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.
Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment
Abstract The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100 °C for 20 min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si + Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.