Branching in columnar cacti features morphological and anatomical characteristics specific to the subfamily Cactoideae. The most conspicuous features are the pronounced constrictions at the branch-stem junctions, which are also present in the lignified vascular structures within the succulent cortex. Based on finite-element analyses of ramification models, we demonstrate that these indentations in the region of high flexural and torsional stresses are not regions of structural weakness (e.g. allowing vegetative propagation). On the contrary, they can be regarded as anatomical adaptations to increase the stability by fine-tuning the stress state and stress directions in the junction along prevalent fibre directions. Biomimetic adaptations improving the functionality of ramifications in technical components, inspired, in particular, by the fine-tuned geometrical shape and arrangement of lignified strengthening tissues of biological role models, might contribute to the development of alternative concepts for branched fibre-reinforced composite structures within a limited design space.
The branching of arborescent (tree‐like) monocotyledonous plants of the genus Dracaena or of columnar cacti differ considerably from that observed in other dicotyledonous or gymnosperm trees. The investigated ramifications exhibit distinctive morphological and anatomical features. In arborescent monocotyledons the side branches are attached to the main stem by a fiber‐reinforced tissue newly formed during secondary growth, clasping the main stem and finally resulting in a “flange‐mounted” structure. In the case of columnar cacti the most obvious feature is the pronounced constriction at the attachment point of the branches that is also mirrored in the lignified vascular tissue. One might argue that these characteristic morphological and anatomical features in regions exposed to high mechanical stresses represent structural weaknesses. However, the outer shape and the inner structures of the ramifications cause considerable stability and structural integrity of the stem‐branch connection under static and dynamic loading. Our results allow concluding that load‐adaptation in ramified plant structures is a result of a combination of optimization in outer shape and fiber arrangement within the ramifications. Numerical methods simulating the mechanical behavior based on data obtained from the studied plants support this assumption. A deeper understanding of the outer shape of the connection between shoot and branch as well as of the arrangement of the lignified vascular tissues in the branching region, may contribute toward alternative concepts for branched technical light‐weight‐structures. In particular for braided fiber‐reinforced composites this biomimetic approach might help to keep the demand on the available design space as small as possible.
The junctions between stems and branches in arborescent monocotyledons and columnar cacti are structurally and functionally poorly understood to date. Therefore, the functional anatomy and morphology of these junctions as well as the arrangement and biomechanics of mechanically relevant tissues were investigated. Both plant groups share distinctive anatomical features. Due to restricted secondary growth, newly formed tissues connecting stem and branch clasp around the main shoot, resulting in a “flange-mounted” structure. In addition, an indentation or a distinct necking forming a specific shape characterizes the area of attachment. As a result, the distribution of mechanical stresses is modified by increasing the resistance against high stresses upon static and presumably also upon dynamic loading. The mechanically important fibrous bundles or wood lamellae are collinearly aligned with the occurring stresses leading to a lateral shift of the stress trajectories and a shape adjustment. This particular branching type shows a remarkable potential to improve joints in braided fiber-reinforced composites. In the near future, a fully automated fabrication of biomimetic branched composites will be possible. First promising demonstrators have already been produced on lab scale.
The aim of this study is the biomimetic optimisation of branched fibre-reinforced composites based on the detailed analysis of biological concept generators. The methods include analyses of the functional morphology and biomechanics of arborescent monocotyledons and columnar cacti as well as measurements and modelling of mechanical properties of biomimetic fibre-reinforced composites. The key results show evidence of notch stress reduction by optimised stem-branch-attachment morphology in monocotyledons and columnar cacti. It could be shown that some of these highly interesting properties can be transferred into biomimetic fibre-reinforced composites.
The manufacturing of nodal elements and/or ramifications with an optimised force flow is one of the major challenges in many areas of fibre-reinforced composite technology. Examples are hubs of wind-power plants, branching points of framework constructions in the building industry, aerospace, ramified vein prostheses in medical technology and the connecting nodes of axel carriers. Addressing this problem requires the adaptation of innovative manufacturing techniques and the implementation of novel mechanically optimised fibrereinforced structures. Consequently, the potential of hierarchically structured plant ramifications as concept generators for innovative, biomimetic branched fibre-reinforced composites was assessed by morphological and biomechanical analyses. Promising biological models were found in monocotyledons with anomalous secondary growth, i.e. Dracaena and Freycinetia, as well as in columnar cacti, such as Oreocereus and Corryocactus. These plants possess ramifications with a pronounced fibre matrix structure and a special hierarchical stem organization, which markedly differ from that of other woody plants by consisting of isolated fibres and/or wood strands running in a partially lignified parenchymatous matrix. The angles of the Y-and T-shaped ramifications in plants resemble those of the branched technical structures. Our preliminary investigations confirm that the ramifications possess mechanical properties that are promising for technical applications, such as a benign fracture behaviour, a good oscillation damping caused by high energy dissipation, and a high potential for lightweight construction. The results demonstrate the high potential for a successful technical transfer and will lead to the development of concepts for producing demonstrators in the lab-bench and pilot plant scales that already incorporate solutions inspired by nature.
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