A metallic tin plate with a given surface finish of wavelength λ ≃ 60 μm and amplitude h ≃ 8 μm is explosively driven by an electro-detonator with a shock-induced breakout pressure PSB = 28 GPa (unsupported). The resulting dynamic fragmentation process, the so-called “micro-jetting,” is the creation of high-speed jets of matter moving faster than the bulk metallic surface. Hydrodynamic instabilities result in the fragmentation of these jets into micron-sized metallic particles constituting a self-expanding cloud of droplets, whose areal mass, velocity, and particle size distributions are unknown. Lithium-niobate-piezoelectric sensor measured areal mass and Photonic Doppler Velocimetry (PDV) was used to get a time-velocity spectrogram of the cloud. In this article, we present both experimental mass and velocity results and we relate the integrated areal mass of the cloud to the PDV power spectral density with the assumption of a power law particle size distribution. Two models of PDV spectrograms are described. The first one accounts for the speckle statistics of the spectrum and the second one describes an average spectrum for which speckle fluctuations are removed. Finally, the second model is used for a maximum likelihood estimation of the cloud’s parameters from PDV data. The estimated integrated areal mass from PDV data is found to agree well with piezoelectric results. We highlight the relevance of analyzing PDV data and correlating different diagnostics to retrieve the physical properties of ejecta particles.
Laser-induced pressure generation and surface modifications induced by laser-shock processing were investigated with the use of different laser conditions including a large range of pulse durations and wavelengths. With the use of several lasers sources, original results could be pointed out concerning the precise influence of laser parameters on the shock generation in water-confined plasma regime. The beneficial influence of short pulse durations (1 to 10 ns) and the deleterious effect of short wavelengths (below 0.532 micrometer) were demonstrated with the use of a VISAR Doppler velocimetry system. Lastly, beneficial effects of LSP were shown on many surface applications such as fatigue and fatigue contact resistance, pitting corrosion and stress corrosion cracking behaviors.
To face High Velocity Impacts, the aerospace industry has a constant need for innovative materials that can be used as debris shielding components. Bulk Metallic Glasses (BMG) revealed interesting mechanical properties in case of static and quasi-static loading conditions: high elasticity, high toughness, low density and high fracture threshold. The department of Mechanics and Glass of the Institute of Physics Rennes conducted laser shock experiments (ELFIE facility) to study compositional effects on the behavior of a ternary ZrCuAl BMG under high strain rate, up-to the fragmentation process. Ejecta velocities were measured by Photonic Doppler Velocimetry and debris morphologies were observed by shadowgraphy. Scanning Electron Microscopy observations revealed the influence of the composition on fracture surfaces morphologies. A dependency of spalling process has been correlated to Poisson's ratio (i.e. zirconium content).
Debris ejection upon shock breakout at a rough surface is a key issue for many applications, including pyrotechnics and inertial confinement fusion. For a few years, we have used laser driven shocks to investigate microjetting in metallic samples with calibrated grooves in their free surface. Fast transverse optical shadowgraphy, time-resolved measurements of both planar surface and jet tip velocities, and post-shock analysis of recovered material have provided data over ranges of small spatial and temporal scales, short loading pulses (ns-order) and extremely high strain rates. The new experiment presented here involves two laser beams in a pump-probe configuration. Picosecond laser irradiation of a thin copper wire generates x-rays which are used to radiograph the microjets expanding from single grooves in tin and copper samples shock-loaded by a longer, nanosecond laser pulse. Such ultrashort radiography can be used to infer the density gradients along the jets as well as inside the samples deep beneath the grooves. Thus, combining this x-ray probe with the other experimental techniques mentioned above provides a more complete insight into the physics of microjetting at very high loading rates and the ballistic properties of the resulting ejecta.
This paper investigates the different physical processes involved during laser-delayed double shock-wave generation in water-confined geometry. With this technique, two laser pulses, separated by a Δt duration, irradiate a target immersed in water at an intensity of a few GW/cm2 and form a high pressure plasma which results in a double shock-wave generation. This 2 pulses configuration is currently being investigated as an attractive method for improving the LASer Adhesion Test (LASAT) [L. M. Barker and R. E. Hollenbach, J. Appl. Phys. 43, 4669–4674 (1972)] technique by adapting the time delay Δt to the position of interfaces. The LASAT technique is a noncontact adhesion test allowing to generate a high-level tensile stress near interfaces with the use of laser-driven shock wave. The generation of two delayed high-intensity shock waves by laser plasma in the water-confinement regime has been investigated at 10ns@532 nm with the new Nd:YAG laser GAIA from Thales Laser company in the new facility HEPHAISTOS. For each incident Gaussian laser impulsion, the characterization of the high-amplitude laser-plasma-generated shock wave and its propagation through the target has been performed using a velocity interferometer system for any reflector [L. Berthe et al., “State-of-the-art laser adhesion test (LASAT),” Nondestr. Test. Eval. 26(3–4), 303–317 (2011)]. The new laser facility allows us a nanosecond-control of the time delay between the two laser pulses and a precise control of each laser maximum-intensity. Therefore, the influence of the first laser-induced plasma, on the second shock-wave generation has been studied by modifying different parameters such as the delay Δt and the intensity I1 and 12 of each pulse and different aluminum plate thicknesses from 0.2 to 1.5 mm. Preliminary tests show that the maximum pressure level of the second generated shock wave is sensitive to the time delay between the two impulsions and influenced by the plasma generated by the first laser pulse.
Coating characteristics are highly dependent on substrate preparation and spray parameters. Hence, the surface must be adapted mechanically and physicochemically to favor coating–substrate adhesion. Conventional surface preparation methods such as grit blasting are limited by surface embrittlement and produce large plastic deformations throughout the surface, resulting in compressive stress and potential cracks. Among all such methods, laser patterning is suitable to prepare the surface of sensitive materials. No embedded grit particles can be observed, and high-quality coatings are obtained. Finally, laser surface patterning adapts the impacted surface, creating large anchoring area. Optimized surface topographies can then be elaborated according to the material as well as the application. The objective of this study is to compare the adhesive bond strength between two surface preparation methods, namely grit blasting and laser surface patterning, for two material couples used in aerospace applications: 2017 aluminum alloy and AISI 304L stainless steel coated with NiAl and YSZ, respectively. Laser patterning significantly increases adherence values for similar contact area due to mixed-mode (cohesive and adhesive) failure. The coating is locked in the pattern.
