Full or partial melting under shock compression or upon release following a shock wave and subsequent fragmentation in the melted state are still essentially open questions in most metals. We present laser shock experiments performed on tin and aluminium, to pressures ranging from about 60 to 250 GPa. Diagnostics include Photonic Doppler Velocimetry (PDV) measurements of the free surface velocity, transverse observations of the expanding cloud of droplets sometimes referred to as 'micro-spall', and soft recovery of such droplets within a low density gel. Multi-phase equations of state are used to infer the evolution of the thermodynamic state along shock propagation distance, accounting for the decay of the loading pressure pulse, and for the presence of mixed regions in the phase diagrams. Experimental observations are interpreted on the basis of hydrodynamic simulations of laser-matter interaction and shock response involving the above multi-phase equations of state.
The laser shock adhesion test (LASAT) in its first configuration works by applying direct high power laser irradiation onto a substrate/coating to yield controlled tensile stress at the interface leading possibly to debonding. Using high power lasers implies rather short pulse durations, resulting in a strong hydrodynamic decay through the thickness of the bulk materials. Hence, in this configuration, the LASAT test is limited to rather thin samples within the millimetric range. A new technique derived from LASAT, the flier laser shock adhesion test (F-LASAT) applied to a Cu/Al system, is introduced. This new technique consists in impacting a substrate/coating system 272by a thin flier which is accelerated by laser irradiation. In this way, it is possible to produce high level shocks and consecutively high traction levels into the system to be debonded. The duration of shock loading being increased by this technique, the decay is reduced and debonding of coating is achieved on thicker samples, up to 3 mm on Cu/Al system. The present paper discusses the relevant parameters of this technique and shows its potential to extend the possibilities of the LASAtest to a thicker range of samples.
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 pressure measurements and plasma observations allow explaining the current limit of high amplitude shock-waves generation by laser plasma in the water-confinement mode and open new research areas for the understanding of breakdown plasma effects at the surface of the confining water.
Laser ablation propulsion and orbit cleaning are developing areas of research. The general aim of laser-based techniques applied to this field is to maximize the momentum transfer produced by a laser shot. This work presents results from ballistic pendulum experiments under vacuum on aluminum, copper, tin, gold, and porous graphite targets. The work has focused on the metrology of the laser experiments to ensure good stability over a wide range of laser parameters (laser intensity ranging from 4 GW/cm2 to 8.7 TW/cm2, pulse duration from 80 ps to 15 ns, and wavelengths of 528 or 1057 nm). The results presented compile data from three experimental campaigns spanning from 2018 to 2021 on two different laser platforms and using different pulse durations, energies, and wavelengths. The study is complemented by the simulation of the momentum from the mono-dimensional Lagrangian code ESTHER. The first part of this work gives a detailed description of the experimental setup used, the ESTHER code, and the treatment of the simulations. The second part focuses on the experimental results. The third part describes the simulation results and provides a comparison with the experimental data. The last part presents possible improvements for future work on the subject.
Laser shock processing (LSP) is an emerging industrial process in the field of surface treatment with particular application to the improvement of fatigue and corrosion properties. In the standard configuration, the metal sample is coated with a sacrificial layer in order to protect it from detrimental thermal effects, and a water overlay is used to improve the mechanical coupling by a confining like effect. Whereas the induced mechanical effects are now well understood, very few studies have been realized concerning the thermal effects. For this purpose, the knowledge of the confined plasma microscopic parameters has a great importance. A complete model describing the laser-liquid-metal interaction is presented. The model predicts the time evolution of the plasma parmmeters (temperature, density, ionization) and allows us to compute the induced pressure and temperature in the metal sample. By comparing the numerical results with various experimental measurements, predictions can be made concerning the best laser irradiation conditions for LSP.
In previous works, both experimental and numerical investigations have shown the potential of the laser shock technique for disassembling adhesively bonded composite/metal coupons. This study extends this concept to upscale the method, focusing on the full disassembly of a foreign object damage (FOD) panel designed to replicate an aircraft engine fan blade. Utilizing LS-Dyna explicit FE software, the numerical simulation incorporates various material models: a progressive damage model for the 3D CFRP substrate, Johnson–Cook plasticity model combined with Grüneisen equation of state for the titanium layer, and a cohesive zone model with a bilinear traction separation law for the adhesive layer. Investigating the FOD panel's inclined geometry, a preliminary analysis examines the impact of inclination angles on shock wave propagation and back face velocity, revealing minimal alterations. Initial simulations are conducted to determine double pulse delay times and evaluate spot location effects. Automation of the multi-step process simulation is achieved through a Python script. After approximately 30 shots, complete disassembly of the FOD strip is achieved, with observed damage primarily limited to matrix cracking in the composite substrate. Numerical findings support the efficacy of the double-shot laser shock method for disassembling adhesively bonded metallic/CFRP components, contingent upon appropriate experimental arrangements.
The laser shock adhesion test (LASAT) is a technique allowing the generation of high tensile stresses in materials. The LASAT consists in focusing a pulsed laser beam on a water-confined target. The laser pulse crosses the water transparent layer and is absorbed by the target. High energetic plasma is created at the surface of the sample. As a response to the expansion of the plasma, a shock wave is generated and propagates through the sample. This shock wave leads to the generation of high tensile stresses in the sample. These stresses allow the interface solicitation in order to evaluate the dynamic adhesive bond strength of coated systems. In order to determine interface strengths, this technique has already proven its feasibility. In this paper, the adhesion strength of coated system was evaluated using LASAT for two surface pretreatments of substrates obtained by grit-blasting and laser surface texturing techniques. The generation of the high-intensity shock wave by laser plasma in the water-confinement regime has been performed at 7.1 ns at 532 nm with the new Nd:YAG laser facility HEPHAISTOS. This paper shows that surface treatments have a great influence on the adherence results of the coated systems obtained with laser adhesion test. However, the LASAT is efficient on thin coating. In that sense, thicker industrial coatings are not adapted for the conventional LASAT anymore. Therefore, a new technique was designed to improve and extend the conventional technique. This technique consists of varying the delay Δt between two incident pulses to adjust the location of the maximum tensile stresses near the interface. Some preliminary results on the improved configuration are presented in this paper and the problematic of the laser-matter interaction with two time-delayed laser pulses which has arisen is discussed.
Nowadays, laser machining of metals has some advantages over more traditional processes. For example, there is greater flexibility of use and no mechanical contact with the surface. In micro-machining, shorter pulses reduce heat-affected damage of the material and open new ways for nanometer accuracy. This study presents a new use of pulsed lasers. Indeed, a laser usually used for marking (a few microns deep) has achieved scour over ten millimeter deep over an area of a few square centimeters. The objective of this study is to better understand the potential of the process and by extension to define new ways to develop the applicability of this method. An experimental study was carried out using a pulsed laser with an energy of up to 1 mJ at 1064 nm (pulse duration from 130 to 180 ns, repetition rate of 50 to 100 kHz). The influence of various operating parameters of laser processing has been studied. This paper will discuss preliminary results that concern the optimization of the machining process through improving material removal rate.
The generation of shock waves by laser-plasma in Water Confinement Regime (WCR) has been investigated for the first, second and third harmonics of Nd:Glass laser (1,064, 0,532, 0,355 μn) with 0.6, 10 and 25 ns laser pulse durations. Pressure measurements have been mainly performed using a Velocimetry Interferometer System for Any Reflector (VTSAR). It appears that, depending on laser parameters, above a given laser intensity threshold, the peak pressure is saturated and the pressure duration is reduced due to laser-induced breakdown plasma in the confining water. The observation of the interaction zone (confined plasma and confining water) with a fast intensified camera shows that this parasitic breakdown occurs exclusively at the surface of water and limits the efficiency of the process. The time-resolved transitivity of this plasma has been measured with a continuous Argon laser beam for a 1.064 μm/25 ns incident laser irradiation. Above 10 GW/cm2, the peak power density transmitted through the breakdown plasma saturates and the laser pulse transmitted is reduced in agreement with pressure measurements. The relative influence of main physical mechanisms occurring during the generation of the laser breakdown at the surface of water have been discussed. According to the wavelength effect which tends to authorize higher pressure with longer wavelengths, the influence of multiphotoionic processes seem to dominate the effect of avalanche ionization. The protective coatings and laser spot size influences have been also investigated regarding the stress levels induced in the targets. About 30 % stresses increase in the material is shown to occur with adapted coating. On the contrary, the laser spot size has no specific effects on the pressure induced in WCR. Residual Stresses (RS) field have been determined for different laser conditions.
