Robust and precise identification of the hygro-expansion of single fibers: a full-field fiber topography correlation approach

Over the past decades, natural fibers have become an important constituent in multiple engineering- and biomaterials. Their high specific strength, biodegradability, low-cost production, recycle-ability, vast availability and easy processing make them interesting for many applications. However, fiber swelling due to moisture uptake poses a key challenge, as it significantly affects the geometric stability and mechanical properties. To characterize the hygro-mechanical behavior of fibers in detail, a novel micromechanical characterization method is proposed which allows continuous full-field fiber surface displacement measurements during wetting and drying. A single fiber is tested under an optical height microscope inside a climate chamber wherein the relative humidity is changed to capture the fiber swelling behavior. These fiber topographies are, subsequently, analyzed with an advanced Global Digital Height Correlation methodology dedicated to extract the full three-dimensional fiber surface displacement field. The proposed method is validated on four different fibers: flat viscose, trilobal viscose, 3D-printed hydrogel and eucalyptus, each having different challenges regarding their geometrical and hygroscopic properties. It is demonstrated that the proposed method is highly robust in capturing the full-field fiber kinematics. A precision analysis shows that, for eucalyptus, at 90% relative humidity, an absolute surface strain precision in the longitudinal and transverse directions of, respectively, 1.2 × 10 -4 and 7 × 10 -4 is achieved, which is significantly better than existing techniques in the literature. The maximum absolute precision in both directions for the other three tested fibers is even better, demonstrating that this method is versatile for precise measurements of the hygro-expansion of a wide range of fibers. Graphic abstract