V50 determination challenges for state-of-the-art body armour

The material constructions in personal protective equipment (PPE) have shifted over the last decades. In soft armour ballistics the construction shifted from plain wave fabrics towards more use of Unidirectional (UD) sheets. Also, the use of Ultra High Molecular Weight Polyethylene (UHMWPE) fabric constructions increased significantly compared to aramid. Most state-of-the-art combat helmets are currently constructed from UHMWPE UD composite. All these changes were made to achieve weight reduction and increased ballistic protection for body armour equipment. The ballistic limit velocity (V50) is widely used as a measure of the ballistic performance of a ballistic protective material or construction. This measure does depend on the standard or method used to determine the V50. Four different standards for V50 determinations are discussed and compared to show that the ballistic limit velocity of a material is dependent on the test procedure, test requirement and the statistical analysis method used even if the threat, the mounting of the sample and the sample size were the same. Several challenges to accurately determine a V50 value have already been reported, such as the effect of start velocity and total number of shots. This paper focusses on effects observed due to the increasing ballistic performance: deformation of the rigid fragment simulating projectile, larger affected impact zone, and increasing Zone of Mixed Results (ZMR). Experimental data is given to support the effects. For all observed effects, solutions are proposed like using a hardened FSP and specifying a minimum shot-to-shot distance. The consequences of these challenges are discussed, including experimental challenges if high V50 values must be determined. It is questioned if the V50 value is always the consistent and reliable evaluation parameter to be used, especially when a large ZMR is observed. A solution could be, not to use the V50 value as a measure, but instead the percentage of perforations for one or a few specified velocities.