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

连续纤维增强热塑性塑料，即所谓的有机板材，是机械性能优异的半成品，通过高效的热成型工艺，可以非常经济地加工成壳形部件。然而，纤维增强材料有限的可垂性导致其几何成分的复杂性相对有限。具有连续碳或玻璃纤维增强的有机片材不会表现出塑性变形行为，因为连续纤维的加载几乎肯定会导致材料失效，因为断裂伸长率低。使用短纤维纱线提供了一种解决这个问题的方法，因为它们可以在热成型有机板材时实现额外的变形自由度。短纤维纱线是由长度约为60至80毫米的单个增强纤维（所谓的短纤维）在不同位置重叠而成。这提供了一种特殊的结构，与高粘性的热塑性基体结合，形成了一种复合材料，在这种复合材料中，单个增强纤维可以在基体的熔融状态下相互滑动。这是由于在纱线的纵向上施加拉伸应力造成的。用技术术语来说，这意味着短纤维有机片材在熔融状态下具有伪塑性变形。这第一次使深拉深成形成为可能，因为它是目前已知的金属板加工。该项目侧重于确定短纤维纱线（由回收的碳纤维制成）在生产有机板材中的使用，以及评估其在热成型过程中的加工过程。为了提高短纤维有机片材的工艺性能和力学性能，需要在两个工艺阶段将生产过程中产生的纤维起伏降到最低。首先，生产主动拉伸短纤维带，在铺带工艺中进一步改进单纤维取向，将其加工成有机片材。对这些有机短纤维片材的伪塑性性能进行了详细的分析。为此，需要进行基础研究，以对材料和工艺行为进行一致的描述，从胶带生产到热成型过程中的伪塑性变形。为了研究伪塑性材料行为的基本机理，重点研究了单向短纤维有机片材的行为。如果这项研究成功，将在后续项目中探索多向层压板的扩展，以及工具和特定热成型的影响。

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.