Determining Detonation Threshold for Multiple Fragment Impacts

Dual fragment impacts were studied using the Eulerian hydrocode CTH. The purposes of the study were to gain insight into the physical phenomena underlying multiple fragment impacts, and to develop an equation that predicts critical fragment velocity, the velocity at which a fragment must travel to cause a detonation in a covered explosive, for dual fragment impacts. Identical steel fragments were used, and the target coverplate material, coverplate thickness, and the energetic material were held constant. The first analysis studied fragment separation in the dimension parallel to the target surface, with zero time separation. The next analysis was conducted varying the time between fragment impacts, with zero distance separation. The next analysis was performed varying the fragment separation in time and in distance simultaneously. Finally, the analysis was extended to other fragment sizes to determine if a general equation could be developed. The analyses showed that the distance separating fragments (in the dimension parallel to the target) can significantly affect the critical velocity required to initiate a covered charge. The effect of separating fragments in time can be even more detrimental to the target than simply placing fragments close together in space. A general critical velocity relation which accounts for variation in impact times may depend on more variables than are easily determined for or employed in an empirical relation. BACKGROUND Multiple fragment impact is one of the U.S. Navy’s critical insensitive munitions tests. In this test, 250-grain steel fragments are launched against ordnance items. At least two fragments must impact the test item. To pass the test, a burning reaction from the ordnance item is the most violent reaction allowed. The empirical Jacobs-Roslund relation and hydrocode analysis are used to predict the explosive response. The Jacobs-Roslund equation is given by       + + = d CT B d A Vc 1 1 ) ( cosθ [1] where Vc is the critical fragment velocity required for a fragment to initiate an explosive charge A is an empirical constant defining the explosive sensitivity d is the critical fragment dimension