The anatomy of a pipe bomb explosion: the effect of explosive filler, container material and ambient temperature on device fragmentation.

Understanding the mechanical properties of different piping material under various conditions is important to predicting the behavior of pipe bombs. In this study, the effect of temperature on pipe bomb containers (i.e., PVC, black steel and galvanized steel) containing low explosive fillers (i.e., Pyrodex and double-base smokeless powder (DBSP)) was investigated. Measurements of fragment velocity and mass were compared for similar devices exploded in the spring (low/high temperature was 8°C/21°C) and winter (low/high temperature range was -9°C/-3°C). The explosions were captured using high speed filmography and fragment velocities were plotted as particle vector velocity maps (PVVM). The time that elapsed between the initiation of the winter devices containing double-base smokeless powder (DBSP) and the failure of their pipe containers ranged from 5.4 to 8.1 ms. The maximum fragment velocities for these devices ranged from 332 to 567 m/s. The steel devices ruptured and exploded more quickly than the PVC device. The steel devices also generated fragments with higher top speeds. Distributions of fragment masses were plotted as histograms and fragment weight distribution maps (FWDM). As expected, steel devices generated fewer, larger fragments than did the PVC devices. Comparison to devices exploded in the spring revealed several pieces of evidence for temperature effects on pipe bombs. For example, the mean fragment velocities for the winter devices were at or above those observed in the spring. The maximum fragment velocity was also higher for the winter steel devices. Although there were no significant differences in mean relative fragment mass, the fragment weight distribution maps (FWDMs) for two winter devices had anomalous slopes, where lower energy filler caused more severe fragmentation than higher energy filler.