Evaluation of the three-dimensional properties of Kevlar across length scales
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Mark R. VanLandingham Weapons & Materials Research Directorate-Materials and Manufacturing Sciences Division, U.S. Army Research Laboratory, ATTN: RDRL-WMM-B, Aberdeen Proving Ground, Maryland 21005-5069 (Received 9 December 2011; accepted 21 February 2012)
In this study, nanoindentation was utilized to measure the local, three-dimensional properties of Kevlar 49 and Kevlar KM2 on the length scales of the fiber microstructure. First, atomic force microscopy-based methods were used to explore the extent of property changes with respect to radial position in the fibers’ axial and hoop planes. From these measurements, no significant change in response was found for Kevlar 49 fibers, consistent with transverse isotropy. However, a reduced stiffness “shell” region (up to ;300–350 nm thick) was observed for KM2 fibers. Instrumented indentation was then used to evaluate fiber response with respect to orientation and contact size and establish a critical contact size above which the response is independent of indenter size (i.e., “homogeneous” behavior). A previously proposed analytical method for indentation of a transversely isotropic material was used to estimate the local material properties of the Kevlar fibers from the measured homogeneous response.
I. INTRODUCTION
KevlarÒ fabrics have historically been used for protective systems to make use of their exceptional axial specific strengths. Recent developments in flexible protective textile composite systems have shown increased energy dissipation upon impact through incorporation of nanosized particulates.1–3 From these studies, a primary source of improvement appeared to be a particle-fiber interaction mechanism. Upon impact, particles were embedded into adjacent fiber surfaces and restricted the ability of the fabric elements to move out of the way of the impactor. The extent of embedding is highly dependent on the local material properties of the Kevlar monofilaments and on the size, shape, and properties of the particles. The nanosized particulates used in textile composite systems impart contact sizes on the fiber surface within the nano- to micrometer regime. In this work, nanoindentation techniques based on atomic force microscopy (AFM) and instrumented indentation were performed to study smallscale contact on Kevlar fibers. Probes with sizes ranging from nanometers to micrometers were used to determine mechanical properties (e.g., elastic modulus, E ) and deformation with respect to radial position, contact size, and orientation. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.80 1824
J. Mater. Res., Vol. 27, No. 14, Jul 28, 2012
http://journals.cambridge.org
Downloaded: 14 Mar 2015
A. Kevlar microstructure and bulk material properties
Kevlar fibers show distinctive microfibril orientation and core-shell structure (Fig. 1), where the near-surface region of the fiber shows a higher degree of crystalline block orientation relative to the fiber interior.4–7 Kevlar 49, in particular, has received extensive morphology characteri
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