Application of a new type of laser doppler velocimeter
Photonic Doppler velocimetry had been developed before 2005 in the USA. Since that time PDV has gained a lot of attention and its usage has spread within several countries including, China, France, Russia,United Kingdom, and Czech Republic. The photonic Doppler velocimeter (PDV) allows measuring velocity profiles (velocity-time dependencies) of moving objects with high time resolution of up to 5 ns and accuracy of the measured velocity of about 0.1-0.5 %.
In recent years various uses of PDV have been tested and evaluated at the Institute of Energetic Materials,University of Pardubice. Three different PDV instruments have been employed in testing of the PDV application possibilities: single channel prototype PDV, four-channel custom made PDV and at the end,commercial four-channel Optimex-PDV (OZM Research). The voltage signals were recorded using a high bandwidth oscilloscope. The oscilloscope records were analyzed using short-time Fourier transform (STFT) with a Hamming window. The use of PDV for various applications is reviewed in this contribution.
Flyer plate acceleration by high explosives
One way to obtain detonation zone parameters is a measurement of initial velocity of explosively accelerated thin metal disc. The velocity steps caused by shock reverberations in the disc can also be used to determine isentropic expansion path of the detonation products which is an important information for numerical modelling of blast loading processes. The limiting (“coast”) velocity at the rear part of the profile corresponds to the energy transferred to the flyer from the detonation products – the acceleration ability of the explosive. The characteristic Gurney velocity can also be calculated.
Flyer plate acceleration by high explosives
One way to obtain detonation zone parameters is a measurement of initial velocity of explosively accelerated thin metal disc. The velocity steps caused by shock reverberations in the disc can also be used to determine isentropic expansion path of the detonation products which is an important information for numerical modelling of blast loading processes. The limiting (“coast”) velocity at the rear part of the profile corresponds to the energy transferred to the flyer from the detonation products – the acceleration ability of the explosive. The characteristic Gurney velocity can also be calculated.
Cylinder expansion test
Cylinder expansion test is a classical procedure which has been used since 60’ for determination of metal acceleration ability of explosives. An explosive charge is confined in a copper tube and expansion of the explosively accelerated tube walls is recorded in time. The PDV instrumentation is superior compared to the outdated methods of wall expansion tracking by electric contact pins or streak cameras in view of accuracy, resolution and simplicity of operation. Characteristic Gurney velocity can be obtained from this experiment as well as the JWL equation of state constants of the detonation products.
Falling hammer impact
The falling hammer behaviour was observed at the moment of impact on a high explosive sample. PDV probe recorded velocity of the hammer directly before the moment of impact on the sample which exhibits as a short velocity drop to a zero value. Various types of reflected hammer velocity profiles were obtained for different explosives and depending on type of the explosive and mechanism of its initiation
Blasting chamber vibrations
A large blasting chamber was loaded with high explosive charge placed in its center. The PDV allowed to measure velocity of the chamber wall as it was accelerated by shock waves generated by the explosion.Ringing of the wall continued until the end of recording time window. Displacement was obtained by integrating the velocity profile for the pair of peaks with the highest amplitude. The maximum displacement of agreed well with preliminary numerical model predictions
Spalling of blast loaded concrete slab
A reinforced concrete slab subjected to detonation of a high explosive charge placed above it to simulate a terrorist attack. A large spall fracture occurred on the soffit of the slab and the spalled material was accelerated towards the PDV probe which was able to capture the spall movement including its early stages. The resulting velocity profile provides input data for numerical modelling of blast response of the material
Cladder plate tracking in explosion welding
Explosion welding allows to weld practically any combination of metallic materials with excellent joint quality. However, parameters of the explosive and spatial relations of all the parts must be adjusted correctly. The probe aimed against the cladder plate allows to determine the whole velocity profile of the plate including its terminal velocity at which the impact with the base plate occurs. The PDV proved to be applicable even in a full scale open field explosion welding
Impedance window for particle velocity measurement
The method of choice for determination of particle velocity and therefore pressure profiles in the detonation waves is the impedance window method. A layer of transparent window material such as plexiglass with a thin aluminium mirror layer at the interface is attached to the explosive charge and the interface velocity is recorded by the PDV. The particle velocity profile of the interface may subsequently be converted to the pressure profile using impedance matching technique and the Chapman-Jouguet (CJ) detonation pressure of the explosive can be determined.
