Multiaxial mechanical behavior of aramid fibers and identification of skin/core structure from single fiber transverse compression testing

Abstract

International audienceThe transverse and longitudinal mechanical properties of aramid fibers like Kevlar™ 29 (K29) fibers are strongly linked to their highly oriented structure. Mechanical characterization at the single fiber scale is challenging especially when the diameter is as small as 15 µm. Longitudinal tensile tests on single K29 fibers and single fiber transverse compression test (SFTCT) have been developed. Our approach consists of coupling morphological observations and mechanical experiments with SFTCT analysis by comparing analytical solutions and finite element modeling. New insights on the analysis of the transverse direction response are highlighted. Systematic loading/unloading compression tests enable to experimentally determine a transverse elastic limit. Taking account of the strong anisotropy of the fiber, the transverse mechanical response sheds light on a skin/core architecture. More importantly, results suggest that the skin of the fiber, typically representing a shell of one micrometer in thickness, has a transverse apparent modulus of 0.2 GPa. That is around more than fifteen times lower than the transverse modulus of 3.0 GPa in the core. By comparison, the measured longitudinal modulus is about 84 GPa. The stress distribution in the fiber is explored and the critical areas for damage initiation are discussed

Production of Synthetic, Para-Aramid and Biopolymer Nanofibers by Immersion Rotary Jet-Spinning

“…Poly(para‐phenylene terephthalamide) (PPTA, the polymeric material of commercial Kevlar and Twaron) is a class of ultrastrong temperature‐resistant para‐aramids with broad uses that include ballistic apparel, brake and transmission friction parts, ropes and cables, and reinforcement of rubbers and other composites . Commercial PPTA fiber diameters are on the order of ≈10 μm and possess an inhomogeneous core‐skin morphology that depends on proprietary production processes . Reducing fiber diameter will be of interest for use in composites, where the surface area‐to‐volume ratio of nanofibers can lead to improved adhesion to the matrix and strengthening of the composite .…”

In Situ tensile tests to analyze the mechanical response, crack initiation, and crack propagation in single polyamide 66 fibers

“…As this inclined surface can be supposed to be the zone where the convex blunting of the initial crack was located, three reasons might be considered to explain this shape, beyond the projection effects: Possible skin–core effects like residual stresses already reported in previous work. The bending effect due to the asymmetry of the crack in a SENT like geometry: the neutral fiber location due to this bending being different at the surface from that in the central part. The viscoelastic effect that may change the fracture surface aspects in the observed conditions, that is, in the fully relaxed state (long‐time elapsed after test). …”

“…As in most studies on this subject, focus was given to the change of the microstructure related to the level of deformation in the longitudinal direction using the engineering stress–strain curve. Transverse mechanical properties of fibers have received even less attention due to the aforementioned difficulties related to their fineness …”

“…First, by inducing a highly oriented structure, this is especially the case for polymers, a great enhancement of the specific modulus along the fiber axis can be achieved compared to unoriented materials. [1][2][3] The second feature is the high aspect ratio of the structure: a fiber diameter is typically tens of micrometers, with monofilaments (MFs) for particular uses having diameters up to hundreds of microns, whereas fiber lengths can be up to several meters or virtually continuous, depending on applications. Thus, despite high tensile moduli, fibers as elementary mechanical structures are highly flexible and can easily be organized into complex multi-assemblies like yarns or fabrics.…”

“…Transverse mechanical properties of fibers have received even less attention due to the aforementioned difficulties related to their fineness. 3 Regarding failure properties of polymer fibers, single fiber stress (and strain) at break often remain the gold standards as quantitative descriptors of ultimate properties. 11,12 Yet, often neither stress nor strain at break can provide a satisfactory failure criterion due to the lack of knowledge on the distributions of defects, their nature and sizes.…”