Process analysis of pseudo-plastic deformation behavior of unidirectional reinforced staple fiber organic sheets

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Continuously fiber-reinforced thermoplastics, so-called organic sheets, are mechanically high-performance semi-finished products, which are processed very economically into shell-shaped components in an efficient thermoforming process. However, the limited drapability of the fiber reinforcement results in comparatively limited geometric component complexity. Organic sheets with continuous carbon or glass fiber reinforcement do not exhibit plastic deformation behavior, since loading of the continuous fibers almost certainly leads to material failure due to the low elongation at fracture. Using staple fiber yarns provides one approach to solve this problem, as they make it possible to achieve an additional degree of freedom in deformation when thermoforming organic sheets. The staple fiber yarns are made of individual reinforcing fibers (so-called staple fibers) with an approximate length of 60 to 80 mm, which are overlapping at different positions. This provides a specific architecture that - in combination with a highly viscous thermoplastic matrix - results in a composite material, in which the individual reinforcing fibers can slide off from each other in the molten state of the matrix. This occurs due to the tensile stress applied in the longitudinal direction of the yarn. In technical terms, this means, that staple fiber organic sheets are pseudo-plastically deformable in the molten state. For the first time, this makes deep-drawing forming possible as it is currently known in metal sheet processing. The project focusses on qualifying the use of staple fiber yarns (made from recycled carbon fibers) in producing organic sheets, as well as on evaluating their processing in the thermoforming process. The fiber ondulation, caused by the production process, requires to be minimized in two process stages, thus improving the process behavior and mechanical properties of staple fiber organic sheets. First, an actively stretched staple fiber tape is produced, which is further processed into organic sheets in the tape laying process with further improvements of the single fiber orientation. These staple fiber organic sheets are analyzed in detail with regard to their pseudo-plastic material behavior. For this purpose, basic research is required for a consistent description of the material and process behavior from tape production to pseudo-plastic deformation in the thermoforming process. In order to investigate the basic mechanisms of pseudo-plastic material behavior, the investigations focus specifically on the behavior of unidirectional staple fiber organic sheets. If this research is successful, an extension to multidirectional laminates, as well as tool and specific thermoforming influences, will be explored in follow-up projects.