Wood structure and mechanics

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Group-leader: Markus Rüggeberg

The mechanical behaviour of wood is largely influenced by the structure and biochemistry of the cell wall. By mechanical testing and correspondingly analysing native, genetically and chemically modified wood we aim at elucidating structure property relationships of the wood cell wall. Hereby, we cover the different hierarchical levels in mechanical testing and analysing structure and biochemistry.

On macroscopic scale testing is being done with universal mechanical testing machines and with a custom-built microtensile testing machine. On microscopic scale we use in-situ micropillar compression tests (collaboration with Empa Thun) and on nanoscale force mapping with the AFM. Structure and biochemical analysis is done by EM, X-ray diffraction, Raman and FTIR-Analysis. 

The swelling and shrinking of wood is commonly seen as disadvantage in the use of wood as construction material. We perceive this inherent property of wood as a genuine capacity which can thus serve as a highly valuable material for the generation of smart responsive structures. Inspired by the responsive behaviour of different plant organs such as the opening and closing of pine cones we analyse the principle behaviour of wooden bilayers for its use in responsive elements in architecture and construction. Plant tissues take up and loose water when exposed to fluctuating relative humidity. this hygroscopic nature comes along with dimensional changes. Nature makes use from these properties and has developed plant organs which are actuated in response to changes in relative humidity. Pine cones, wheat awns and orchid tree seedpods generate complex and reversible movements following changes in relative humidity for effective seed dispersal (Dawson, Vincent et al. 1997, Elbaum, Zaltzman et al. 2007, Erb, Sander et al. 2013). In many cases, the actuation is achieved by bi-layered tissue structures with specific fibre orientations in the individual layers. The differential fibre orientation leads to a transformation of the dimensional changes to reversible bending and twisting movements. 

In our research we explore nature's materials and design principles which allow such autonomous water-actuated movement. In this respect wood is unique as it inherits responsiveness to function as a smart material and excellent mechanical properties and  a good workability to function as an engineering and building material for creating large scale components. By taking wood and understanding the underlying design principles, we are able to develop biomimetic convertible elements that move as a result of daily changes in air humidity or rain exposure without needing additional engines or control. Such elements could be used in architecture or construction for automatically moving facade elements, sunshades or for solar tracking solar panels.

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