Experimental and numerical study of peculiarities at high-velocity interaction between a projectile and discrete bumpers

A high-velocity impact interaction of a polyethylene projectile (15 mm diameter) and aluminium projectile (6.35 mm diameter) with string and mesh bumpers (made of steel strings of 0.5–1.0 mm in diameter) was investigated experimentally and numerically. The study was aimed on detecting the projectile fragmentation peculiarities during projectile interaction with discrete bumpers. Since polyethylene has lower penetration resistance than aluminium, the effects inherent to discrete bumper penetration into the projectile must be more obvious for polyethylene. The string bumper is a set of parallel strings lying in a plane. The geometry of the string bumper which is simpler than the geometry of the mesh one also allowed one to get more understandable distribution of fragments on a thick aluminium witness-plate which was imposed behind the studied bumper to register the results of impact interaction for further analysis. The projectile velocity varied in the range of 1.7–3.8 km/s. The geometrical properties of such bumper-projectile system were characterized by two geometrical parameters: the parameter κ characterizing the bumper discreteness and equal to cell aperture-string diameter ratio, and the parameter ɛ defining the average number of cells falling within the projectile diameter.According to the experiments, the projectile destruction on string and mesh bumpers is especially distinguished by the accompanying formation of jets of fragments which are ejected from the front part of the projectile along and across its movement direction. The number of the jets correlates with the number of discrete bumper cells arriving at the projectile. The jets intensity depends on the impact velocity and values of the both geometrical parameters κ and ɛ; the action of the jets on the witness plate (evaluated by the craters depth) can exceed the action of the remaining projectile mass. The numerical modelling evaluates the revealed cumulative effect as follows: the velocity of the fragments in the jets exceeds the original projectile velocity up to factor of ˜1.5 in case when the projectile is aimed towards a cell centre.The experiments and calculations show that projectile destruction and fragmentation on the string and mesh bumpers are characterized not only by the formation of the jets ejected from the front part of the projectile but also by the shock-wave destruction of rear part of the projectile. The latter is the mechanism, which is inherent to a high-velocity impact on continuous bumpers. Which of these mechanisms prevails depends on the κ and ɛ parameters values. Frontal fragmentation dominates if the bumper has higher aperture of cells while at lower aperture the part of the projectile mass fragmented due to the formation of the jets diminishes significantly and the shock-wave destruction of projectile prevails. Numerical modelling also explains the mechanism of the formation of craters groups linearly distributed over the witness surface which were observed in the earlier experiments on projectile fragmentation upon the mesh bumpers.In addition, experiments on the high-velocity interaction of an aluminium projectile with two spaced string bumpers were carried out in case when the aperture of the string cell exceeded the projectile radius. Despite the lower areal density and small inter-bumper distance, the successive string bumpers revealed extremely higher ability to break up the projectile. Intensive fragmentation occurred even in the range of velocities that was lower than a threshold velocity at which the fragmentation of the projectile would have begun on a continuous aluminum bumper with the equal areal density. Numerical modelling makes clear the mechanics of the interaction of the projectile with the spaced string bumpers of the higher aperture.