This paper presents the first extensive study of the performances of solid polymers used as confinement materials for laser shock applications such as laser shock peening (LSP) as opposed to the exclusively used water-confined regime up to now. The use of this new confinement approach allows the treatment of metal pieces needing fatigue behavior enhancement but located in areas which are sensitive to water. Accurate pressure determination in the polymer confinement regime was performed by coupling finite element simulation and experimental measurements of rear free-surface velocity using the velocity interferometer system for any reflector (VISAR). Pressure could reach 7.6 and 4.6 GPa for acrylate-based polymer and cross-linked polydimethylsiloxane (PDMS), respectively. At 7 and 4.7 GW/cm ${}^2$ , respectively, detrimental laser breakdown limited pressure for acrylate and PDMS. These results show that the pressures produced were also as high as in water confinement, attaining values allowing the treatment of all types of metals with LSP and laying the groundwork for future determination of the fatigue behavior exhibited by this type of treated materials.

This article presents the study of the shock wave propagation through aluminum alloys (pure aluminum and aluminum 2024-T3) produced by laser plasma using experimental and numerical tests. Water confinement regime interaction, pulse duration (7.2 ns), and power density (1–5 $\mathrm{GW}/{\mathrm{cm}}^2$ ) range correspond to laser shock peening process configuration and parameters. To that scope, we simulate the shock wave propagation using non-linear explicit code LS-DYNA, which we validate with experimental results. Thereupon, we present a descriptive analysis that links separately the material model and loading conditions to the dynamic response of aluminum alloys under high strain rate laser shock by coupling the Johnson-Cook (J-C) material model with the Grüneisen equation of state (MAT_015 and EOS_GRUNEISEN accordingly). In addition, we make use of stress propagation into target thickness to analyze the origin of different points on the Back Face Velocity (BFV) profile during shock propagation. Finally, we provide evidence that 2D compressive effects do not depend only on the focal spot size or target thickness such as the edge effects but also on power density and material initial yield strength.

This work aims at demonstrating the ability of an acoustic linear code to model the propagation of a shock wave created by a laser impact over a metallic surface. In this process, a high pressure surface level is reached using a ns laser pulse that heats the surface of the material and generates a dense plasma expansion. The pressure reaches few GPa so shock waves are generated and propagate into the bulk of the material. Currently, shock wave propagation is modeled using continuity equations and an ad hoc equation of state for the illuminated material, very limiting because it is numerically intensive. Here, we propose to model the shock wave bulk propagation using a linear acoustic code. A nonlinear surface pressure term, resulting from the laser–matter interaction, is used as a boundary condition. The applied numerical scheme is based on the Virieux scheme, including a fourth order finite difference discretization of the linearized elastomechanical equations. The role of longitudinal and transverse waves and their origins are highlighted. The importance of considering 3D geometries is pointed out. Simulations are finally confronted with experimental results obtained with the Hephaistos Laserlab facility (energy up to 14 J at 532 nm wavelength laser; pulse duration: 7 ns). Illuminations up to the optical breakdown in water are easily achieved with laser focal spots of 5 mm width. Excellent agreement between experiments and simulations is observed for several sets of experimental parameters for titanium, a material of high elastic limit, while limitations are founded for aluminum. The code is available in the MetaData.

The authors present a new configuration for laser-induced plasmas in confined regimes for a 10 ns-range laser pulse in the green wavelength (532 nm) that repulses the breakdown threshold above 20 GW/cm2 compared to 8 GW/cm2 as generally indicated in previous works. Using this new configuration, pressures above 12 GPa have been reached for the first time in confined regimes. This can enlarge the range of applications of laser shock such as the range of treatable materials (very high strength materials) or facilities’ costs since neither vacuum nor heavy laser systems will be needed to reach these levels of pressure. The proposed configuration mainly consists of the usage of a water tank. Hence, a great thickness of water is used instead of the extensively used thin water layer. Therefore, the water breakdown plasma will not initiate at the surface of the water, as the laser beam is still not focused there. Instead, it will occur in the depth of the water. In that case, the breakdown threshold value is increased as either the avalanche breakdown or the multiphoton ionization may start at higher laser intensities than at the air/water interface. The authors experimentally demonstrated this new breakdown threshold with the measurement of transmitted intensity, transmitted pulse duration, and the indirect measurement of the plasma pressure. Multiple shots (laser shock treatment) were also performed, and the specimen surface deformation was measured, leading to the same conclusion.