Nanoindentation of freestanding single Kevlar® fibers with an adjusted indentation area function

Abstract

High-performance, anisotropic, single Kevlar fibers, K29, KM2, and K119, are evaluated by a newly developed modified Curved Area Function (mCAF). Indentations at various loads and depths are carried out to obtain the mechanical properties across the diameter of the fibers. To avoid substrate effects, the 10% substrate thickness rule of thumb was followed. Targeted Poisson ratios are assumed in the calculations of the indentation area function to match published data. The indentation moduli of Kevlar fibers at 10% fiber thickness measures: KM2 at 5.10 ± 0.4 GPa (ν = 0.30) and 4.91 ± 0.4 GPa (ν = 0.35), K29 at 2.95 ± 0.3 GPa (ν = 0.30) and 2.84 ± 0.3 GPa (ν = 0.35), and K119 at 2.43 ± 0.4 GPa (ν = 0.27). For all studied cases, the indentation moduli do not differ by more than ±0.4 GPa, indicating a small measurement error, contrasting the ±0.86 to ±1.0 GPa error range attained by traditional area function calculations. The results are compared with the semi-plane analysis proposed by Oliver and Pharr to illustrate the necessity of the mCAF method. Additional experiments and indentation moduli calculations at 2–13%, 7–13%, and 25–40% indentation depth of each fiber diameter are performed, using both the corrected and the traditional contact areas. The indentation moduli computed from the flat area function are generally less than those determined from the mCAF. As expected, maximum indentation moduli measurements occur closer to the surface of the fibers.