Modeling non-isothermal thermoforming of fabric-reinforced thermoplastic composites

The correct modeling of the sheet forming of fabric reinforced thermoplastic composites, so called organosheets, is still a challenge. In the past it was possible to predict accurately and efficiently the fiber orientation during the draping of dry reinforcement or constant temperature organosheet (reinforcement and molten polymer) using the explicit FEM-Software LS-DYNA®. Until now, the developed model was only able to simulate the right material behavior for an isothermal process. However, the draping of an organosheet is in reality a non-isothermal process that takes place at elevated temperatures. During thermoforming, the sheet cools down by contact with the forming tools and changes its temperature dependent mechanical behavior. With several enhancements, it is now possible to implement the temperature dependent shear and bending stiffness of the thermoplastic material into the model. This allows the modeling of a non-isothermal forming process. The method is based on a hybrid model which uses a combination of shell and beam elements. The bending stiffness of the composite is represented by the beam elements. The bending behavior is characterized using a three-point bending test, performed inside a heating chamber in the range of the forming temperature of the thermoplastic resin. The shell elements represent the shear behavior of the fabric reinforcement which is influenced by the temperature dependent stiffness of the resin. Using a horizontal picture frame test, it is possible to characterize the shear behavior of the composite for different sheet temperatures. With the new extended modeling approach, is it possible to predict fiber orientation and occurring defects, such as wrinkling, even more accurately. Many defects which occur during thermoforming are caused and can even be controlled by temperature changes due to the ongoing tool contact which can now be considered in the calculation. The verification of the developed modeling approach is carried out through the simulation of material characterization tests (shear and bending behavior) for a commonly used commercially available material TEPEX® dynalite. The material model parameters derived from the verification models are used to simulate the forming behavior of a novel automotive crash element geometry. Afterwards, the results are directly compared with real thermoformed parts.