European Journal of Mechanics - A/Solids

A method coupling experiments and simulations, is developed to characterize the yield stress and strain hardening of several metals loaded at 106 s−1 and < 25 ns, typically involved during Laser Shock Peening. It was applied to four materials: pure aluminum, 2024-T3 and 7175-T7351 aluminum alloys and Ti6Al4V-ELI titanium alloy. Thin foils have been irradiated with high-power laser to induce high-pressure shock wave. Plastic deformation is activated through the thickness up to the rear free-surface of the foils. These experiments have been simulated using three material constitutive equations: Elastic–Perfectly Plastic model considering static yield stress, Johnson–Cook model without strain hardening and Johnson–Cook model with strain hardening. The material parameters of Johnson–Cook law were identified by comparison of the experimental and calculated velocity profiles of the rear-free surface. Results are shown and discussed

Propagation of laser-generated shock waves in metals: 3D axisymmetric simulations compared to experiments

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 mate-rial, 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

Novel Confinement Possibility for Laser Shock: Use of Flexible Polymer Confinement at 1064 nm Wavelength

Through the years, laser shock peening became a treatment of choice in the aerospace industry to prolong the life of certain critical pieces. Water flow is commonly used as a confinement to improve the process capability but some applications cannot allow for water presence in the area of interest. In a previous article, an alternative to the water confinement was presented, a flexible polymer confinement was used and demonstrated the production of pressures equivalent to the water configuration treatment. However, laser parameters have been restricted to a wavelength in the visible range at 532 nm. In this paper, the study is extended to 1064 nm which is commonly used in LSP applications and with two different pulse durations. A 1064 nm near infra-red laser is used to do pressure characterization of shots with polymer confinement through Velocity Interferometer System for Any Reflector (VISAR) measurements coupled with Finite Element Modelling on Abaqus software. The results show that the pressures produced by the confinement is slightly lower with the 1064 nm wavelength, similar to what is observed with the classic water confined regime when switching from 532 nm to a near infra-red wavelength. Nevertheless, the high level of pressure produced by laser shock under the polymer confinement configuration allows for the treatment of common types of metal alloys used in the aerospace industry. Although the use of such a confinement has yet to be applicable to peening setups, it has already uses in some single shot configurations such as LasAT where it allows the avoidance of the water flow optimization

Laser induced plasma characterization in direct and water confined regimes: new advances in experimental studies and numerical modelling

Optimization of the laser shock peening (LSP) and LASer Adhesion Test (LASAT) processes requires control of the laser-induced target's loading. Improvements to optical and laser technologies allow plasma characterization to be performed with greater precision than 20 years ago. Consequently, the processes involved during laser-matter interactions can be better understood. For the purposes of this paper, a self-consistent model of plasma pressure versus time is required. The current approach is called the inverse method, since it is adjusted until the simulated free surface velocity (FSV) corresponds to the experimental velocity. Thus, it is not possible to predict the behavior of the target under shock without having done the experiments. For the first time, experimental data collected in different labs with the most up-to-date laser parameters are used to validate a self-consistent model for temporal pressure-profile calculation. In addition, the parameters characterizing the plasma (temperature, thickness and duration) are obtained from the ESTHER numerical code, together with the amount of ablated matter. Finally, analytic fits are presented that can reproduce any pressure-temporal profiles in the following domains of validity: Intensities, I, ranging from 10 to 500 GW cm-2 and pulse durations, T pul, between 5 and 40 ns for the direct-illumination regime at 1053 nm, I ranging from 1 to 6 GW cm-2 and T pul between 10 to 40 ns in the water-confined regime at 1053 nm, and I from 1 to 10 GW cm-2 and T pul between 7 and 20 ns in the water-confined regime at 532 nm. These temporal pressure profiles can then be used to predict the aluminum target's behavior under laser shock using mechanical simulation software

Towards selective laser paint stripping using shock waves produced by laser-plasma interaction for aeronautical applications on AA 2024 based substrates

Laser stripping is a process which typically includes different forms of ablation phenomena. The presented work investigates a mechanical stripping process using high pressure laser-induced shock waves in a water confined regime. Power density is studied as a parameter for selective laser stripping on painted specimens and for adhesion relations with single layer epoxy targets. A flashlamp-pumped Nd:YAG laser with fixed spot size (4 mm) is shot on single layer epoxy and several layers of polymeric paint applied on a AA 2024-T3 (Aluminium) substrate. After laser treatment, samples are investigated with optical microscopy, profilometer and chemical analysis (FTIR & TGA). The results show that selective laser stripping is possible between different layers of external aircraft coatings and without any visual damage on the substrate material. In parallel to the experimental work, a numerical model has been developed to explain the background of the physical mechanisms and to qualitatively evaluate the detailed stress analysis and interfacial failure simulation for a single layer of epoxy on an aluminium substrate. The predicted failure patterns agree with the surfaces of the tested specimens observed by a microscope

Beam size dependency of a laser-induced plasma in confined regime: Shortening of the plasma release. Influence on pressure and thermal loading

Processes using laser-shock applications, such as Laser Shock Peening or Laser Stripping require a deep understanding of both mechanical and thermal loading applied. We hereby present new experimental measurements of the plasma pressure release regarding its initial dimension, which depends on the laser beam size. Our data were obtained through shock waves’ velocity analysis and radiometric assessments. A new model to describe the adiabatic release behavior of a laser-induced plasma with a dependency to the beam size is developed. The results and the associated model exhibit that the plasma release duration is shortened with smaller laser spots. As a consequence, with chosen smaller laser spots (0.6 mm to 1 mm), the thermal loading applied during the plasma lifetime will also decrease. These new results shall help for a better understanding of laser-matter interaction for laser-shock applications by giving more accurate plasma profiles. Thus, process simulations can be improved as well. Eventually, by considering recent developments with high-power Diode Pumped Solid-State lasers (DPSS), we now expect to develop a new configuration for LSP which could be applicable both without any thermal coating and deliverable by an optical fiber.This research was funded by Thales company, institutions (CEA,NRS, ENSAM), and by the ANR (Agence Nationale de la Recherche), Forge Laser Project (Grant No.: ANR-18-CE08-0026)

Laser shock peening: toward the tse of pliable polid polymers for confinement

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/cm2, 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

Interaction laser/matière en régime confiné pour des petites taches focales (inférieures au mm). Application au renforcement de surfaces par grenaillage laser.

Laser Shock Peening (LSP) is a surface treatment process used mainly in the aeronautical, naval and nuclear fields in order to strengthen used metals, extend their lifetime and thus lighten the whole structures. This key issue of metal reinforcement has been studied since the 1970s, and functional industrial configurations have been effective for more than 20 years. In this thesis, we wished to study with more accuracy the interaction between the laser and the matter (in its fourth state, called a plasma), in the water confinement regime. Especially, this study was carried out for submillimetre laser spots, with the aim of being able to address new issues: treatment of sheltered areas, beam transportation through optical fibre and development of a high-repetition rate configuration without using a thermal protective coating. To answer these needs identified in the ANR Forge Laser project and by exploiting the recent developments achieved by Thales regarding diode-pumped solid-state lasers (DPSS) with the THEIA system (200 Hz, 10 ns, 1 J), we have implemented and studied a new platform to perform LSP treatments.On the first hand, the work of this thesis aims at demonstrating the interest of submillimetre spots through the experimental and theoretical study of laser-matter interaction; new models have been developed to reproduce experimental results obtained, namely a faster plasma release at small laser spots, and thus a reduced thermal loading. On the other hand, we investigate the behaviour of the water confinement used both under a high repetition rate configuration and under a new (patented) configuration allowing to increase the breakdown threshold inside the water, and thus the maximum pressure generated by the plasma on the target. Finally, we are using these results to unlock current limits and to carry out a LSP treatment without any thermal coating and at a high repetition rate. The used materials are aluminium alloy 2024 and titanium alloy Ti-6AL4V. These treatments have been analysed by X-ray diffraction both in laboratory and in large instruments (Diamond synchrotron), hence allowing conclusions to be drawn regarding the best process parameters to be used for this new configuration.Le grenaillage laser (LSP – Laser Shock Peening) est un procédé de traitement de surface utilisé principalement dans les domaines aéronautique, naval et nucléaire afin de renforcer les métaux utilisés, prolonger leur durée de vie et in fine alléger les structures obtenues. Cette problématique clef de renforcement des métaux est étudiée depuis les années 1970, et des configurations industrielles fonctionnelles sont effectives depuis 20 ans. Dans le cadre de cette thèse, nous avons souhaité étudier plus finement l’interaction entre le laser de puissance et la matière sous sa quatrième forme (plasma), dans le régime de confinement eau. En particulier, cette étude a été menée pour les taches laser submillimétriques, avec comme objectif de pouvoir adresser de nouvelles problématiques : traitement de zones claustrées, transport du faisceau laser par fibre optique et développement d’une configuration haute cadence sans revêtement de protection thermique. Face à ces besoins identifiés dans le projet ANR Forge Laser et conjointement aux développements récents atteints par Thales concernant les lasers solides de puissance pompés par diodes (DPSS – Diode Pumped Solide State) avec le système THEIA (200 Hz, 10 ns, 1 J), nous avons mis en œuvre et caractérisé une nouvelle plateforme de traitements par grenaillage laser.Dans un premier temps, les travaux de cette thèse visent à démontrer l’intérêt des taches submillimétriques à travers l’étude expérimentale et théorique de l’interaction laser matière ; de nouveaux modèles ont été développés pour traduire les résultats expérimentaux obtenus, à savoir une détente du plasma plus rapide aux petites taches, et donc une charge thermique réduite. Dans un second temps, nous nous intéressons au comportement de l’eau de confinement à la fois sous un régime haute cadence, et à la fois sous une nouvelle configuration brevetée permettant d’augmenter le seuil de claquage, et donc la pression maximale générée par le plasma sur la cible à renforcer. Enfin, nous mettons en œuvre ces résultats pour débloquer les verrous actuels et réaliser un traitement par grenaillage laser sans revêtement thermique et à haute cadence. Les matériaux d’intérêts concernent l’alliage 2024 d’aluminium et l’alliage Ti-6AL4V de titane. Ces traitements ont été caractérisés par diffraction de Rayons X à la fois en laboratoire et sur grands instruments (synchrotron Diamond), et permettent de conclure quant aux paramètres procédés à utiliser pour cette nouvelle configuration

Interaction laser/matière en régime confiné pour des petites taches focales (inférieures au mm). Application au renforcement de surfaces par grenaillage laser.

Laser Shock Peening (LSP) is a surface treatment process used mainly in the aeronautical, naval and nuclear fields in order to strengthen used metals, extend their lifetime and thus lighten the whole structures. This key issue of metal reinforcement has been studied since the 1970s, and functional industrial configurations have been effective for more than 20 years. In this thesis, we wished to study with more accuracy the interaction between the laser and the matter (in its fourth state, called a plasma), in the water confinement regime. Especially, this study was carried out for submillimetre laser spots, with the aim of being able to address new issues: treatment of sheltered areas, beam transportation through optical fibre and development of a high-repetition rate configuration without using a thermal protective coating. To answer these needs identified in the ANR Forge Laser project and by exploiting the recent developments achieved by Thales regarding diode-pumped solid-state lasers (DPSS) with the THEIA system (200 Hz, 10 ns, 1 J), we have implemented and studied a new platform to perform LSP treatments.On the first hand, the work of this thesis aims at demonstrating the interest of submillimetre spots through the experimental and theoretical study of laser-matter interaction; new models have been developed to reproduce experimental results obtained, namely a faster plasma release at small laser spots, and thus a reduced thermal loading. On the other hand, we investigate the behaviour of the water confinement used both under a high repetition rate configuration and under a new (patented) configuration allowing to increase the breakdown threshold inside the water, and thus the maximum pressure generated by the plasma on the target. Finally, we are using these results to unlock current limits and to carry out a LSP treatment without any thermal coating and at a high repetition rate. The used materials are aluminium alloy 2024 and titanium alloy Ti-6AL4V. These treatments have been analysed by X-ray diffraction both in laboratory and in large instruments (Diamond synchrotron), hence allowing conclusions to be drawn regarding the best process parameters to be used for this new configuration.Le grenaillage laser (LSP – Laser Shock Peening) est un procédé de traitement de surface utilisé principalement dans les domaines aéronautique, naval et nucléaire afin de renforcer les métaux utilisés, prolonger leur durée de vie et in fine alléger les structures obtenues. Cette problématique clef de renforcement des métaux est étudiée depuis les années 1970, et des configurations industrielles fonctionnelles sont effectives depuis 20 ans. Dans le cadre de cette thèse, nous avons souhaité étudier plus finement l’interaction entre le laser de puissance et la matière sous sa quatrième forme (plasma), dans le régime de confinement eau. En particulier, cette étude a été menée pour les taches laser submillimétriques, avec comme objectif de pouvoir adresser de nouvelles problématiques : traitement de zones claustrées, transport du faisceau laser par fibre optique et développement d’une configuration haute cadence sans revêtement de protection thermique. Face à ces besoins identifiés dans le projet ANR Forge Laser et conjointement aux développements récents atteints par Thales concernant les lasers solides de puissance pompés par diodes (DPSS – Diode Pumped Solide State) avec le système THEIA (200 Hz, 10 ns, 1 J), nous avons mis en œuvre et caractérisé une nouvelle plateforme de traitements par grenaillage laser.Dans un premier temps, les travaux de cette thèse visent à démontrer l’intérêt des taches submillimétriques à travers l’étude expérimentale et théorique de l’interaction laser matière ; de nouveaux modèles ont été développés pour traduire les résultats expérimentaux obtenus, à savoir une détente du plasma plus rapide aux petites taches, et donc une charge thermique réduite. Dans un second temps, nous nous intéressons au comportement de l’eau de confinement à la fois sous un régime haute cadence, et à la fois sous une nouvelle configuration brevetée permettant d’augmenter le seuil de claquage, et donc la pression maximale générée par le plasma sur la cible à renforcer. Enfin, nous mettons en œuvre ces résultats pour débloquer les verrous actuels et réaliser un traitement par grenaillage laser sans revêtement thermique et à haute cadence. Les matériaux d’intérêts concernent l’alliage 2024 d’aluminium et l’alliage Ti-6AL4V de titane. Ces traitements ont été caractérisés par diffraction de Rayons X à la fois en laboratoire et sur grands instruments (synchrotron Diamond), et permettent de conclure quant aux paramètres procédés à utiliser pour cette nouvelle configuration

Laser/matter interaction in a confined geometry for small focal spots (less than a mm).Application to surface reinforcement by laser shock peening.

Le grenaillage laser (LSP – Laser Shock Peening) est un procédé de traitement de surface utilisé principalement dans les domaines aéronautique, naval et nucléaire afin de renforcer les métaux utilisés, prolonger leur durée de vie et in fine alléger les structures obtenues. Cette problématique clef de renforcement des métaux est étudiée depuis les années 1970, et des configurations industrielles fonctionnelles sont effectives depuis 20 ans. Dans le cadre de cette thèse, nous avons souhaité étudier plus finement l’interaction entre le laser de puissance et la matière sous sa quatrième forme (plasma), dans le régime de confinement eau. En particulier, cette étude a été menée pour les taches laser submillimétriques, avec comme objectif de pouvoir adresser de nouvelles problématiques : traitement de zones claustrées, transport du faisceau laser par fibre optique et développement d’une configuration haute cadence sans revêtement de protection thermique. Face à ces besoins identifiés dans le projet ANR Forge Laser et conjointement aux développements récents atteints par Thales concernant les lasers solides de puissance pompés par diodes (DPSS – Diode Pumped Solide State) avec le système THEIA (200 Hz, 10 ns, 1 J), nous avons mis en œuvre et caractérisé une nouvelle plateforme de traitements par grenaillage laser.Dans un premier temps, les travaux de cette thèse visent à démontrer l’intérêt des taches submillimétriques à travers l’étude expérimentale et théorique de l’interaction laser matière ; de nouveaux modèles ont été développés pour traduire les résultats expérimentaux obtenus, à savoir une détente du plasma plus rapide aux petites taches, et donc une charge thermique réduite. Dans un second temps, nous nous intéressons au comportement de l’eau de confinement à la fois sous un régime haute cadence, et à la fois sous une nouvelle configuration brevetée permettant d’augmenter le seuil de claquage, et donc la pression maximale générée par le plasma sur la cible à renforcer. Enfin, nous mettons en œuvre ces résultats pour débloquer les verrous actuels et réaliser un traitement par grenaillage laser sans revêtement thermique et à haute cadence. Les matériaux d’intérêts concernent l’alliage 2024 d’aluminium et l’alliage Ti-6AL4V de titane. Ces traitements ont été caractérisés par diffraction de Rayons X à la fois en laboratoire et sur grands instruments (synchrotron Diamond), et permettent de conclure quant aux paramètres procédés à utiliser pour cette nouvelle configuration.Laser Shock Peening (LSP) is a surface treatment process used mainly in the aeronautical, naval and nuclear fields in order to strengthen used metals, extend their lifetime and thus lighten the whole structures. This key issue of metal reinforcement has been studied since the 1970s, and functional industrial configurations have been effective for more than 20 years. In this thesis, we wished to study with more accuracy the interaction between the laser and the matter (in its fourth state, called a plasma), in the water confinement regime. Especially, this study was carried out for submillimetre laser spots, with the aim of being able to address new issues: treatment of sheltered areas, beam transportation through optical fibre and development of a high-repetition rate configuration without using a thermal protective coating. To answer these needs identified in the ANR Forge Laser project and by exploiting the recent developments achieved by Thales regarding diode-pumped solid-state lasers (DPSS) with the THEIA system (200 Hz, 10 ns, 1 J), we have implemented and studied a new platform to perform LSP treatments.On the first hand, the work of this thesis aims at demonstrating the interest of submillimetre spots through the experimental and theoretical study of laser-matter interaction; new models have been developed to reproduce experimental results obtained, namely a faster plasma release at small laser spots, and thus a reduced thermal loading. On the other hand, we investigate the behaviour of the water confinement used both under a high repetition rate configuration and under a new (patented) configuration allowing to increase the breakdown threshold inside the water, and thus the maximum pressure generated by the plasma on the target. Finally, we are using these results to unlock current limits and to carry out a LSP treatment without any thermal coating and at a high repetition rate. The used materials are aluminium alloy 2024 and titanium alloy Ti-6AL4V. These treatments have been analysed by X-ray diffraction both in laboratory and in large instruments (Diamond synchrotron), hence allowing conclusions to be drawn regarding the best process parameters to be used for this new configuration