Experimental Investigations of Yarn Pull-Out Behavior in Kevlar®: Influence of Applied Metallic Coatings and Effect of Dynamic Loading
Date of Award
Open Access Thesis
Flexible woven Kevlar® textile fabrics are widely used in protective armor applications. Yarn pull-out, comprised of yarn uncrimping (i.e., straightening) and yarn translation, is a major energy absorption mechanism during ballistic impact onto these fabrics, especially for impact velocities below the ballistic limit. Yarn pull-out is influenced by various parameters including inter-yarn friction, pull-out distance, pull-out velocity, transverse pre-tension, fiber diameter, fabric architecture, fabric waviness, fabric count and yarn modulus.
The first part of this thesis investigates yarn pull-out behavior of commercially available Kevlar® KM2+ individual yarns coated with metallic layers (copper, aluminum, aluminum nitride, silver) via a directed vapor deposition process. The uncoated control and metal-coated Kevlar® yarns are hand-woven into fabric swatches for quasi-static pull-out experiments. To perform these experiments, a yarn pull-out fixture is custom-designed and fabricated to apply transverse pre-tension to the fabric. Three levels of transverse pre-tensions are studied at 100 N, 200 N, and 400 N. Both peak pull-out force and energy absorption during the pull-out process is found to increase with increase in transverse pre-tension. All the metal-coated groups showed an approximately 200% increase in peak pull-out force and a 20% reduction in tenacity compared to uncoated control. Furthermore, all the metal-coated groups showed an increase in energy absorption, with aluminum-coated yarns showing the highest increase of 230% compared to control. These results suggest enhanced frictional interactions during yarn pull-out in metal-coated yarns compared to uncoated control as evidence by the frictional calculations.
The second part of this thesis investigates the yarn pull-out response of dynamic loading on Kevlar® K2+ woven specimens. A custom testing woven fabric fixture and single yarn grip fixture was designed and fabricated. The dynamic loading rates are studied at 15 m/s, 20 m/s, and 25 m/s. Peak force and energy absorption increased as the displacement rate increased for both tested L=27 mm and L=5 mm specimens. In the L=27 mm dynamic tested specimens, there was 33% increase in peak force and approximately 110% increase in energy absorbed with an increase in displacement rate. Quasi-static L=27 mm compared to dynamic L=27 mm specimens there was 528% increase in peak force, and 600% increase in energy absorbed. Studying dynamic experimental loading rates L=5 mm specimens show a 50% increase with an increase from 15 m/s to 20 m/s, but a slight decrease in peak force once reaching highest tested displacement rate. L=5 mm dynamic tested specimens energy absorption values show approximately a 130% increase with an increase in displacement rate. For Quasi-static to dynamic loading rates L=5 mm specimens presented a 900% increase from 1 mm/s to 20 m/s with a slight percentage decrease once reaching 25 m/s. The energy absorption from quasi-static to dynamic loading rates in L=5 mm experiments display a 700% increase in energy absorbed results of S706 fabric. All dynamic loading test showed higher peak forces and energy absorption values than quasi-static experiments. The results insinuate higher peak forces and energy absorption results which can be directly correlated to improved ballistic performance.
Roark, J. A.(2022). Experimental Investigations of Yarn Pull-Out Behavior in Kevlar®: Influence of Applied Metallic Coatings and Effect of Dynamic Loading. (Master's thesis). Retrieved from https://scholarcommons.sc.edu/etd/7052