Polycarbonate in transparent armour – mechanical properties, ballistic performance and ageing behaviour

Polycarbonate is an engineering plastic that is frequently used as a component in, or as a stand-alone, transparent armour for many military applications. One important application of polycarbonate is in transparent armour systems for front and side windows in military ground vehicles. Other examples are goggles, visors and ballistic shields for individual soldiers. All these different transparent armours are designed to provide ballistic protection against specific projectile and fragment threats. Transparent armours on ground vehicles are made of layers of materials, each with very different physical properties and behaviours during ballistic impact, with the most common materials being glass and polycarbonate. These armour systems are designed to be transparent but still provide effective ballistic protection despite being constrained by factors such as final thickness, weight and cost. Polycarbonate is usually chosen as the backing layer, and its main role is to contain eroded projectiles and armour fragments. This report gives an overview of the mechanical properties, ballistic performance and ageing behaviour of polycarbonate. Polycarbonate is a ductile material and the deformation behaviour of polycarbonate is similar to that of many engineering plastics. Several material properties are strain rate dependent, and there is some increase in the yield stress and hardening with increasing strain rate. Sheets that are impacted by projectiles will normally fail by plastic deformation, and several penetration mechanisms have been identified. Overall, thinner plates favour more extensive dishing of the target, while thicker plates may favour plugging failures more. Although polycarbonate has some excellent properties that make it suitable for use in armour systems, it also has some major drawbacks. The material is known to degrade on a molecular level when exposed to ageing conditions, such as ultraviolet light, humidity and temperature, ultimately resulting in embrittlement and premature failure. In the ageing of transparent armour, both the degradation of the polycarbonate (critical for the ballistic performance), and also of the adhesive interlayers (critical to transparency), may contribute to the failure of the armour being unable to fulfil its role. Ageing results in the transparent armours having a lifetime that is much shorter than that of the vehicles, resulting in high replacement costs and significantly increased operational costs. These topics are addressed in this report.