High Velocity Impact Properties of Fabrics Treated with Shear Thickening Fluid

Abstract

yarn pullout is dominant at lower impact velocity while stress localization is dominant at high impact velocity. The hybridization of STF treated fabrics improved both blunt trauma resistance and perforation resistance against deformable projectiles (e.g. 9mm full metal jacketed (FMJ) bullets) compared to an all neat fabric panel having the same areal density. The layering sequence of the neat and STF treated fabrics in a hybrid panel was found to play an important role in improving the ballistic performance, where the panel with neat fabric layers backed by STF impregnated fabric layers showed a better performance. Such superior ballistic performance is presumed to be due to the better coupling of yarn elongation in the frontal neat fabric layers and the rear STF treated fabric layers, and thus, increased bullet expansion. A conceptual analysis was carried out by adopting a method of accumulating successive line segments to present the energy dissipation route of each panel during the impact. For another series of hybrid ballistic materials (i.e. unidirectional (UD) and woven fabrics), the effect of layering sequence on the ballistic performance was further investigated with two different kinds of bullets

an unexpandable 5.56mm FSP and an easily expandable .44 Magnum semi-jacketed hollow point (SJHP) bullet. Some of the woven fabric layers were treated with STF to modify their properties. When the layers with smaller in-plane constraint (neat woven fabric) were laminated behind the layers with larger in-plane constraint (UD or STF treated woven fabric), an increase in perforation resistance against the FSP was observed due to the decreased out-of-plane constraint. When the layering sequence was reversed, an increase in both the blunt trauma resistance and perforation resistance against the .44 Magnum round was observed due to the better coupling of yarn elongation in the frontal and rear layers. The effect of fabric count and shot location on the ballistic performance of hybrid panels containing STF impregnated layers was also investigated. The panels with higher fabric count dissipated a higher fraction of the given impact energy through tensile dissipation and this led to a lower BFS. The decrease in BFS value by the hybridization of neat and STF impregnated fabrics was smaller for panels of densely woven fabric due to a larger difference in the warp and weft crimp ratios. Shot location affected the V50 value as well as the BFS value of the panels, where both values increased as the shot location approached the edge. Finally, the amount of energy transferred to the backing material of oil-based clay (i.e. kinetic dissipation) in ballistic tests of soft body armor panels was assessed. To determine the relationship between penetration depth (or dent volume) and impact velocity (or energy), weight dropping test with a series of steel spheres was carried out at low impact velocities, and direct shooting with a 5.56mm FSP was carried out at high impact velocities. At both high and low impact velocities, the volume of the dent made in the oil-based clay was proportional to the velocity of the impactor. The change in dent volume per the change in impact velocity was found to be proportional to the 1.5th power of the mass of the impactor, while the energy absorption per unit dent volume increased linearly with the impact velocity. The relationship between trauma depth (or dent volume) and kinetic dissipation of a soft body armor panel subjected to a 9mm bullet at 436 m/s is presented, where the trauma diameter approached that of a 1.043 kg steel ball.

Keywords: high velocity impact, soft body armor, shear thickening fluid, laminating sequence, fabric count, crimp, shot location, ballistic limit, backface signature, kinetic dissipation

전단농화유체를 함유한 방탄직물(연질방탄재)의 고속 충격 저항성에 관한 연구를 수행하였다. 연구에 사용된 전단농화유체는 액상의 폴리에틸렌글리콜에 고상의 경질 실리카 나노입자가 임계치에 근접하게 충전된 연질의 혼합응축상 물질로서 임계치 부근의 전단변형속도에서 전단응력이 가역적으로 급격히 변하는 거동을 보인다. 전단농화의 개시점은 입자의 크기에 의존하며 글로벌한 항복응력은 입자의 크기 및 부피분율에 의존한다. 전단농화유체를 아라미드직물에 함침하여 구성사의 뽑힘성질 및 인장성질을 조사한 결과 이 물질이 직물내부의 공극을 점유하는 효과 및 전단농화 효과가 복합적으로 작용하여 구성사의 뽑힘저항성이 증가하고, 인장 시 크림프에 의한 인장지연 현상을 억제하며 단일층에서 경위사 크림프율 차이에 의한 경위사간 장력불균일 현상을 해소하는 긍정적인 측면을 보이는 반면 응력집중의 부정적인 측면도 동시에 보였다. 이러한 효과들의 크기는 이 물질의 유변특성 및 직물에 함침되는 양에 의해 결정된다. 전단농화유체가 함침된 아라미드직물을 함유하는 연질 방탄재의 관통저항성과 둔상억제력을 조사하였다. 단일성분으로 구성된 방탄재간의 성능을 비교한 결과 비교적 저속의 영역에서는 전단농화유체를 함유한 아라미드직물로 이루어진 방탄재의 모의파편탄 방호성능이 상대적으로 우수하였으나, 속도의 증가에 따라 성능의 우열이 바뀌는 현상을 관찰할 수 있었으며 이는 방탄재의 주요 파단거동이 속도에 따라 변하는 데서 기인한다. 미처리직물과 전단농화유체 함침 직물을 혼용한 방탄재의 경우 미처리직물을 앞쪽에 적층한 경우 9밀리 볼탄에 대한 관통저항성과 둔상억제력이 동시에 증가하는 현상을 관찰할 수 있었으며, 이는 앞쪽에 위치한 미처리직물과 뒤쪽에 위치한 전단농화유체 함침 직물중의 탄자와 마주치는 구성사들간의 장력결합이 좀 더 효율적으로 일어나는 점으로 인한 탄자의 뭉개짐 증가에서 기인하는 것으로 보여진다. 교차 일방향 직물을 보통의 제직물과 혼용한 방탄재에 대해서도 44구경 매그넘 탄자로 실험한 결과 9밀리 볼탄과 유사한 결과를 보인 반면 모의파편탄과 같이 뭉개짐이 없는 탄자의 경우는 정반대의 결과를 보였으며, 그 이유는 다층적층물의 층간 간섭의 증가로 인한 파단거동의 변화에 의한 것으로 보여진다. 제직밀도가 높은 직물로 동일한 시험을 수행한 결과 경위사간의 크림프율의 격차가 상대적으로 증가하여 혼성화에 의한 성능향상의 정도는 감소한 반면 절대적인 성능에 있어서는 둔상억제력 측면에서 제직밀도가 낮은 직물에 비해 좀 더 나은 결과를 보였다. 따라서 제직밀도는 높이되 경위사 크림프율의 관리 (경위사간 크림프 격차를 줄이고 크림프율 자체를 낮게함)에 좀 더 세심한 노력을 기울일 필요가 있음을 확인할 수 있었다. 또한 방탄복에 적용할 경우 유효 방호면적과 직접 관련되는 사격위치에 따른 영향을 살펴본 결과 변부에서 가까운 위치일수록 관통저항성은 증가하는 반면 둔상억제력은 감소함을 확인할 수 있었고, 전단농화유체 적용 시 나타나는 현상들은 사격위치에 무관함을 확인하였다. 끝으로 연질 방탄재의 방탄시험 시 후면재로 전달되는 에너지를 정량화 하기 위해 강구의 자유낙하실험과 모의파편탄을 유점토에 직접 사격하는 방법을 병행하여 충격자의 치수(중량)에 따른 실험식을 얻고, 운동방정식과 탄성한계치를 이용하여 얻어진 실험식의 물리적 의미를 파악함으로써 이의 확장응용 가능성을 제시하였다.

주요어: 고속 충격, 연질 방탄재, 전단농화유체, 적층순서, 제직밀도, 크림프, 사격위치, 관통저항성, 둔상억제력, 후면전달 에너지

The high velocity impact properties of p-aramid fabrics treated with shear thickening fluid (STF) was investigated for soft body armor application. The STF used in this study was a mixed soft condensed matter which was composed of solid phase hard silica nanoparticles and liquid phase polyethylene glycol (PEG) where the volume fraction of the solid phase was just below the critical packing fraction. The matter showed a reversible shear thickening behavior in which the shear stress jumps dramatically (i.e. discontinuous shear thickening) as the shear rate was increased. The onset stress of shear thickening of the matter was found to be dependent mainly upon particle size, while the global failure stress of the shear thickened matter was dependent upon both particle volume fraction and particle size. The pullout and elongational properties of a single yarn within p-aramid fabrics that were treated with STF were investigated. The occupation of the interstitial volume within the fabrics by the impregnated STF resulted in an increase in yarn pullout force. Additional increase in yarn pullout force was observed above the critical pullout rate, which is presumed to be due to shear thickening of the impregnated matter. Furthermore, the occupation of the interstitial volume within the fabrics by STF also less retarded the tension increase upon elongation (i.e. less retardation of the propagation of the elongational wave). Otherwise, upon elongation, the increase in tension would have been more retarded due to the yarn crimp. Both the single yarn pullout and elongational properties of the fabrics were affected by the rheological property of the STF, the add-on of the matter as well as the fabric count. The ballistic performance of p-aramid fabrics treated with STF was investigated using the two most widely adopted methods, the ballistic limit (V50) test and the backface signature (BFS) test. For single component ballistic panels (i.e. all neat fabrics or all STF treated fabrics), STF treatment improved the impact resistance of the panels against an undeformable projectile such as 5.56mm fragment simulating projectile (FSP) at low velocity (< 250 m/s), but at increased velocity, the V50 performance decreased even with the same number of layers. This is presumed to be due to the different failure mechanisms involved at each impact velocity range