Modeling and Experiments on Ballistic Impact into UHMWPE Yarns Using Flat and Saddle-Nosed Projectiles

Abstract

Abstract:Yarn shooting experiments were conducted to determine the ballistically-relevant, Young's modulus and tensile strength of ultra-high molecular weight polyethylene (UHMWPE) fiber. Target specimens were Dyneema ® SK76 yarns (1760 dtex), twisted to 40 turns/m, and initially tensioned to stresses ranging from 29 to 2200 MPa. Yarns were impacted, transversely, by two types of cylindrical steel projectiles at velocities ranging from 150 to 555 m/s: (i) a reverse-fired, fragment simulating projectile (FSP) where the flat rear face impacted the yarn rather than the beveled nose; and (ii) a 'saddle-nosed projectile' having a specially contoured nose imparting circular curvature in the region of impact, but opposite curvature transversely to prevent yarn slippage off the nose. Experimental data consisted of sequential photographic images of the progress of the triangular transverse wave, as well as tensile wave speed measured using spaced, piezo-electric sensors. Yarn Young's modulus, calculated from the tensile wave-speed, varied from 133 GPa at minimal initial tension to 208 GPa at the highest initial tensions. However, varying projectile impact velocity, and thus, the strain jump on impact, had negligible effect on the modulus. Contrary to predictions from the classical Cole-Smith model for 1D yarn impact, the critical velocity for yarn failure differed significantly for the two projectile types, being 18% lower for the flat-faced, reversed FSP projectile compared to the saddle-nosed projectile, which converts to an apparent 25% difference in yarn strength. To explain this difference, a wave-propagation model was developed that incorporates tension wave collision under blunt impact by a flat-faced projectile, in contrast to outward wave propagation in the classical model. Agreement between experiment and model predictions was outstanding across a wide range of initial yarn tensions. However, plots of calculated failure stress versus yarn pre-tension stress resulted in apparent yarn strengths much lower than 3.4 GPa from quasi-static tension tests, although a plot of critical velocity versus initial tension did project to 3.4 GPa at zero velocity. This strength reduction (occurring also in aramid fibers) suggested that transverse fiber distortion and yarn compaction from a compressive shock wave under the projectile results in fiber-on-fiber interference in the emerging transverse wave front, causing a gradient in fiber tensile strains with depth, and strain concentration in fibers nearest the projectile face. A model was developed to illustrate the phenomenon.

The Effect of Projectile Nose Shape on the Critical Velocity of High-Performance Yarn

“…As pointed out in the previous study, the critical velocity predicted by Smith theory was around 731 m/s [3,6]. Such a discrepancy between the predicted and experimental critical velocities for all cases agreed with the results reported in the literature [1,2,7,8]. The load cells mounted to the grips allowed the force to be measured when the yarn specimen was subjected to transverse impact.…”

“…Phoenix et al [1] performed ballistic experiments on Dyneema ® SK76 yarns with various initial pretension using two types of projectiles (flat and saddle noses). The results showed that as the initial pretension increased, the Young's modulus also increased.…”

“…To the best of our knowledge, the circular nose shape projectiles (round, saddle or spherical) used in most of the transverse impact experiments contained only one radius of curvature [1,7,8,10,11]. The only exception was the experimental work done by Hudspeth et al [2] where a razor blade and a 0.30-cal round projectile were used.…”

“…Projectile nose shape is an important parameter that dominates the ballistic performance of a high-performance fiber [1][2][3]. The projectile striking velocity that causes instantaneous rupture on a yarn under transverse impact is defined as critical velocity [2,3].…”

“…A factor that causes the overestimation of the predicted critical velocity by the Smith theory is the projectile's nose shape [1,2]. Hudspeth et al [2] shot a razor blade, a 0.30-cal fragmented simulation projectile (FSP) and a 0.30-cal round onto high-performance yarns and found out that the 0.30-cal round had the highest critical velocity, followed by the 0.30-cal FSP and the razor blade.…”