The interaction between a shaped charge jet and a single moving plate

Reactive armour is a very efficient add-on armour against shaped charge threats. Explosive reactive armour consists of one or several plates that are accelerated by an explosive. Similar but less violent acceleration of plates can also be achieved in a completely inert reactive armour. To be efficient against elongated jets, the motion of the plates needs to be inclined against the jet such that a sliding contact between the jet and the plates is established. This sliding contact causes a deflection and thinning of the jet. Under certain circumstances, the contact will become unstable, leading to severe disturbances on the jet. These disturbances will drastically reduce the jet penetration performance and it is therefore of interest to study the conditions that leads to an unstable contact.Previous studies on the interaction between shaped charge jets and flyer plates have shown that it is mainly the forward moving plate in an explosive reactive armour that is effective in disturbing the jet.This is usually attributed to the higher plate-to-jet mass flux ratio involved in the collision of the forward moving plate compared to the backward moving plate. For slow moving plates, as occurs in inert reactive armour, the difference in mass flux for the forward and backward moving plate is much lesser, and it is therefore of interest to study if other factors than the mass flux influences on the protection capability.In this work, experiments have been performed where a plate is accelerated along its length, interacting with a shaped charge jet that is fired at an oblique angle to the plate’s normal, either against or along the plate’s velocity. The arrangement corresponds to a jet interacting with a flyer plate from a reactive armour, with the exception that the collision velocity is the same for both types of obliquities in these experiments. The experiments show that disturbances on the jet are different in the two cases even though the collision velocities are the same. Numerical simulations of the interaction support the observation. The difference is attributed to the character of the contact pressure in the interaction region.For a backward moving plate, the maximum contact pressure is obtained at the beginning of the interaction zone and the contact pressure is therefore higher upstream than downstream of the jet while the opposite is true for a forward moving plate. A negative interface pressure gradient with respect to the jet motion results in a more stable flow than a positive, which means that the jet-plate contact is more stable for a backward moving plate than for a forward moving plate. A forward moving plate is thus more effective in disturbing the jet than a backward moving plate, not only because of the higher jet to plate mass flux ratio but also because of the character of the contact with the jet.