Effects Of Crimp And Slip On Laminar And Woven Fabrics Subjected To Ballistic Impact
In the past decade, there has been growing interest and research on improving the performance of soft body armor materials subjected to high-speed ballistic impact. One "by-product" of the production process for these high strength polymer fibers, which are bundled into yarns, is the existence of undulation or waviness in the yarns, known as crimp. While this has always been treated as undesirable, few comprehensive studies have been done on the true effects of crimp in conjunction with yarn slip in ballistic fabrics. We first develop an in-house Finite-Difference (FD) numerical model to study the post-impact but pre-failure behavior of crimped fabrics made with Dyneema® yarns. While there has been past literature that attempted to numerically model crimp in ballistic fabrics, we note that the results provided little insight regarding the strain profile of individual yarns, the growth and evolution of tension and cone waves, and the yarn de-crimping process. Our first fabric model has laminar geometry with outof-plane zigzag crimp, and we validate our results through comparison with previous analytical models developed by the Cornell Phoenix Group. We present the following findings: (i) the peak strain attained by yarns in the fabric is lowered with increase in crimp, but with small sacrifices in terms of velocity deceleration and out-of-plane projectile displacement, (ii) yarn strain build-up towards its maximum value can vary significantly depending on projectile mass and size dimensions, (iii) tension and cone waves' velocities and shapes are influenced by various parameters including [ETA], the rocking viscosity coefficient, and (iv) allowing for frictional slip between the projectile and the yarns beneath it causes a shock wave effect, which changes the very early response behavior of the fabric post-impact. With our first fabric model, we also ensure that local strain concentrations and other dynamic artifacts resulting from the discretization of the structure are suppressed or smoothed. Since most fabrics in reality are woven with interlaced, over-under yarn structure, we develop a second crimp model with woven geometry. In addition to the forces from our first model, we introduce six new forces to describe the contact motion between weft and warp yarns in-plane (viscoelastic), and out-of-plane (allowing for compression but imposing a "Hertzian" condition). Correspondingly, we introduce six new parameters to control for yarn slip, crossover forces, and restoring forces. Comparing our two models, we observe that the second model provides greater flexibility and is more realistic in its ability to accurately portray the phenomenon of crimp interchange. Within the woven crimp model, we find many of the same trends in results provided in (i)-(iv) from the laminar model. We also discover that most of these parametric effects are not independent of each other, and provide case studies to show that varying combinations of multiple parameter values can in some cases produce nearly identical simulation behavior, but in other cases significantly different simulation behavior from one another.
ballistic impact; crimp; yarn slip
Phoenix, Stuart Leigh
Hui, Chung-Yuen; Sachse, Wolfgang H
Ph. D., Mechanical Engineering
Doctor of Philosophy
dissertation or thesis