Laser-induced Shocks Research Articles

Dynamic behavior of porous graphite under laser-induced shocks

The present study focuses on developing reliable numerical models to describe the mechanical behavior of isotropic polycrystalline graphite subjected to laser shocks. Isotropic graphite of nuclear grade, which was extensively used in Beam Intercepting Devices of particle accelerators, is characterized by a high porosity enabling effective absorption of laser-induced shockwaves. Accurate numerical models which aim at describing shockwaves traveling inside porous graphite must take into account the effect of pores on the structural response of the material. In this work the Fu Chang foam material model was calibrated to capture the behavior of R4550 graphite. Based on the foam material model, hydrodynamic simulations were developed and compared to experimental data from literature and from PHELIX experimental campaign.

Characterization of a carbon fiber composite material for space applications under high strains and stresses: Modeling and validation by experiments

The authors present the characterization of the behavior of a Carbon Fiber Reinforced Polymer (CFRP) under dynamic loading. The study deals with thin shields representative of the shields employed for satellite protection. The process was divided into two steps. First, experiments over a wide scale of strain rates were performed, including planar plate impacts, electron beam shots, and laser-induced shocks. The loads generated by these experiments can be unidirectional or two-dimensional with durations from a tenth of nanoseconds up to several microseconds. Rear surface velocities were recorded by Doppler Laser Interferometry, and damaged target was recovered when possible. Analysis of these results provides data about equation of state, elasto-plasticity behavior, and the damage process. In a second part, those data were used to develop an isotropic numerical model for the CFRP composite. This model is assessed in simulations with the hydrocode HESIONE (CEA) for all the previous experiments. Although slight differences were highlighted for laser-induced shocks, the model provides relevant results for simulating dynamic behavior of the CFRP composite.

Laser-Stimulated Phase Transformations in Thin Layers of SiOx and CNx –Ni

We demonstrate the possibility of laser-assisted phase transformations in thin layers of SiOx from the amorphous nonstoichiometric phase into a nanocomposite layer with Si nanocrystals embedded in the silicon-oxide matrix. The formation of Si nanoparticles (NPs) in SiOx films occurs due to their irradiation with nanosecond pulses of a Nd+3:YAG laser with base wavelength λ = 1.064 μm and the second harmonic λ = 0.532 μm. The nanostructurization of the surface in the form of nanoparticles with mean sizes varying within the range 5–85 nm was detected with the help of atomic force microscopy and optical transmission spectra. The sizes of nanoparticles and their surface distribution depend on the intensity of the laser and its wavelength λ. We determined the mechanism of formation of Si nanocrystals by laser-induced thermal shocks. By analyzing the results of atomic force microscopy, as well as the data of Raman and polarization-modulation spectroscopies, we detected the increase in the amount of single-crystal phase in the film of CNx–Ni nanocomposite with simultaneous increase in the sizes of CNx–Ni nanoparticles caused by the action of laser pulses. The mechanism of laser-stimulated nanocrystallization is proposed. This mechanism is based on the local annealing of the CNx shell as a result of laser heating of the metallic Ni core of CNx–Ni nanoparticles and a more intense sublimation of the amorphous phase of CNx–Ni due to the lower temperature of dissociation of the CNx shell as compared with the Ni core.

Nondestructive prediction of point source pyroshock response spectra based on experimental conditioning of laser-induced shocks

Pyroshock can easily cause failures in electronic and optical components that are sensitive to high-frequency energy. Pyroshock is generated during explosive-based pyrotechnical events, such as the separation of boosters from a space shuttle and the separation of satellites from a space launcher. Therefore, the prediction of high-frequency structural response, particularly the shock response spectrum (SRS), is important for safe operation of pyrotechnical devices. In general, real explosive testing using distributed accelerometers is widely used. This paper proposes a technology to replace the expensive, dangerous, low-repeatability explosive test with a laser-induced shock test based on a laser beam and in-line filter conditioning. This method does not use any special numerical signal processing. Two different experiments based on explosive and laser excitation were performed with a 2-mm thick aluminum plate. The optimum laser-induced shock experimental conditions to predict real pyroshock were investigated while considering the size, energy, and fluence of the laser beam as parameters. The similarity of the SRS of the laser-induced shock to that of the real explosive pyroshock was evaluated based on the mean acceleration difference (MAD, %). The experimentally determined optimal conditions were also applied to four points on the path of a pyroshock propagation. To match the SRS at each point, the laser-induced shock was amplified, for which three different gain concepts are proposed: the initial gain, optimized gain, and constant gain. The proposed technology enables nondestructive pyro SRS prediction by conditioning the laser-induced shock to obtain an SRS with high similarity to the real pyroshock.

Laser shock peening and mechanical shot peening processes applicable for the surface treatment of technical grade ceramics: A review

Laser shock peening and conventional mechanical shot peening are both comparable processes generally applicable to surface treat various metals and alloys. Commercial advantages offered by the laser systems such as flexibility, deep penetration of laser-induced shocks with precise control of the thermal pulses, shorter process times, high speeds, accuracy and aesthetics are attractive in comparison with the mechanical shot peening technique. Laser shock peening in the recent years has proved to be successful with steels, aluminium and titanium surfaces and metallic alloys in general. Nevertheless, minimal research has been conducted on laser shock peening and mechanical shot peening of technical grade ceramics. This article presents an update of the theory and to-date relevant literature within the two subject areas, as well as a comparison and a contrast between the mechanical and laser shock peening techniques. In addition, various gaps in knowledge are identified to propose further research for the development of both the techniques applicable to the surface treatment of technical grade ceramics.

Nucleating bubble clouds with a pair of laser-induced shocks and bubbles

AbstractLaser-induced optical breakdown at two spatial locations in ultrapure water saturated with ambient gas is used to nucleate microscopic bubble clouds with lifetimes of tens of nanoseconds. The liquid is ruptured via the interaction of a pair of laser-induced shocks and bubbles. We find that the acoustically nucleated micro-bubbles appear in a localized region defined by the plane that bisects the pair of foci, where rarefaction waves (reflected from the laser-induced bubbles) merge. We measure the probability for acoustic nucleation as a function of the separation between the foci, and the minimum pressures for each separation are calculated with Euler flow simulations. The simulations show that the liquid is exposed to negative pressures for 3–17 ns. A statistical threshold pressure for cavitation inception (0.5 probability) of $- 20. 1\pm 3. 4~\mathrm{MPa} $ is extracted from the measured probabilities and the calculated minimum pressures.

Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks

A method for efficient laser acceleration of heavy ions by electrostatic shock is investigated using particle-in-cell (PIC) simulation and analytical modeling. When a small number of heavy ions are mixed with light ions, the heavy ions can be accelerated to the same velocity as the light ions so that they gain much higher energy because of their large mass. Accordingly, a sandwich target design with a thin compound ion layer between two light-ion layers and a micro-structured target design are proposed for obtaining monoenergetic heavy-ion beams.

Laser Shock Processing of Al-SiC Composite Coatings

Laser shock processing (LSP) is a technique of surface treatment (similar to shot peening) in which laser-induced mechanical shocks develop compressive stresses in the material. The stresses are of sufficient intensity to modify microstructure and properties of the coatings. In the present study, laser shocks of power density of 5 to 8 GW/cm2 power density, generated by means of a neodymium-glass laser, were used to treat Al + SiC composite coatings deposited by means of a HVOF spraying technique. The laser processed samples were metallographically prepared, and their microstructure was investigated by optical microscope and SEM. The latter was also used to investigate the surface morphology of the laser treated specimens. Finally, the microhardness and oscillating wear resistance of the coatings were tested and compared to data obtained for as-sprayed samples.

Reflectivity and Optical Brightness of Laser-Induced Shocks in Silicon

We report the first simultaneous measurement of the reflectivity and optical emission of a strong (4\char21{}8 Mbar) shock front emerging at a free surface of a solid. Planar shock waves were driven by thermal x rays from a laser-heated cavity. The inferred model-independent brightness temperature of the shock front in silicon turns out to be significantly below the expected Hugoniot temperature. We find that our data cannot be explained within the two-temperature model which assumes instantaneous metallization of silicon in the density jump.

Some applications of laser-induced shocks on the dynamic behavior of materials

High-power pulsed lasers are used widely nowadays as shock generators. They settle as a complementary technique to the conventional shock generators by the high peak pressure and short duration shocks they deliver. They are used to investigate the feasibility of new industrial processes and to get information on the behavior of matter in specific conditions of extremely high strain rate. In this paper, some studies about typical applications of laser shocks are presented: surface densification of porous materials, spallation in ductile and brittle materials, α-ε phase change in iron, and an estimate of the relaxation time for this transition. Laser shock experiments provide additional data on the dynamic behavior of materials at the nanosecond time scale.

Shock waves

The first volume of the Journal of Fluid Mechanics contained nine articles (of 39) on shock waves. Some of these pioneered new branches of fluid mechanics. Others dealt with older problem areas. Surprising is one's realization that important elements of all topics are still of current interest. The subjects treated were shock structure, diffraction, refraction, waves in supersonic and hypersonic flows, large-amplitude acoustic and blast waves, and astrophysical processes. The subsequent addition of work on chemically reactive flows, radiating and laser-induced shocks, the effects of electric and magnetic fields on shock propagation in ionized media and the development of computer-based methods of analysis have greatly broadened the scope of shock wave investigations during the ensuing twenty-five years.The paper traces some of the principal lines of investigation from early motivations to the present state of understanding and application. Motivation is not often consciously expressed in the scientific literature. Usually an external motivation in terms of identifiable needs for better understanding for the solution of practical problems can be identified; though much excellent work must be ascribed to that ubiquitous trait curiosity.The topics covered in this article were chosen as representative of the basic elements of shock wave interactions and effects. They are: shock structure, refraction, diffraction, shocks in liquid helium, and condensation and liquefaction shocks. The paper closes with an assessment of how approximate and computational methods developed for handling complex flow problems fare when applied to some of the basic shock interactions considered here. Most of the emphasis will be on shock waves in gases, for which knowledge of an equation of state has been key to the significant advances made during the last twenty-five years. For liquids and solids, shock waves have been used the other way around; to study state properties.

Laser-induced shocks and their capability to produce fracture

The fracture potential of pulsed laser light is investigated for brittle and ductile fracture. The occurrence of fracture depends on the power density of the beam, the pulse duration, the target thickness, and a number of material parameters, ranging from the plasma state to the solid state of the target material. An essential part of the fracture analysis is the attenuation of the induced shock, which is rapid for short pulses. Using aluminum as a target material it will be shown that fluence levels of 104 J/cm2 are necessary to break targets of centimeter thickness, assuming that the pulse duration is 10−7 sec. It appears that an optimal situation for the achievement of fracture exists, since the peak pressure is inversely proportional to a power of pulse duration, whereas attenuation is proportional to pulse duration.