Laurent Berthe

Shock waves from a water-confined laser-generated plasma

Generation of a high amplitude shock wave by laser plasma in a water confinement regime has been investigated for an incident 25–30 ns/40 J/λ=1.064 μm pulsed laser beam. Experimental measurements of temporal and spatial profiles of induced shock waves for this regime of laser shock processing of materials were performed using a velocimetry interferometer system for any reflector system. Above a 10 GW/cm2 laser intensity threshold, a saturation of the peak pressure is shown to occur while the pressure pulse duration is reduced by parasitic plasma occurring in the confining water. The observation of the interaction zone with a fast camera system shows that this breakdown plasma, which mainly occurs at the very surface of the water rather than within the water volume, limits the efficiency of the process. This plasma absorbs the incident laser energy, and the power density reaching the target gradually decreases with increasing power densities while the shock-wave duration is correspondingly reduced. Both pr...

Surface modifications induced in 316L steel by laser peening and shot-peening. Influence on pitting corrosion resistance

The influence of laser peening (LP) on the electrochemical behavior of AISI type 316L stainless steel in a saline environment was evaluated. Surface modifications were investigated as they might have beneficial effects on the corrosion behaviour. Low residual stress and work hardening levels were found, when compared with a conventional shot-peening (SP) treatment, mainly because of the absence of martensite transformation in the case of LP. Surface changes were accompanied by small roughening effects and a global preservation of the surface chemistry after treatment. Therefore, electrochemical tests performed on samples after LP and SP treatments showed increases in rest potentials, reductions of passive current densities and anodic shifts of the pitting potentials evidenced by a stochastic approach of pitting. The better pitting resistance was observed after LP treatment, which seems to reflect a reduction or an elimination of active sites for pitting at lower potentials. Even though the deleterious surface state of shot peened surfaces possibly counterbalances the beneficial influence of residual stresses, a beneficial influence of mechanical surface treatments has been demonstrated regarding the localized corrosion properties.

Physics and applications of laser-shock processing

The first part of this article presents a review of the main process parameters controlling pressure generation in a confined mode. The effect of laser intensity, target material, laser pulse duration, and laser wavelength are, therefore, discussed. An optimized process can then be defined. The second part of this article deals with the surface modifications induced by laser-shock processing. The generation of residual compressive stresses is then highlighted. Finally, in the third part, the interest of laser-shock processing is discussed for several typical applications. A conclusion will present the future trends of this technique.

Wavelength dependent of laser shock-wave generation in the water-confinement regime

The generation of high-intensity shock waves by laser plasma in the water-confinement regime has been investigated at 1.064, 0.532, and 0.355 μm laser wavelengths. Experimental characterizations of pressures induced by laser plasma have been performed with a velocimetry interferometer for any reflector. For each incident laser wavelength, above a laser power density threshold, maximum pressure levels saturate and the pressure durations are reduced due to parasitic plasma occurring in water. However, it is shown that this threshold is lower at the 0.532 and 0.355 μm wavelengths than at the 1.064 μm wavelength. The generation of the parasitic plasma in water is easier with a short wavelength because it would be dominated by multiphotoionization mechanisms. Below the saturation pressure threshold, the pressure levels are significantly higher at the 0.532 and 0.355 μm wavelengths than at the 1.064 μm wavelength. Unlike the detrimental effect of short laser wavelengths on water breakdown plasma, the confined l...

Laser-shock processing of aluminium-coated 55C1 steel in water-confinement regime, characterization and application to high-cycle fatigue behaviour

55C1 steel was irradiated with a high-power neodymium-glass laser with application to induce plastic shock waves within targets, through the expansion of a laser-induced surface plasma. Laser-shock processing experiments were conduced in the plasma-confined regime with water to increase the laser-induced peak stresses. Physical, mechanical and processings aspects were reviewed, such as the characterization of stress waves in coated steels with a VISAR velocimeter system, and the mechanical changes induced in 55Cl in terms of compressive residual stresses or work-hardening levels. With the use of convenient protective coatings, some 7-8 GPa peak stress levels could be achieved which authorized the generation of high compressive residual stress levels (nearly 80% of the compressive yield strength), but preserved the surface integrity from detrimental roughening. Surface modifications performed under different shock conditions were shown to display some 30% increase on the bending fatigue limits of 55C1 at R=0.1.

Laser shock processing of materials, physical processes involved and examples of applications

Laser shock processing (LSP) consists of irradiating a metallic target with a short (about 20 ns) and intense (>1013W m−2) laser light in order to generate, through a high pressure surface plasma (>1 GPa), a plastic deformation and a surface strengthening within materials. This paper initially reviews the physical processes involved in the analytical modeling of the generation pressure mechanism in a confined plasma regime. Limiting factors such as the dielectric breakdown in the confining medium are also discussed together with current research directions aimed at improving the laser—material coupling such as using short rise time pulses instead of Gaussian ones or shorter wavelengths than the traditional λ = 1.06 μm. Surface mechanical effects are also theoretically and experimentally presented. They consist mainly of compressive residual stresses generated in the first 1–2 mm of depth that are the key to enhanced mechanical properties. The application of LSP to two new areas is presented. These areas a...

Experimental study of laser-driven shock waves in stainless steels

Two industrial stainless steels were investigated under laser-shock loading in plasma confined regime with water at nearly 10 GW/cm2 irradiance, driving to stress loadings close to 7 GPa—20 ns at the surface of the targets. A velocimetry interferometer system for any reflector was used to measure shock wave decay and estimate, through the determination of elastic precursors, the dynamic yield strengths at strain rates approaching 106 s−1. Some 50%–100% increases could be found between dynamic yield strengths and static plastic flow limits at 10−3 s−1. Different behaviors were noticed: on AISI 316L Austenitic steel, elastic precursors were shown to have constant values, whatever the propagation depths, whereas on a Martensitic X12CrNiMo12-2-2 steel, precursor attenuations in depth were evidenced. Concerning the application of laser-shock waves to reinforce surfaces, the determination of Hugoniot elastic limits was shown to be useful to predict the plastified depths and the general evolution of plastic defo...

Spallation generated by femtosecond laser driven shocks in thin metallic targets

Spallation induced by a laser driven shock has been studied for two decades on time scales of nanosecond order. The evolution of laser technologies now opens access to sources whose pulse duration is under the picosecond, corresponding to characteristic times of numerous microscopic phenomena. In this ultra-short irradiation regime, spallation experiments have been performed with time-resolved measurements of the free surface. These measurements, complemented with post-test observations, have been compared with numerical simulations to check the consistency of modelling of the laser–matter interaction, shock propagation and to the study of dynamic damage at this ultra-short time scale, inducing strong tensile stress states at very high strain rates.

Physical approach to adhesion testing using laser-driven shock waves

This paper deals with an adhesion test of coatings using laser-driven shock waves. Physical aspects concerning laser–matter interaction, shock wave propagation and interface fracture strength are described. This comprehensive approach using two numerical codes (HUGO and SHYLAC) allows the determination of mechanisms responsible for coating debonding and a quantitative evaluation of fracture strength. From this description, a coating test protocol is also designed. To diagnose coating debonding, it is based on the analysis of experimental rear free surface velocity profiles measured by velocity interferometer system for any reflectors (VISAR). Ni electrolytic coating (70–90 µm) deposited on a Cu substrate (120–190 µm) is used for the experimental validation of the test. The fracture strength is 1.49 ± 0.01 GPa for a laser pulse duration of 10 ns at 1.064 µm.

Experimental determination by PVDF and EMV techniques of shock amplitudes induced by 0.6-3 ns laser pulses in a confined regime with water

With the objective to envisage short pulses for laser-shock hardening of materials, this paper reports experiments where laser-shock amplitudes P generated with 0.6-3 ns laser pulses at = 1.06 µm in a confined regime with water have been compared with those achieved with the usual 10-30 ns configuration. First, the experimental characterization of shock waves with polyvinylidene fluoride (PVDF) and electromagnetic (EMV) gauges shows that the short durations allow the generation of higher shock amplitudes than longer duration pulses (10 GPa versus 5 GPa) because of an increase of the pressure saturation intensity threshold Ith with short pulses (up to 100 GW cm-1 at 0.6 ns). Above Ith , a pressure pulse shortening accompanies the saturation. The P = f (I ) curves have been confirmed by surface deformation measurements induced on a Al12Si alloy. Second, the use of 10 µm aluminium coatings on 316L steel targets impacted at 40 GW cm-2 irradiance was shown to provoke a 25% maximum increase of the peak pressures by type mismatch acoustic impedance effects. Lastly, the PVDF technique is shown to be an accurate method to measure laser shock wave profiles in the 0-200 GW cm-2 regime, whereas the EMV technique is limited to intensity values of less than 20 GW cm-2 .