Observation of the shock wave propagation induced by a high-power laser irradiation into an epoxy material

The propagation of laser-induced shock waves in a transparent epoxy sample is investigated by optical shadowgraphy. The shock waves are generated by a focused laser (3 ns pulse duration—1.2 to 3.4TWcm−2) producing pressure from 44 to 98.9 GPa. It is observed that the shock wave and the release wave created by the shock reverberation at the rear face are both followed by a dark zone in the pictures. This corresponds to the creation of a tensile zone resulting from the crossing on the loading axis of the release waves coming from the edge of the impact area (2D effects). After the laser shock experiment, the residual stresses in the targets are identified and quantified through a photoelasticimetry analysis of the recovered samples. This work results in a new set of original data which can be directly used to validate numerical models implemented to reproduce the behaviour of epoxy under extreme strain rate loading. The residual stresses observed prove that the high-pressure shocks can modify the pure epoxy properties, which could have an influence on the use made of these materials

Experimental and numerical investigations of shock and shear wave propagation induced by femtosecond laser irradiation in epoxy resins

In this work, original shock experiments are presented. Laser-induced shock and shear wave propagations have been observed in an epoxy resin, in the case of femtosecond laser irradiation. A specific time-resolved shadowgraphy setup has been developed using the photoelasticimetry principle to enhance the shear wave observation. Shear waves have been observed in epoxy resin after laser irradiation. Their propagation has been quantified in comparison with the main shock propagation. A discussion, hinging on numerical results, is finally given to improve understanding of the phenomenon

Dynamic fragmentation of graphite under laser-driven shocks: Identification of four damage regimes

This study presents the results of a large experimental campaign conducted on the Luli2000 laser facility. Thin targets of a commercial grade of porous graphite were submitted to high-power laser-driven shocks leading to their fragmentation. Many diagnostics were used such as high-speed time- and space-resolved imaging systems (shadowgraphy and photography), laser velocimetry (PDV and VISAR), debris collection and post-mortem X-ray tomography. They provided the loading levels into the targets, the spall strength of the material, the shape and size of debris and the localization of the subsurface cracks. The crossed data reduction of all the records showed their reliability and allowed to get a better insight into the damage phenomena at play in graphite. Thereby, four damage regimes, ranked according to their severity and loading level, were identified. It confirms that laser shocks are very complementary to classical impact tests (plates and spheres) since they ally two-dimensional loadings to the possibility of using both, in-situ and post-mortem diagnostics. Finally, the campaign shall be able to provide large and consistent data to develop and adjust reliable models for shock wave propagation and damage into porous graphite

Etude des plasmas générés par interaction laser-matière en régime confiné. Application au traitement des matériaux par choc laser.

In this work, we study the plasmas generated by laser-matter interaction in a water confinement regime, and more particularly, their application to laser-shock processing of materials. With the aim to get a better understanding of the physical processes involved in this particular interaction regime, an original modeling has been developed. First, a numerical code describing the breakdown process in water allows us to compute the characteristics (peak intensity and full width at half maximum pulse duration) of the laser pulse which is transmitted through the confinement window. These parameters are then used in an auto-consistent hydrodynamic model describing the confined plasmas (dense cold correlated and partially degenerated plasmas), in order to compute the thermal and mechanical loadings induced at the surface of the processed sample. Last, these loadings are used as input parameters in the FEM code ABAQUS to simulate the thermal and mechanical residual stresses induced by the process.The results of the simulations have been validated by comparison with various experimental measurements carried out with laser irradiation conditions (1064 nm and 532 nm wavelengths, 3 ns and 10 ns pulse durations) similar to those used for the industrial application of the process. These results show that small focal spot sizes allow to greatly reduce the heating of the target by the confined plasma, and thus to avoid the thermal effects induced by the process. This highlights the fact that such reduced focal spot sizes could be used in order to realize laser shock processing without any thermal protective coating. Furthermore, they allow to explain recent results obtained with the process configuration developed by Toshiba (very small focal spot diameter, high repetition rate, no protective coating), which were not clearly understood until now.Ce travail de thèse est consacré à l'étude des plasmas générés par interaction laser-matière en régime confiné, et plus particulièrement à leur application au traitement des matériaux par choc laser. Afin de mieux comprendre les phénomènes physiques mis en jeu dans ce régime d'interaction particulier, une modélisation originale du procédé a été développée. Un code numérique traitant les processus de claquage dans l'eau de confinement permet dans un premier temps de déterminer les caractéristiques (intensité crête et durée à mi-hauteur) de l'impulsion laser transmise à travers la fenêtre de confinement. Un modèle hydrodynamique auto-consistant traitant les plasmas confinés (plasmas froids et denses, corrélés et partiellement dégénérés) utilise ensuite ces paramètres pour calculer les chargements mécaniques et thermiques induits à la surface de la cible traitée. Pour terminer, ces chargements sont utilisés en entrée du code aux éléments finis ABAQUS afin de simuler les contraintes résiduelles d'origine mécanique et thermique induites par le traitement. Les résultats de ces simulations ont été validés par comparaison avec différentes mesures expérimentales réalisées pour des conditions d'irradiation laser (longueurs d'ondes de 1064 nm et de 532 nm, durées d'impulsion de 3 ns et 10 ns) typiques des conditions opératoires réelles utilisées au niveau industriel. Ces résultats montrent que les petites taches focales permettent de limiter fortement le chauffage de la cible par le plasma confiné, et donc de s'affranchir des effets thermiques induits par le traitement. Ils ouvrent donc de nouvelles perspectives quant à la réalisation du traitement par choc laser sans utiliser de revêtement thermo-protecteur. Par ailleurs, ils permettant d'expliquer les résultats obtenus avec la configuration de traitement développée par Toshiba (très petites taches focales, haute cadence, pas de revêtement protecteur), qui demeuraient incompris jusqu'alors

Characterization of the ballistic properties of ejecta from laser shock-loaded samples using high resolution picosecond laser imaging

International audienceA high resolution picosecond laser imaging diagnostic has been developed for making high-resolution spatial measurements of ejecta particles moving at high velocities (a few km.s$^{-1}$). Preliminary results obtained with both visible (532 nm) and UV (355nm) lighting are presented for laser shock-driven tin ejecta experiments performed with different kind of surface defects. These results are compared with those obtained at LANL under high explosive loading using ultraviolet in-line Fraunhofer holography, and also with molecular dynamics (MD) simulations performed at lower space and time scales

Etude des plasmas générés par interaction laser-matière en régime confiné (application au traitement des matériaux par choc laser)

In this work, we study the plasmas generated by laser-matter interaction in a water confinement regime, and more particularly, their application to laser-shock processing of materials. With the aim to get a better understanding of the physical processes involved in this particular interaction regime, an original modeling has been developed. First, a numerical code describing the breakdown process in water allows us to compute the characteristics (peak intensity and full width at half maximum pulse duration) of the laser pulse which is transmitted through the confinement window. These parameters are then used in an auto-consistent hydrodynamic model describing the confined plasmas (dense cold correlated and partially degenerated plasmas), in order to compute the thermal and mechanical loadings induced at the surface of the processed sample. Last, these loadings are used as input parameters in the FEM code ABAQUS to simulate the thermal and mechanical residual stresses induced by the process.The results of the simulations have been validated by comparison with various experimental measurements carried out with laser irradiation conditions (1064 nm and 532 nm wavelengths, 3 ns and 10 ns pulse durations) similar to those used for the industrial application of the process. These results show that small focal spot sizes allow to greatly reduce the heating of the target by the confined plasma, and thus to avoid the thermal effects induced by the process. This highlights the fact that such reduced focal spot sizes could be used in order to realize laser shock processing without any thermal rotective coating. Furthermore, they allow to explain recent results obtained with the process configuration eveloped by Toshiba (very small focal spot diameter, high repetition rate, no protective coating), which were not clearly understood until now.VERSAILLES-BU Sciences et IUT (786462101) / SudocSudocFranceF

Review on Laser Interaction in Confined Regime: Discussion about the Plasma Source Term for Laser Shock Applications and Simulations

International audienceThis review proposes to summarize the development of laser shock applications in a confined regime, mainly laser shock peening, over the past 50 years since its discovery. We especially focus on the relative importance of the source term, which is directly linked to plasma pressure. Discussions are conducted regarding the experimental setups, experimental results, models and numerical simulations. Confined plasmas are described and their specific properties are compared with those of well-known plasmas. Some comprehensive keys are provided to help understand the behavior of these confined plasmas during their interaction with laser light to reach very high pressures that are fundamental for laser shock applications. Breakdown phenomena, which limit pressure generation, are also presented and discussed. A historical review was conducted on experimental data, such as pressure, temperature, and density. Available experimental setups used to characterize the plasma pressure are also discussed, and improvements in metrology developed in recent years are presented. Furthermore, analytical and numerical models based on these experiments and their improvements, are also reviewed, and the case of aluminum alloys is studied through multiple works. Finally, this review outlines necessary future improvements that expected by the laser shock community to improve the estimation of the source term

Skew photonic Doppler velocimetry to investigate the expansion of a cloud of droplets created by micro-spalling of laser shock-melted metal foils

International audienceDynamic fragmentation in the liquid state after shock-induced melting, usually referred to as micro-spallation, is an issue of great interest for both basic and applied sciences. Recent efforts have been devoted to the characterization of the resulting ejecta, which consist in a cloud of fine molten droplets. Major difficulties arise from the loss of free surface reflectivity at shock breakout and from the wide distribution of particle velocities within this cloud. We present laser shock experiments on tin and aluminium, to pressures ranging from about 70 to 160GPa, with complementary diagnostics including a photonic Doppler velocimeter set at a small tilt angle from the normal to the free surface, which enables probing the whole cloud of ejecta. The records are roughly consistent with a one-dimensional theoretical description accounting for laser shock loading, wave propagation, phase transformations, and fragmentation. The main discrepancies between measured and calculated velocity profiles are discussed in terms of edge effects evidenced by transverse shadowgraphy

Recent PDV Implementations

Author Institution: Commissariat à l'énergie atomique et aux énergies alternatives (France)Slides presented at the 6th Annual Photonic Doppler Velocimetry (PDV) Workshop held at Lawrence Livermore National Laboratory, Livermore, California, November 3-4, 2011