Near simultaneous multiple fragment impacts on aramid fabrics: Effects of velocity, dispersion and time intervals

Protection against fragment impacts is a critical concern across various applications. Conventional standardised methods use single-impact tests as a simplified approach for evaluating material resistance against impacting fragments. An alternative, near simultaneous triple impact test was implemented for testing protective equipment and materials against fragmentation, effectively simulating the impact of a dense cluster of fragments. Aramid fabrics, the most common protective measure against fragments, exhibit up to 13 % reduced ballistic resistance under triple impacts opposed to single impact tests signifying the importance of a multi-hit test method when evaluating protection against fragments. In this article, a finite element model is developed to conduct a detailed analysis of the dynamic interactions within a triple impact test. This analysis decouples and isolates the various contributing factors, including: the impacting velocity, the dispersion between impacts, the impact positions with respect to the main axes of material symmetry and the time intervals between the impacting projectiles. The model describes 15 layers of aramid fabric, in a detailed discrete mesoscale format, subjected to single, dual and triple impacting 1.102 g Fragment Simulating Projectiles (FSP) in different configurations using the finite element package LS-Dyna. As anticipated, penetrability increases when the distance between two impacting projectiles decreases. Additionally, on-axis impacts are more likely to result in perforation due to increased stress wave destructive–constructive interferences. Furthermore, the time interval between impacts is found to play a major role on the target's eventual ballistic performance. The oscillating phase caused by the excitation from the preceding impacting projectile, can amplify or diminish the relative impacting velocity of the subsequent projectile by as much as 40 %, resulting in either an over-perforation with residual velocity 55 % of the initial impacting velocity, or no-perforation with 0 % penetration depth.