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Fiber-optic sensors in explosion and detonation experiments
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For the detection of shock waves and high-speed phenomena in ex- plosive materials, fiber-based systems are safer and cheaper than electronics. At TNO, considerable effort has been invested into explosion and detonation experiments for energetic materials research. These investigations are required for both the development of new materials and the control optimization of existing materi- als. Typical measurements include detonation speed, response to high temperature and impact, and shock wave propagation in materials. It is obvious that the measurement systems must be designed with great care to minimize risk. They also need to be protected from the test environment. Furthermore, high- speed measurement is also required to record experimental phe- nomena. TNO has developed a number of measurement systems based on fiber optic technology. Relative to conventional elec- trical measurement technology, they offer significant advantages in terms of safety and/or cost. Fiber optic (FO) sensors are currently generating significant interest for special applications. This is because they basically have no electrical components at the measurement location and can be used without risk of interaction with the explosive mate- rials under investigation. Furthermore, since an optical fiber has a typical diameter of 0.25mm, measurements can also be done at locations that are not accessible to other type of sensors. An example is the measurement of shock waves in explosives using a fiber Bragg grating (FBG). The loss in an optical fiber is gener- ally extremely low. This allows placement of the detection unit and its electronics at a safe distance from the experiment without requiring expensive protection and shielding. These advantages allows these sensors to outperform their conventional electrical counterparts. We now describe some of our FO sensor systems developed for explosion or detonation experiments.
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... However, the robustness of the concept means that the results are reliable and data acquisition is simple and easy to analyse. Fibre optic systems based on the same discrete TOA concept include fibre-coupled light detection [21,22], drilled fibre-optic probes [21,23] and the aluminium coated time of arrival diagnostic (TOAD) [24]. ...
Throughout this project, a number of fibre optic systems have been developed in order to measure detonation velocity — the propagation speed of a shockwave through an explosive medium. The aim of this work has been to utilise the small size and high speed infrastructure of fibre optic systems to develop precise, high spatial resolution, embedded fibre optic velocity probes.
The three key developments, demonstrated through a combination of simulations, laboratory testing and explosive trials are: an optimised chirped fibre Bragg grating (CFBG) measurement system; a brand new uniform fibre Bragg grating (UFBG) measurement system, and; a simplified, low-cost active fibre measurement system.
It has been discovered that CFBG velocity measurements are prone to innate nonlinearities due to Fourier limitations on the grating bandwidth. These effects can be mitigated if the CFBGs are designed with a high chirp-rate and a low reflectivity. This is shown using transfer-matrix simulations and in explosive tests — where the longest continuous CFBG detonation velocity measurement (24 cm) is also demonstrated.
Explosive test results from a new UFBG velocity probe show a maximum noise level that is an order of magnitude lower than similar CFBG tests — demonstrating a noise-limited spatial uncertainty below 10 m.
Tests on Er and Er/Yb doped fibres demonstrate the potential for these fibres to be used as strain-insensitive detonation velocity probes. This is put into practice by implementing the fibres in a helical geometry — amplifying the spatial precision tenfold and resulting in a 2 mm uncertainty over a 100 mm measurement range.
... In this method optical fiber is used which is capable of detecting and transmitting a light signal accompanying the detonation wave front[1] [5]. This method is point to point to type wherein the first cable signals the start whereas the second cable placed at a known fixed distance stops the timing clock. ...
The development of insensitive energetic materials with stability, high performance, reliability, safety, and low toxicity requires measurement and prediction of thermophysical and thermochemical properties. The measurement and estimation of properties such as the enthalpy of formation, density, detonation velocity, detonation pressure and sensitivity are used to screen potential energetic candidates. Experimental data are also needed to test prediction methods and molecular models. This chapter outlines experimental methods to measure some of the important properties. Different methods of determining a property and the theory associated have been outlined.
Velocity of Detonation (VOD) is an important measure characteristics parameter of explosive material. The performance of explosive invariably depends on the velocity of detonation. The power/ strength of explosive to cause fragmentation of the solid structure determine the efficiency of the Blast performed. It is an established fact that measuring velocity of detonation gives a good indication of the strength and hence the performance of the explosive. In this survey various VOD measurement techniques such as electric, nonelectric and fibre optic have been discussed. To aid the discussion some commercially available VOD meter comparison are also presented. After review of the existing units available commercially and study of their respective merits and demerits, feature of an ideal system is proposed.
Detonation velocity is an important property to consider when rating an explosive. It is an established fact that measuring velocity of detonation gives a good indication of the strength and hence the performance of the explosive. This paper presents and discusses the design and implementation of continuous wire discrete resistance VOD measuring technique (sensor)for bulk explosives. In this technique step change in resistance is preferred to increase the system reliability as well as it reduces the system complexity. Sensor used in discrete point's method is based on resistance change in the region of several kilo ohms improving the noise immunity.
A new technique for the dynamic measurement of detonation pressures by use of a Fiber Bragg Grating sensor is reported. A variation in pressure changes the wavelength of the FBG reflected light. On a detonation the shock wave passes the explosive with a velocity of ca 7 km/s and the pressure builds up to ca. 10 GPa within 100 ns. Hence huge differences in pressure exist in a small area. To prevent measuring the average pressure in a few mm, short FBG with a length of about 0.5mm is developed by Hongkong PolyU. The FBG is inserted in the explosive. With a Fiber Optic Probe the exact distance between the shock wave and the FBG can be determined simultaneously for comparison. The light reflected by the FBG is sent to a fiber optic Mach-Zehnder interferometer with a 3x3 beam combiner. Pressure induced wavelength shift will cause a phase change of the interferometric signal. Using the 3 output signal of the interferometer, the phase can be calculated. Interaction between the FBG and the ionization light generated by the shockwave is also demonstrated.
For the development of an Exploding Foil Initiator for Insensitive Munitions applications the following topics are of interest: the electrical circuit, the exploding foil, the velocity of the flyer, the driver explosive, the secondary flyer and the acceptor explosive. Several parameters of the EFI have influences on the velocity of the flyer. To investigate these parameters a Fabry-Perot Velocity Interferometer System (F-PVIS) has been used. The light to and from the flyer is transported by a multimode fibre terminated with a GRIN-lens. By this method the velocity of very tiny objects (0.1 mm), can be measured. The velocity of flyer can be recorded with nanosecond resolution, depending on the Fabry-Perot etalon and the streak camera. With this equipment the influence of the dimensions of the exploding foil and the flyer on the velocity and the acceleration of the flyer are investigated. Also the integrity of the flyer during flight can be analyzed. To characterize the explosive material, to be used as driver explosive in EFI's, the initiation behaviour of the explosive has been investigated by taking pictures of the explosion with a high speed framing and streak camera. From these pictures the initiation distance and the detonation behaviour of the explosive has been analyzed. Normally, the driver explosive initiates the acceptor explosive (booster) by direct contact. This booster explosive is embedded in the main charge of the munitions. The combination of initiator, booster explosive and main charge explosive is called the detonation train. In this research the possibility of initiation of the booster by an intermediate flyer is investigated. This secondary flyer can be made of different materials, like aluminium, steel and polyester with different sizes. With the aid of the F-PVIS the acceleration of the secondary flyer is investigated. This reveals the influence of the thickness and density of the flyer on the acceleration and final velocity. Under certain circumstances the flyer breaks up in several parts and several velocities at the same time have been recorded. Several flyer materials and dimensions exist that are able to initiate very insensitive explosives like TATB.
A Cook-off test is an experiment to investigate the sensitivity of munition to high temperatures. A Cook-off tube is filled with the explosive material under investigation. During the Cook-off test, the tube will be heated up to several hundreds of degrees and, upon explosion, the velocity of the change in the diameter of the Cook-off tube will be monitored to obtain information of the reaction process in the tube. A conventional strain gauge only measures the local strain of the tube. Therefore, a conventional strain gauge is not suitable for measuring the diameter change of the Cook-off tube. TNO has developed a system based on Fiber Optic interferometry. This system includes two different types of Fiber Optic interferometer: a Sagnac interferometer and a Mach-Zehnder interferometer. A part of each of the interferometers is wound 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.
Fiber Optic sensors are found to be very suitable for explosion and shock wave measurements because of the immunity to Electromagnetic Interference (EMI). In the past few years, TNO has developed a number of sensors systems for explosion and shock wave measurements in which the optical fiber is a vital component. This paper presents an overview of these systems. The basic design of all these systems, the experimental results and some comparisons with other techniques are presented in this paper.
Measurement techniques in cook-off research at the TNO Prins Maurits Laboratory
- G Scholtes
G. Scholtes, Measurement techniques in cook-off research at the TNO Prins Maurits
Laboratory, Proc. 32nd Int. Ann. Conf. ICT, 2001.