Optimization of detonation velocity measurements using a chirped fiber Bragg grating

Abstract

Dynamic measurements of detonation velocity profiles are performed using long Chirped Fiber Bragg Gratings (CFBGs). Such thin probes, with a diameter of typically 150 µm, are inserted directly into a high explosive sample or simply positioned laterally. During the detonation, the width of the reflected optical spectrum is continuously reduced by the propagation of the wave-front, which physically shortens the CFBG. The reflected optical intensity delivers a ramp down signal type, which is directly related to the detonation velocity profile. Experimental detonation velocity measurements were performed on the side of three different high explosives (TNT, B2238 and V401) in a bare cylindrical stick configuration (diameter: 2 inches, height: 10 inches). The detonation velocity range covered was 6800 to 9000 m/s. The extraction of the detonation velocity profiles requires a careful calibration of the system and of the CFBG used. A calibration procedure was developed, with the support of optical simulations, to cancel out the optical spectrum distortions from the different optical components and to determine the wavelength-position transfer function of the CFBG in a reproducible way. The 40-mm long CFBGs were positioned within the second half of the three high explosive cylinders. The excellent linearity of the computed position-time diagram confirms that the detonation was established for the three high explosives. The fitted slopes of the position-time diagram give detonation velocity values which are in very good agreement with the classical measurements obtained from discrete electrical shorting pins.

Ultrafast Fiber Bragg Grating Interrogation for Sensing in Detonation and Shock Wave Experiments

“…In addition to our group, this approach has also been adopted by Barbarin et al in France with excellent results on detonation propagation measurements on several types of high explosives [3]. This approach is illustrated in Figure 1a.…”

Shock to Detonation Transition of Plastic Bonded Aluminized Explosives

“…The distance-time diagram is recorded thanks to piezoelectric pins whose coordinates were determined in the wedge system of reference, by means of a coordinate measuring machine. Fiber Bragg gratings ( [10,11]) and a radio-interferometer ( [12,13]) are also used to register the x-t diagram. The advantage of the radio-interferometer and the fiber Bragg gratings is that the history of the shock speed is continuously recorded.…”

“…Each phase follows the governing equations of mass (5), momentum (6) and energy balance (7) plus the closure relations (8) to (10). 1 ¼…”

“…The notations u k , h k , 1 k , p k , E k , a k and l k are respectively the velocity, the enthalpy, the density, the pressure, the total energy, the volume fraction and the mass fraction of the phase k while 1 is the density of the mixture and M k , G k and V k are the mass, momentum and energy debit of phase k. Actually, those debits should respect the constitutive relation (10).…”

Development of a Shock and Detonation Velocity Measurement System Using Chirped Fiber Bragg Gratings

“…Early on, the calibration, i.e., the amplitude of the CFBG reflecting signal through the complete measurement system as a function of its length, was carried out in numerous steps by cutting the CFBG with a laser [3,7] or by polishing it [9,12]. Another method, non-destructive, was to characterize the linearity of the chirp as a function of the position using a local heating technique thanks to a small tip touching the CFBG [4,7,9].…”