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A Cook-off test is an experiment to investigate the sensitivity and response of an explosive subtonic to high temperatures. A steel cylindrical Cook-off tube is filled with the explosive material under investigation. During the Cook- off test, the Cook-off tube will be heated up to several hundreds of degrees and, upon explosion, the rate of change of the diameter of the Cook-off tube is monitored to obtain information about the reaction process in the tube. To detect these expansion velocities, TNO has developed a system based on Fiber Optic interferometer. This system includes a Sagnac interferometer and a Mach-Zehnder interferometer. Part of the fiber loop of each interferometer is wrapped around the tube and is indicated as the sensing fiber. The change in the length of the sensing fiber is measured by analyzing the interferometric signals. The phase of the Sagnac interferometric signal is proportional to the rate of change in length of the sensing fiber, while the phase of the Mach- Zehnder interferometric signal is proportional to the absolute length change of the sensing fiber. The simplest method to analyze the interferometric signal is counting the fringes as a function of time. A more dedicated signal-processing scheme is proposed to obtain additional information. This scheme is designed for interferometers with 3 phase shifted output signals. The output signals from both the interferometers were recorded by a Digital Signal Analyzer (DSA). The signal to noise ratio was found to be sufficient. We have carried out several Cook-off experiments with the explosive Comp. B. for different configurations of the Cook-off tube, difference in the reaction process can indeed be observed. One or more oscillations in the expansion velocity of the tube were found at the start of the expansion process. As far as known, the presence of these oscillations has never been reported before. A discrepancy between the results of the two interferometer systems was observed. This is probably caused by the application of two different sensing fibers. A configuration applying a single sensing fiber for both interferometer systems is proposed.
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