditions. Two existing experimental methodologies based on torsion were evaluated, revealing anisotropic and inhomogeneous deformations that rendered the results unreliable. As an alternative, a novel bias extension experiment was developed based on specimens with a cross-ply layup, proving effective to introduce deformations similar to woven composites and enabling local deformation measurements using a video extensometer. Datasets were collected for two materials at various speeds and temperatures. Additionally, a new constitutive relation was developed to accurately describe the bias extension results for forming simulations. The proposed relation is based on the transversely isotropic “Ideal Fibre Reinforced Fluid” description with rate- and temperature-dependent viscosities, where a one-dimensional Maxwell model was introduced to capture the observed start-up behavior.

The acquired models for in-plane shear were validated through updated simulation predictions, where an improved correlation was observed for the local deformation on parts with dominant in-plane deformation. However, only updating the in-plane shear models was found insufficient to fully resolve the deficient wrinkling predictions. Additional modifications to the material models for the bending and ply-ply friction mechanisms were shown to be promising for further improving the correlation with wrinkling defects. Moreover, an analysis of the forming and characterization conditions was used to recommend additional improvements with potential for future research.

The research presented has resulted in an effective combination of a characterization method and a constitutive relation for an enhanced description of the in-plane shear behavior for unidirectional thermoplastic composites. The enhanced in-plane shear models are an essential first step, but more research is required to further detail the overall material description. The 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.