Measuring The Velocity Distribution Of Shaped Charge Jet With Flash X-Ray Photography

Abstract

Measuring the velocity distribution of shaped charge jet with flash x -ray photographyWang YepingXian Mining Institute, Xian, ChinaABSTRACTThis paper presented a new method of measuring the velocity distribution of shapedcharge jet according to flash x -ray pictures of the broken jet- Stretching and Breaking upMethod, this method not only saves materials but also reduces experimental work consider-ably. Its results are in agreement with those from Truncating Method . Its accuracycan meet the needs of engineering purpose.1. THE PURPOSE OF MEASURING THE VELOCITY DISTRIBUTION OF SHAPED CHARGE JETWhen shaped charge with a metallic liner detonates, the liner would form a thin andlong jet. The velocity of the jet is not uniform. The front part always has higher veloc-ity and the rear part lower. That is, there is velocity gradient along the jet axis. Tocalculate or measure the velocity distribution is to determine the velocity distributionalong the jet axis at certain moment (curve V1-Z) or to determine the spatial distributionof jet elements with various velocities at different moments (curve t -Z).When materials of target and liner are given, the penetrating effect of jet (the penet-ration depth, the shape and the size of the hole generated) depends on its velocity andmass distribution, and the penetration depth mainly depends on the velocity distribution.Since jet has velocity gradient, it would stretch and become thinner while flying freely inthe air, and finally it would neck and break up. Therefore jet velocity distribution wouldinfluence its mass distribution. So, to some extent, the velocity distribution plays thedecisive role in penetration.Jet velocity and mass distribution is a middle link which connects jet forming processwith jet penetrating process. That is why result of jet forming process and the main init-ial condition of jet penetrating process. That is why measuring or calculating jet velocitydistribution is one of the main subjects in the studies of penetration mechanism.2. THE ESTABLISHMENT OF STRETCHING AND BREAKING UP METHODAccording to this method two flash x -ray pictures of a free broken jet would be taken attwo different instants. Matching each shadow image in one picture with another image in ano-ther picture and making sure that they are photos of same segment of the broken jet, we thencan get the moving distance of each jet segment Z= Z2 -Z1, and furthermore, according to thetime difference between these two flash x -ray pictures, we can get the velocity of each jetsegment, the whole velocity distribution curve in Vi -Z plane and corresponding equation. Wealso can get the velocity distribution curve in t -Z plane and corresponding equation by co-ordinate transformation.For setting up this method, the following three suppositions are introduced:(1). Jet elements and segments after breaking up move uniformly in the testing space.That is to say, air resistance is neglected.(2). Necking and breaking process does not influence jet velocity distribution, namely,necking and breaking process does not influence the velocities of jet elements.(3). The erosion due to the friction of jet and its segments with air can be neglected.According to the experimental observation and theoretical analysis the suppositionsabove are feasible.In order to take the picture of a broken jet, it is required that the instrument haslonger time delay and large field of vision. Experiments show that for a shaped charge ofmiddle calibre the rear part of the jet can not completely break up until 3O0us after det-onation. The field of vision at the place where film locates should be more than one meter.If the whole picture of a broken jet can not be put on one film, the experiment must be donetwice; for the first time a shorter time delay is used for taking the picture of those seg-ments formed from the front part of the jet; for the second time a longer time delay shouldbe used in order to take the picture of those segments from the rear part of the jet. Thesetwo sets of data are converted and unified to calculate the velocity distribution at time T.