Examining the Relationship Between Ballistic and Structural Properties of Lightweight Thermoplastic Unidirectional Composite Laminates
Abstract : The pursuit for lightweight personal protection (helmets, body armor, etc.) has been a key issue for the Army in recent years. Previous efforts have shown that the variation of orientation and architecture in ultra-high molecular weight polyethylene (UHMWPE) composite panels can significantly affect ballistic performance characteristics. In our experiments, an architecture now referred to as ARL X Hybrid emerged as the clear leader in the compromise between ballistic performance and back face deformation (BFD). For this work, thermoplastic unidirectional materials (both aramid and polyethylene based) were evaluated using 17 grain FSP V50 and 9 mm BFD testing in both typical ([0 deg/90 deg]) and X Hybrid architectures, at an areal density target 27% lighter (7.8 kg/sq m) than the currently fielded state of the art. Digital Image Correlation (DIC) was employed to characterize the materials during testing and to help elucidate the panel response as a function of architecture. Polyethylene materials in the X Hybrid architecture were shown to retain 96-99% of the [0 deg/90 deg] architecture's V50 ballistic performance, while also reducing the BFD by 36-41%. Higher fiber involvement, increased interaction area, and increased membrane stress and through-thickness compression are proposed as factors for this phenomenon.
Ballistic impact response of Ultra-High-Molecular-Weight Polyethylene (UHMWPE)
Abstract In this paper we report findings from ballistic experiments conducted on Ultra-High-Molecular-Weight Polyethylene (UHMWPE) flat panels. We measured ballistic limit velocities, V50 of panels of different thicknesses using a fragment simulating projectile (FSP) and characterized back face deformation (BFD) using a spherical projectile. UHMWPE panels with different architectures (hybrid and non-hybrid) were evaluated. For the back face deformation (BFD) experiments, the thickness of post-impact intact material and interior delamination were characterized using X-ray Computer Tomography method. These measurements were used to evaluate the effects of fiber orientation and boundary constraints on material deformation and failure response to ballistic loading conditions. The expansion rate of the back face deformation zone in the transverse direction (to the direction of impact) decreased with time. This transverse expansion speed was higher in hybrid panels compared to the [0/90] cross-ply panels, which resulted in earlier influence of boundary constraints on back face deformation for hybrid panels. One major delamination occurred in the hybrid panels at cross-ply [0/90] and non-cross ply interface. The lateral extent of this delamination was larger when the panel edges were clamped as compared to the free panel edge case.
Dynamic tensile testing of aramid and polyethylene fiber composites
Dynamic properties of materials such as aramid and polyethylene fiber reinforced composites are rarely found in the literature, in spite of their significance in ballistic design. It is also difficult to find detailed descriptions of the testing techniques and procedures to characterize these materials. This paper describes a dynamic tensile testing technique for these composite materials, and discusses problems such as the specimen size and the clamping system. A full numerical simulation is performed in Autodyn-2D to elucidate what is happening during the experiment. Finally this testing technique is applied to show that reliable data can be obtained from the Hopkinson bar for aramid and polyethylene fiber reinforced composites. Stress-strain, stress versus rate of strain and strain versus rate of strain curves are included. The results indicate that for these materials the tensile strength rises with the strain rate while the maximum strain diminishes in the range studied.