Atomic force microscopy-based nanoindentation technique for characterizing the transverse and shear moduli of flax fibers

Experimentally evaluating the elastic properties of flax fibers is challenging due to their complex hierarchical structure, and a standard test procedure for measuring their transverse and shear moduli is currently not reported in the literature. Hence, this study presents an atomic force microscopy (AFM) based nanoindentation technique to evaluate the transverse and shear moduli of flax fiber. A high-precision focused ion beam (FIB)-milling process was used to fabricate a flat surface for indentation along the longitudinal fiber cross-section of the fiber and transverse fiber cross-section by polishing the unidirectional (UD) lamina in order to evaluate the indentation modulus. Further, Swanson's numerical contour approach was adopted to evaluate the elastic properties of the fiber from the measured indentation modulus. The accuracy of the experimentally obtained fiber properties is verified by using it in a micromechanics model for predicting the elastic properties of the UD lamina and comparing it with experimental results reported in the literature.