When the Barrel Fails - A Simulation Study on Natural Fragmentation and Safety Evaluation of Arena Tests

Sammanfattning

Safety is a major concern when testing new weapon systems or prototypes and the fragment danger area poses the highest risk for arena tests. The fragment danger area is described by the K-value, which is the area inside which fragments impact around a detonation. The K-value is empirically determined based on arena-tests which involves detonating the system ought to be tested, then evaluating the fragmentation using witness-plates at incremental distances from the detonation. This is both time-consuming and costly, instead, using simulations to evaluate the fragment impact coordinates would be both cost and time effective. Further aiding safety evaluation, a kinetic energy heatmap would provide a useful tool for incorporating different levels of protection. To accurately describe fragment ballistics, three main values are needed, fragment mass, projected area and initial velocity. Hence, it's essential to properly describe the natural fragmentation caused by explosive loading. This study compared hydro-codes, Impetus FEM, Impetus E2S and Abstrao SPH to evaluate the subsequent ability to describe natural fragmentation. Furthermore, the influence of a fracture energy parameter 𝐺I and an initial distribution of defects 𝐷0 was evaluated. The final material model was used to simulate a barrel detonation failure, after which the ballistic trajectory and subsequent impacts were analyzed. The resulting simulations showed a clear dependency on the 𝐺I and 𝐷0 parameters, where the GI parameter correlated with the ease of crack propagation and 𝐷0 was linked to the number of crack initiation points, allowing for control over the fragment size distribution. Impetus FEM and E2S could both describe the natural fragmentation well when using the 𝐺I and 𝐷0 parameters. The Abstrao SPH solver has not incorporated the 𝐺I parameter into it’s solver, furthermore the solver struggle with too large fragments, a phenomenon which was also seen in Impetus FEM and E2S if no 𝐺I was applied. Impetus FEM solver was used for the final barrel detonations in which a steel and a composite barrel was evaluated. The fragment files were used to calculate the ballistics for each individual fragment which was then plotted together with the K-value. Supplementing the fragment impact plot, a kinetic energy level heatmap was constructed to see the change in kinetic energy compared to energies required for different levels of injury. The results show that hydro-codes can be used to describe natural fragmentation and that the fragmentation can be used to create tools for safety evaluation of arena tests by plotting fragment impacts and kinetic energy heat maps.