A new closed-form expression is derived in this paper for computing the electromagnetic field reflected by two moving parallel interfaces. Next, the interfaces velocities and refractive index between interfaces are derived from measurable frequencies and amplitudes embedded in the reflected electromagnetic field. As an application, the remote analysis of the shock wave phenomenon in solids is reported and shock wave descriptors are estimated. Based on the proposed approach, physical insights on measurement published results are reported. Moreover, new measurement data on shocked polymethyl methacrylate material are presented and discussed.
Radio interferometry techniques are often used to investigate shock and detonation phenomena thanks to the radio‐transparency of high explosives in the gigahertz frequency band. These techniques require the knowledge of the permittivity of studied explosives. Although the permittivity has been thoroughly studied for many materials, very few data are available at high frequencies (>75 GHz) for high explosives. In this paper, we report static measurement data of the permittivity for various reactive materials using the standard line transmission method between 75 GHz and 110 GHz (W frequency Band), and we present dynamic measurement results at 94 GHz obtained from the so‐called detonation wavefront tracking method. It is shown that the measurement results provided by these two methods are in good agreement. As a consequence, this work validates the detonation wavefront tracking method for the dynamic measurement of high explosives permittivity, and shows that the static experimental results are relevant for shock wave propagation analysis from millimeter‐wave measurement techniques.
A multi-channels high resolution dispersive interrogator with at a high sampling rate has been developed to measure shocks pressure levels by Fiber Bragg Gratings (FBGs). Two FBG orientations are compared numerically and experimentally. The first one is along the cylindrical target axis, thus the grating spectrum is "blue shifted". The second orientation is perpendicular to the target axis and the grating spectrum is "red shifted". The interrogator uses a femtosecond laser source to cover the C+L band spectrum. The source repetition rate (100 MHz) fixes the spectra acquisition rate. The wavelengths are basically converted to time using a long telecom fiber. The time-multiplexed spectra are recorded with 400 points by a fast oscilloscope (40 GSa/s). The experimental setup is a Tin plate impact on a PMMA target performed in a 35-mm singlestage gas gun. An impact at 510 m/s generates a pressure level of 1.69 GPa during 5 µs. The performance of the dynamic interrogator and the wavelength shifts in the two FBG configurations are discussed.
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