Insensitive Munitions Thermal Test Instrumentation: Thermocouple Placement, Overpressure Measurement, and Fragment Capture

A CCB burn test rig generates three distinct data streams: thermal history from the munition and its environment, overpressure at defined distances, and fragment projection data. Each requires a different instrumentation approach.

Thermocouple placement is critical for meaningful slow cook-off data. The objective is to characterise the temperature distribution within the munition — specifically whether the explosive fill approaches its initiation temperature before the external casing temperature indicates it. Multiple Type-K thermocouples are positioned at the casing surface, within the fill through an instrumentation port, and in the ambient environment. The data acquisition system samples all channels at 1 Hz during slow cook-off (adequate for the slow thermal evolution) and 100 Hz during the final approach to initiation and the reaction event itself, providing resolution of the initiation transient.

Overpressure measurement uses piezoelectric pressure gauges at defined distances (typically 1m, 2m, 5m, 10m from the munition centre) and at defined azimuth angles. The gauges must survive the test conditions including the pre-initiation fire environment — this typically requires either blast-hardened housings or gauges mounted behind protective screens that are removed immediately before the reaction is expected. Peak overpressure and positive impulse at each gauge location are the primary measurements for reaction violence classification — a Type I detonation produces much higher peak overpressure than a Type IV deflagration at the same distance.

Fragment capture screens — steel plate or witness plate arrays around the test site — intercept and capture fragments projected by the reaction, allowing post-test recovery and characterisation. Fragment count, mass distribution, and projection distance are measured and compared against the IM standard's criteria for acceptable fragment hazard. The capture screens are positioned at the standard exclusion zone radius specified in the applicable test standard.

All instrumentation data, test setup photographs, and post-test damage and fragment assessments form the test report package required for IM qualification submission. The Neometrix CCB Burn Test Rig is designed to generate all required data streams in a single test execution.
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