Smart building skins are biomimetic elements derived from plant movement principles, that react autonomously to changes in environmental conditions with a mechanical movement, e.g. to deflect glare without permanently blocking the sight in a building. Such adaptive elements make a direct use of primary energy to produce motion, which is a great step towards a general reduction of energy consumption in civil engineering applications.
In this context, the adaptive material Cottonid is an efficient combination of a functional and a construction material, bringing together large autonomous movements in reaction to environmental stimuli with mechanical robustness. It is based on chemically modified cellulose and highest mechanical strength and stiffness can be detected in manufacturing direction, respectively micro fibril orientation, whereas humidity-driven passive movements are largest perpendicular to it. By a variation of process parameters during manufacturing, it is possible to realize a local tuning of Cottonid’s properties over the microstructure, to produce stimuli responsive elements with tailored deformation behavior.
This study assesses the actuation behavior, strength and stiffness of Cottonid dependent on microstructure and environmental conditions. Different Cottonid variants were investigated in instrumented cyclic long-term humidity experiments with an optical monitoring of the hygroscopic movements. Further, fatigue tests were performed in tension-tension loading under varying environmental conditions using a servohydraulic testing system with an integrable climate chamber. Tests were instrumented with thermometry and extensometer to detect material’s reactions during loading.
The instrumentation of the experiments leads to a profound understanding of the microstructure based interaction of Cottonid with its environment. It shows a pronounced influence of the superimposed environmental loading during fatigue, leading to varying strength and stiffness correlating with different values of relative humidity. The results are the base for an intelligent adjustment of the microstructure of Cottonid to produce smart functional materials for biomimetic solutions in civil engineering.