Götz T. Gresser

Fiber-Reinforced Plastics with Locally Adapted Stiffness for Bio-Inspired Hingeless, Deployable Architectural Systems

This paper presents results of the investigation of two biological role models, the shield bug (Graphosomaitalicum) and the carnivorous Waterwheel plant (Aldrovandavesiculosa). The aim was to identify biological construction and movement principles as inspiration for technical, deployable systems. The subsequent processes of abstraction and simulation of the movement and the design principles are summarized, followed by results on the mechanical investigations on various combinations of fibers and matrices with regard to taking advantage of the anisotropy of fiber-reinforced plastics (FRPs). With the results gained, it was possible to implement defined flexible bending zones in stiff composite components using one composite material, and thereby to mimic the biological role models. First small-scale demonstrators for adaptive façade shading systems – Flectofold and Flexagon – are proving the functionality.

Hindwings of insects as concept generator for hingeless foldable shading systems

Hingeless shading systems inspired by nature are increasingly the focus of architectural research. In contrast to traditional systems, these compliant mechanisms can reduce the amount of maintenance-intensive parts and can easily be adapted to irregular, doubly curved, facade geometries. Previous mechanisms rely merely on the reversible material deformation of composite structures with almost homogeneous material properties. This leads to large actuation forces and an inherent conflict between the requirements of movement and the capacity to carry external loads. To enhance the performance of such systems, current research is directed at natural mechanisms with concentrated compliance and distinct hinge zones with high load-bearing capacity. Here, we provide insights into our biological findings and the development of a deployable structure inspired by the Flexagon model of hindwings of insects in general and the hierarchical structure of the wing cuticle of the shield bug (Graphosoma lineatum). By using technical fibre-reinforced plastics in combination with an elastomer foil, natural principles have been partially transferred into a multi-layered structure with locally adapted stiffness. Initial small prototypes have been produced in a vacuum-assisted hot press and sustain this functionality. Initial theoretical studies on test surfaces outline the advantages of these bio-inspired structures as deployable external shading systems for doubly curved facades.

Branched Structures in Plants and Architecture

In architecture and construction engineering, a vast number of connections and branched columns in frame structures exist that are exposed to high static and dynamic loads. The manufacture of many of these elaborate structures is both time-consuming and costly. Industry has no solution for cost-effectively producing aesthetic and mechanically stable branched columns. This challenge is addressed by the development of branched structures inspired by branched biological concept generators such as Schefflera arboricola. Here, we present methodological approaches allowing the reconstruction of the outer shape and inner structure of complex branching regions, such as in S. arboricola, by using and combining three-dimensional-image stacking of histological thin sections, micro-computer-tomography (μCT) imaging and laser scanning. Computer-aided design (CAD) and Finite Element (FE) models of such structures can then be produced that not only help to provide a better understanding of the functional morphology and biomechanics of the biological concept generator, but also render the basis for the intended biomimetic transfer to branched columns consisting of a braided hull filled with concrete. The current project results are mainly based on the analysis of S. arboricola branching and the results of a previous research project (SPP 1420) in which biomimetic branched fibre-reinforced plastic (FRP) columns inspired by the branching structure of Dracaena were produced. Currently a biomimetic hull geometry that can be manufactured industrially is developed. Initially, branched FRPs based on triaxial braids with readily adjustable mechanical properties are filled with concrete and thus shall achieve sufficient mechanical properties for application and cost-effective fabrication in the building industry.

Fabrication of Biomimetic and Biologically Inspired (Modular) Structures for Use in the Construction Industry

The transformation of biological paradigms into building construction involves the transfer of structure and system-defining properties from biological role models to construction-specific and innovative non-construction-specific systems and processes. The challenge of manufacturing biomimetic and bio-inspired structures includes the provision of methods and procedures that allow the mapping of biological features on a production-related description. The methodological approach requires the validation and verification of existing production methods at the small scale (model, elementary cell) in order to transfer findings to the production of components at the construction scale. Additionally, the biological features that cannot be reproduced by existing methods require further adjustment or the development of new methods for appropriate transfer. A basic condition for the further development of such production procedures is the possibility of manufacturing complex structures based on biological strategies concerning resource and energy consumption, waste production and greenhouse gas emissions.

Compliant Mechanisms in Plants and Architecture

Plant movements can inspire deployable systems for architectural purposes which can be regarded as ideal solutions combining resilient bio-inspired functionality with elegant natural motion. Here, we first give a concise overview of various compliant mechanisms existing in technics and in plants. Then we describe two case studies from our current joint research project among biologists, architects, construction engineers and materials scientists where the aesthetic movements of such role models from the plant kingdom are analysed, abstracted and implemented in bioinspired technical structures for sustainable architecture. Both examples are based on fast snapping movements of traps of carnivorous plants. The Waterwheel plant (Aldrovanda vesiculosa) captures prey underwater and the Venus flytrap (Dionaea muscipula) snaps in the air. We present results on the motion principles gained by quantitative biomechanical and functional-morphological analyses as well as their simulation and abstraction by using e.g. Finite Element Methods. The Aldrovanda mechanism was successfully translated into a similarly aesthetic and functional technical structure, named Flectofold, which exists in a prototype state. The Flectofold can be used as a facade shading element for complex curved surfaces as existing in modern architecture.

Integration Methods of Sensors in FRP Components

Structural health monitoring is an important research topic in the field of fiber reinforced plastics (FRP). An effective way to detect defects or overloads in these FRP has still not been found. One way to monitor the actual state of FRP components is via integrated sensors. Integrating current standard sensors negatively affects the flux of force. Therefore investigations about integration methods of sensors in FRP components have been made. The integration of an optical fiber sensor into FRP profiles via a pultrusion process was investigated. It could be shown that the pultrusion process is suitable method for the integration of fiber optic sensors for strain measurements. Another investigated sensor principle was the integration of piezoelectric polyvinylidene fluoride (PVDF) fibers via a vacuum assisted process. The PVDF fibers were integrated into 3-point bending specimen and the piezoelectric effect was tested with and without polarization. The investigation showed that it is possible to measure the piezoelectric effect of PVDF fibers integrated into a 3-point bending test specimen. It could also be shown that carbon fibers can be used as textile electrodes for the measurement of the generated charge on the PVDF surface.

Carbon Fiber Reinforced Composite – Toughness and Structural Integrity Enhancement by Integrating Surface Modified Steel Fibers

Carbon fiber reinforced plastic (CFRP) was integrated with steel fibers in order to improve the toughness and to enhance the structural integrity during crash. An epoxy system with internal mold release was chosen as the matrix system. The surface modification of steel fibers was done by sandblasting and twisting in order to improve the fiber-matrix adhesion through mechanical interlocking mechanism. The pull-out test of surface modified steel fiber doubled the adhesive strength. The steel fiber integration increased the maximum bending stress of the composites up to 20% whereas the elongation at break reduced to 2.3%. The energy dissipation factor of the steel fiber integrated CFRPs was also reduced compared to CFRPs without steel fiber. An increase in fracture toughness was observed for the CFRPs with steel fibers that amounts to 17 J.

Organobleche aus recycelten Carbon-Stapelfasergarnen

Für das Jahr 2020 werden rund 20 kt Abfall an carbonfaserverstärkten Kunststoffen (CFK) prognostiziert [1]. Das entspricht einer geschätzten Menge von 12,5 kt (Schätzung basierend auf einem Faservolumengehalt von 50 % und einer mittleren Matrix-Dichte von 1,2 g/cm3) Carbonfasern (CF), was circa 10 % der aktuellen globalen Produktionskapazität ausmacht [2]. Diese nicht vernachlässigbare Menge wird voraussichtlich zu zwei Dritteln in-house (post-industrial) anfallen. Das fehlende Drittel wird als sogenannter End-of-Life (EoL)-Abfall entstehen, auch post-consumer Abfall genannt [1]. Diese Dimensionen veranschaulichen eindrücklich, dass eine Antwort auf die Frage nach dem Recycling von CFK gefunden werden muss.

Organic Sheets Made of Recycled Carbon Staple Fiber Yarns

In order to sustainably establish carbon fiber reinforced polymer composites (CFRPC) in the market on an industry scale, solutions on how to recycle these new materials have to be developed. Quasi-continuously aligned staple fiber structures in organic sheets made of recycled carbon fibers are one approach. The process chain as well as the mechanical properties will be presented. It will be shown that the processing of recycled carbon fibers (rCF) and polyamide 6 fibers into aligned rCF/PA6 staple fiber yarns (rCF-SFY) are a suitable solution for a true recycling of carbon fibers. In the conducted work rCF spooling cut-offs were spun into rCF-SFY and subsequently processed into non-crimp fabrics (NCF). These NCF were pressed into unidirectional (UD) laminates as well as in organic sheets. Achievable tensile strengths for rCF-SFY UD laminates are 1255 MPa, tensile moduli are 94 GPa, which represents a 80% level of virgin CFRPC (vCFRPC). Moreover, the specific feature of rCF-SFY to be able to plastically deform under process temperature of the used PA6 matrix (staple fiber effect), enabling new degrees of deep-drawing of CFRPC organic sheets in the thermoforming process, will be highlighted. The nature of this staple fiber effect was investigated by conducting tensile tests under process temperature. Two different heating methodologies were chosen. First infrared heating panels were used in an open test rig in order to replicate thermoforming conditions. These results were then compared with tensile tests conducted by using a convection oven in a secured area. Finally, the results will be summarized and the impact of the staple fiber effect on the state of the art thermoforming process will be discussed.

Recycled Carbon Fibers in Complex Structural Parts - Organic Sheets Made of rCF Staple Fiber Yarns

In order to sustainably establish carbon fiber reinforced polymer composites (CFRPC) in the market on an industry scale, solutions on how to recycle these new materials have to be developed. Quasi-continuously aligned carbon staple fiber structures in organic sheets made of recycled carbon are one approach which will be dealt with in this article. The process chain as well as the mechanical properties will be presented. Moreover, the specific feature of staple fiber yarns to be able to plastically deform under process temperature, enabling new degrees of deep-drawing of CFRPC organic sheets in the thermoforming process, will be highlighted.