By using electrically powered actuating devices, for example in the field of engineering or construction, the back transformation into a mechanical movement is not efficient, since electrical energy is almost always generated from a primary energy source. It would be desirable, if the actuator directly responds to an external stimulus or an environmental change, like temperature or humidity. Cottonid is a macromolecular cellulose-based composite material manufactured using the parchmentizing process. It is hygroscopic and possesses process-related anisotropic mechanical properties, which makes it an efficient adaptive material for humidity-driven actuators.
In this study, for the first time a direction-dependent fatigue assessment of Cottonid is realized. To evaluate the influence of process-related anisotropy on the cyclic deformation behavior as well as occurring microstructural changes during loading, specimens were cut out of Cottonid sheets in 0°, 45° and 90° according to manufacturing direction, i.e. cellulose micro fibril orientation, and cyclically loaded in stepwise load increase tests. The tests were instrumented with strain measurements as well as thermography to estimate damage development in the material due to proceeding fatigue before failure. Direction-dependent microstructural changes were evaluated with scanning electron microscopy (SEM) on the surface as well as with computed tomography (CT) in the volume to characterize process-structure-property-relationships, which leads to a profound understanding of the biomechanics.
Similar to quasi-static results, specimens orientated in 0° according to manufacturing direction reached highest stresses at failure whereas 90° specimens showed an increased cyclic deformation behavior. Differences in damage development due to alternating micro fibril orientations could be visualized via SEM and CT. The results will be transferred to further develop Cottonid into a structurally optimized biopolymer composite material with pronounced actuation behavior in reaction to alternating humidity over special chemical treatments of natural fibers and directed process control. On this basis, tailor-made functional materials shall be generated in future where anisotropy and hygroscopicity can be adjusted through the manufacturing process.