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Photon Doppler velocimetry (PDV) has become a standard diagnostic in impact and shock experiments, due to its relative flexibility and ease in alignment compared to other optical diagnostics. The conventional approach to fielding PDV in plate-impact experiments is to place probes directly down-range from the projectile and target. In experiments such as these, probes are almost certainly destroyed, resulting in significant replacement cost particularly when multiple probes are used. To overcome these shortcomings, a free-space optical relay system has been developed to focus multiple channels of PDV from outside an experimental tank onto a target of reduced size than the probe array. This system has been successfully tested on plate-impact experiments measuring a Hugoniot state of polycrystal tin, and the peak elastic stress and spall strength in 6082 aluminium alloy, in both cases adding information about the shock tilt to the experimental data generated. [ABSTRACT FROM AUTHOR]

The Photon Doppler Velocimetry (PDV) technique is used to capture simultaneously propagating elastic waves of longitudinal and shear nature in a Tension-Torsion Hopkinson Bar (TTHB) apparatus. The system uses a pair of probes per velocity measurement, which were taken on the opposite sides of the TTHB bar with a laser irradiated spot size of ~35 μm. The collected data were compared to the measurements obtained from the conventional strain gauge technique, and were in good agreement. The PDV method was effective in separating longitudinal and rotation signals even when they were superimposed on each other at the gauge location. This approach is also shown to be effective in detecting and accounting for the presence of bending waves in the TTHB bars. [ABSTRACT FROM AUTHOR]

Pyrometry is an established technique used in high strain rate experiments (such as plate impact experiments) whereby the temperature of a sample is determined from the measurement of collected thermal radiation. By applying Planck's Law, radiometry data can be used to determine temperature. A significant source of uncertainty in applying pyrometry to high strain rate experiments is due to possible changes in emissivity after a sample has been shocked; thus it is desirable to determine emissivity during the experiment. Kirchhoff's Law, which states emissivity is the complement of reflectivity, can be a pplied to determine emissivity in high strain rate experiments. Ideally the sample's reflectivity would be measured at the same wavelength and on the same experiment as the pyrometry measurement. A diagnostic has been developed which produces a modulated reflectivity signal that is insensitive to tilt and surface finish. Following characterisation of the system in the laboratory, dynamic tests were conducted. During the dynamic tests, the reflectivity signal was multiplexed onto the radiance signal allowing simultaneous measurement of radiance and reflectivity. A description of the diagnostic, results from laboratory t esting and r esults and analysis from dynamic experiments are presented. [ABSTRACT FROM AUTHOR]

Calcite and sapphire were shock compressed along the <1 0 1 0> direction (a axis) in a plate impact configuration. Polarimetery and Photonic Doppler Velocimetery (PDV) were used to measure the change in birefringence with particle velocity in the shock direction. Results for sapphire agree well with linear photoelastic theory and current literature showing a linear relationship between birefringence and particle velocity up to 310 m s-1. A maximum change in birefringence of 5% was observed. Calcite however showed anomolous behaviour with no detectable change in birefringence (less than 0.1%) over the range of particle velocities studied (up to 75 m s-1). [ABSTRACT FROM AUTHOR]

We present a study on the dynamic fracture and fragmentation of Ti-6Al-4V cylinders at initial temperatures ranging from approximately 150 K to 750 K. Cylinders with an inner diameter of 50 mm and a wall thickness of 4 mm were driven into uniform axially-symmetric expansion at radial strain rates of 104 s-1 using the ogive-insert gas gun method. Diagnostics consisted of simultaneous high speed imaging and multiple points of laser velocimetry (PDV) along the length of the sample. The imaging and PDV provided a record of the expansion process, giving expansion velocity and the failure strain. Recovered fragments were examined with optical and scanning electron microscopy and electron backscatter diffraction techniques to determine the fracture mechanisms for each initial temperature. The failure strain (radial strain at first fracture) was observed to increase with temperature over the range tested, from 7:4 ± 5:2 percent at 158 K to 24:1 ± 2:4 percent at 724 K. In experiments from 158 K up to 609 K the fracture mechanism was found to be ductile tearing under mode II loading along the planes of maximum shear at 45° to the radius. At an initial cylinder temperature of 724 K the fracture mechanism transitioned to void nucleation and coalescence along adiabatic shear bands, again forming at 45° to the radial direction. The fragmentation toughness Kf was observed to also increase with temperature until the 724 K shot where there was a marked reduction, suggesting the formation of shear bands at high temperatures reduced the energy required to form fragments. The average value of Kf was 101 ± 13 MPa m1/2. [ABSTRACT FROM AUTHOR]

Powders are well known for their dispersive properties, which derive from the many dissipative processes that occur during densification. While numerous studies have been devoted to understand these processes over a wide range of initial densities, the influence of particle morphology has been for the large part overlooked. In this paper, we discuss a new research campaign at the Institute of Shock Physics, to systematically investigate the role of starting configuration on the dynamic densification of metal powders. Multi-target gun loading experiments have been performed on both stainless steel and copper powders of equiaxed- and fiber-shaped morphology. Frequency-shifted PDV was employed to measure the structure and velocity of the dynamic densification wave, to yield the crush strength of the various powders. We find that while the crush strength for the stainless steel powders is reasonably described by a modified Wu-Jing model, this model underpredicts the densification stress for the copper powder. [ABSTRACT FROM AUTHOR]

Shock-induced intermetallic reactions have previously been shown to occur on a nanosecond timescale, with the role of mass mixing processes driven by inter-facial shear at material interfaces suggested as a possible explanation. Experimental setups allowing the role of friction on these mechanisms to be considered are presented. Simulations show that the use of an impedance mismatch, shock driven experimental setup should allow sliding velocities of 50 - 100 m s-1 to be attained with mechanical mixing the main reaction mechanism. A second setup utilising the oblique impact of small spherical projectiles upon a target plate is predicted to enable much higher sliding velocities of up to 2 Km s-1 to be attained with thermally driven diffusion reaction mechanisms becoming dominant. [ABSTRACT FROM AUTHOR]

A combination of parallel-plate impact experiments utilizing stress measurements and mesoscale numerical simulations are used to investigate the effect of particle morphology on the mechanical and chemical response of Ni+Al powder mixtures. The instrumented gas-gun impact experiments were performed at pressures up to 6 GPa. Based on measured shock velocity increases and shock compressibility changes consistent with the Ballotechnic model, the flake-based powder mixture was found to exhibit shock-induced reaction. The particle-level mechanistic details of deformation, mass-flow, and mixing, were explored through discrete particle continuum simulations, validated against the experimental results. The micron-scale spherical and flake mixtures were found to display widely varying configurational changes at several length scales, which give insight into why the flake-Ni morphology is more susceptible to shock-induced reactions under the imposed conditions. [ABSTRACT FROM AUTHOR]