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

Abstract

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

Full-field hygro-expansion characterization of single softwood and hardwood pulp fibers

“…This negative hygro-expansion may suggest that the swelling of the fiber is not solely dominated by the S2 layer, if the primary or the S1 layer are thick, they may induce the negativity that is found, this also affects the major principle strain angles given in Figure 3 (B). A negative longitudinal hygro-expansion was also observed on the viscose fibers tested in Vonk et al (2020) and on cellulose fibrils in Lee et al (2010). Therefore, the large fiberto-fiber variability can be fully attributed to the difference in tested fibers and is properly captured through the high precision of the applied experimental methodology.…”

“…The halftime (t half ) have been extracted from the long-duration experiments by means of exponential fitting the transverse hygro-expansion trend at the wetting or drying plateau and are given in Figure 3 (A), in which fibers 1-2 correspond to the fibers subjected to long-duration tests. Note that the methodology's strain precision (1.0⋅10 −4 and 2.1⋅10 −4 in longitudinal and transverse direction, determined by of computing the standard deviation during the wetting plateau) was significantly increased compared to the values of 1.2⋅10 −4 and 7.0⋅10 −4 reported by Vonk et al (2020). This is mainly attributed to the improved pattern quality, resulting from the applied dedicated mystification setup proposed by Shafqat et al (submitted for publication).…”

“…and five hardwood fibers are tested throughout this research. The considered method that enables measuring the full-field hygro-expansion of single fibers, as proposed by Vonk et al (2020), can be divided into three steps, schematically shown in Figure 1. Figure 1 (A) shows a single fiber fixed by two nylon threads, providing complete kinematic freedom for the hygro-expansion of the fiber while only reducing the global fiber's rigid body motion (fiber swelling, bending and rotation are allowed).…”

“…From this field, the average longitudinal (ϵ ll ) and transverse (ϵ tt ) surface strain can be computed and plotted versus relative humidity as shown in Figure 1 (C). For a more detailed description on the working principles, derivation, applicability and performance of the employed method, the reader is referred to Vonk et al (2020).…”

“…Full-field hygroscopic measurements are experimentally challenging, due to the fiber's relatively low longitudinal hygro-expansion (maximally ∼2 % from dry to completely saturated (Meylan 1972)) combined with the fiber's large kinematic freedom and large release of driedin strain during wetting and drying, causing large displacement modes to occur, i. e. fiber rotation, bending and translation. Therefore, Vonk et al (2020) have developed a experimental methodology, based on Global Digital Height Correlation (GDHC) of surface topographies which allows the determination of the time-resolved full-field hygroexpansion of single fibers, i. e. a map of the surface strain tensor over the full field of view as a function of time or relative humidity. The method's robustness and high precision of 1.2⋅10 −4 in longitudinal and 7.0⋅10 −4 in transverse hygro-expansion was demonstrated for different natural and synthetic fiber types, each with different hygroscopic properties and measurement challenges.…”