Within the scope of the aerospace research project OSFIT (One Shot Fully Integrated Thermoplastic Frame), a process simulation was developed for the thermoforming of a customized tape-laid preform blank into the final geometry of an aircraft fuselage integral frame (figure 1). The goal of the simulation was to be able to predict the fiber orientation during forming along with the part distortion (spring-back/spring-in) which occurs following the forming process. To achieve this goal, a material model was created using an already existing unit-cell approach. The resulting information can then be transferred to static or crash simulations for more accurate stiffness and strength calculations of the part.
During forming, the tailored CF-PEEK preform is subjected to combined bending and shear stresses at elevated temperatures of up to 400°C. For the creation of the material model, it was therefore necessary to determine the temperature-dependent bending and shear behavior of the preform using material characterization tests. For this purpose, the bending behavior of 2 mm thick, 0° UD specimens (unidirectional tailored preform laminates) were investigated in both, fully and partially consolidated forms, using 3-point bending tests at temperatures of 360°C, 380°C and 400°C. In shear frame tests, the shear behavior of 0°/90° laminates in fully and partially consolidated forms at elevated temperatures was also determined. The measured force-displacement and force-shear angle curves were then reproduced in characterization simulations to obtain calibrated material parameters for subsequent forming simulations.
The material model for the simulation is based on a combined element unit-cell approach developed at IVW. Here, the behavior of an individual layer of the preform is not described by a single element, but by a combination of shell and beam elements. Through the interaction of all elements within a unit-cell, the real behavior of the material at the ply-level can be described. Two layers of shell elements and one layer of beam elements defined at common nodal positions are used per layer of tape. The beam elements describe the fiber orientation as well as the bending properties of the tape, while the shell elements simulate the shear behavior. A second layer of shell elements, without mechanical properties, is also defined at common nodes to allow thermal contact between the beam and shell elements of the multiple ply layers (figure 2, left).
To set-up the simulation, a layer of shell elements was first created in ANSA® based on the preform geometry and the radii and flanges of the mesh were refined based on the chosen mesh size. The beam elements and a second layer of shell elements, which provides the thermal contact, were then generated in LS-DYNA® using the machine paths of the tape laying robot. Together, the shell and beam elements form a single layer of the complete preform laminate, represented by a stack of several layers. According to the preform structure, the layers are linked together by means of contact definitions. The upper and lower tool were generated as shell models based on the tooling CAD data. The preform holding frame is realized in the simulation via beam elements, which deform according to the holding forces in the actual forming process representing the sliding of the aluminum strips used in the holding frame. The complete forming simulation model of the demonstrator part, contains about 1.2 million elements.
To carry out a spring-back simulation, the final state of the forming simulation is used as the basis for input. In addition to the fiber orientations, stresses and strains can also be taken directly from the final state of the forming simulation model (figure 3).
The principle of the combined element unit-cell modelling approach allows the representation of temperature-dependent bending and shear behavior of the individual tape layers to be simulated at the component level. Each layer of the preform can be modelled individually and directly from the machine paths of the tape laying robot. This allows the change of the fiber orientation via sliding of the individual layers over one another during forming, to be mapped correctly and closes the gap between simulated preform production and thermoforming simulations. Subsequently part distortion (spring-back/spring-in) simulations along with further structural simulations are then possible, since all the necessary knowledge from the forming simulation is available.
On the basis of a decision by the German Bundestag, the project “OSFIT – One-Shot Fully Integrated Thermoplastic Frame” is funded by the Federal Ministry of Economic Affairs and Energy (BMWi) (funding reference 20W1706C).
Dr. Miro Duhovic
Phone.: +49 631 2017 363
Institut für Verbundwerkstoffe GmbH