Date of Award
Fall 2022
Document Type
Open Access Thesis
Department
Mechanical Engineering
First Advisor
Subramani Sockalingam
Abstract
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.
Recommended Citation
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