J.-P. Cuq-Lelandais

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.

Study of damage phenomena induced by edge effects into materials under laser driven shocks

Spallation of materials induced by laser driven shock waves is generally produced under uniaxial (one-dimensional (1D)) deformation by irradiating a spot of diameter much greater than the sample thickness. Here, two-dimensional (2D) effects are introduced in shock wave propagation by drastically reducing the loaded spot. Experiments performed on aluminium samples detect the effect of lateral wave propagation, both on recovered samples and on time-resolved VISAR measurements. Damage zones are localized completely differently from that under uniaxial condition, according to the presence of 2D effects, and the signature of these 2D effects can be read on VISAR signals. Numerical simulations provide a full understanding of wave propagation and resulting damage in 1D or 2D configuration. Comparisons with experimental VISAR signals show the possibility of validating more accurately the dynamic damage criteria, including the 2D effects.

PDV MEASUREMENTS OF NS AND FS LASER DRIVEN SHOCK EXPERIMENTS ON SOLID TARGETS

We present a new heterodyne velocimeter setup embedding a second low‐power frequency‐tunable laser acting as a local oscillator. We thus double the overall bandwidth of the system and we make the tuning of the laser power levels easier, to achieve good matching between the electric signal matching and the dynamics of the detector. Recently, we used this velocimeter onto metallic target shock driven by high power laser. The aim is to test the ability of this means to reveal shock propagation and effects into materials under extremely high strain rate with fast variations into the loading evolution. Spallation and fragmentation experiments carried out on aluminum samples were performed on the LULI2000 laser (800 J, 3 ns) and on the 100 TW laser (30 J, 300 fs) of the Ecole Polytechnique, with both VISAR and HV diagnostics. Comparisons reveal a very good consistency of experimental results.

Soft recovery technique to investigate dynamic fragmentation of laser shock-loaded metals

With the development of high energy laser facilities dedicated to inertial confinement fusion, the question of debris ejection from metallic shells subjected to intense laser irradiation has become a key issue. We have used two diagnostics to investigate fragmentation processes. Recovery of ejected fragments has been performed in a highly transparent gel of density 0.9 g/cm3. Fragments sizes, shapes, and penetration depths, can be easily observed with a spatial resolution of micrometer-order. Complementary data are provided by transverse shadowgraphy which allows to obtain quasi-instantaneous, successive pictures of the debris clouds and mean ejection velocities.

Investigation of laser shock induced ductile damage at ultra-high strain rate by using large scale MD simulations

Laser driven shocks allow an investigation of materials behavior at very high strain rate (107s-1) and present a great interest for research applications. Microscopic simulations of ultra-short laser driven shock on micrometric Tantalum single-crystals have been performed by using the CEADAM Classical Molecular Dynamics code. This method, complementary to continuum models, provides an analysis the microscopic processes related to damage (ductile pore nucleation and growth) which occurs during spallation. These results are compared to spallation experimental data (VISAR signals, micro-tomography) obtained with the LULI100TW femtosecond laser in order to validate the MD behavior. Moreover, in the framework of a multi-scale approach, we show the possibility to use MD simulation to fit macroscopic damage models. This method is illustrated with an application to the parameters determination of Kanel damage model parameters. This also shows the high strain rates involved during damage process, around 109s-1, ca...

TRANSVERSE SHADOWGRAPHY AND NEW RECOVERY TECHNIQUE TO INVESTIGATE DYNAMIC FRAGMENTATION OF LASER SHOCK‐LOADED METALS

With the development of high energy laser facilities dedicated to inertial confinement fusion, the question of debris ejection from metallic shells subjected to intense laser irradiation has become a key issue. We have used two diagnostics to investigate this phenomenon. Transverse shadowgraphy is an optical time‐resolved diagnostic. It provides successive images that allow characterizing the motion of fragments generated by processes such as microjetting and spallation. Quasi‐instantaneous pictures of the debris clouds are obtained and mean ejection velocities can be derived. Complementary data are provided by post‐shock analysis of recovered fragments. Such recovery can be achieved in aerogels, but their brittleness and low transparency make the analysis difficult. Instead, we have used a new technique, based on a highly transparent gel of density 0.9 g/cm3, which allows soft recovery and easy observation of the fragments sizes, shapes and penetration depths, with a spatial resolution of μm‐order.

Laser Shock Adhesion Test (LASAT) of Electron Beam Physical Vapor Deposited Thermal Barrier Coatings (EB-PVD TBCs)

Damage prediction, adhesion strength and remaining lifetime of TBC are highly important data for understanding and preventing TBC spallation on blades. LAser Shock Adhesion Test (LASAT) is a powerful method to measure adhesion of coating due to its rapidity, simplicity and capabilities to distinguish different strength levels and the easy damage observation in case of TBCs. A new protocol of LASAT has been introduced in order to measure the adhesion level of the ceramic coating from the exploitation of the two-dimensional effects that promotes a shock wave pressure-dependent size of the damage. Finite element modeling, taking into account the TBCs dimensions, showed the edges effect on interfacial stress applied by laser shock.

Damaging of materials by bi-dimensional dynamic effects

Laser shocks are most often used to produce uniaxial stress and strain into materials by irradiating a spot diameter conventionally admitted at least three times larger than the thickness of the shocked sample. By reducing the laser spot versus the sample thickness, 2D lateral waves are created earlier and their crossing during propagation stages generates traction which can yield to voids into materials (near the front loaded face). This phenomenon has been evidenced by an experimental study, including VISAR measurements which exhibit the signature of the fracture generated by these 2D effects. Numerical simulations with the explicit finite element code RADIOSS clearly evidence the origin of the 2D effects on VISAR measurements. This different mode of damaging materials by laser lateral waves can act simultaneously with the classical laser spallation produced by the uniaxial propagation (shock wave reverberation crossing the unloading). This opens new discussed prospects for the development of 2D damage ...

Wave Propagation and Dynamic Fracture in Laser Shock-Loaded Solid Materials

Shock wave loading of solid materials results in specific damage processes at high strain rates. The most widely studied of these processes is probably spall fracture (e.g. Antoun et al., 2002, and references therein), which arises from tensile stresses generated by the interaction of release waves within the material upon reflection of a compressive pulse from a free surface or from an interface with a layer of lower acoustic impedance. If such tensile stresses exceed the dynamic strength of the material, they cause the nucleation and growth of micro-voids or micro-cracks which may eventually coalesce to form a macroscopic fracture and lead to the ejection of one or several fragments (spalled layers) from the sample. Spall damage and wave propagation are tightly coupled. On one hand, the creation of new free surfaces accompanying damage development induces stress relaxation which gives rise to recompression waves. Such waves can be detected in time-resolved velocity (Antoun et al., 2002; Tollier et al., 1998) or piezoelectric (De Resseguier et al., 1997) measurements, and their analysis provides very rich data on the fracture process (location, time and tensile stress at damage initiation, rate of the damage growth, thickness of the spalled layer...). On the other hand, spall fracture results from wave interaction, so that post-test observations of the residual damage in recovered samples (location, sizes and shapes of the damages zones, fracture surface morphology...) may provide key information on the propagation of compression and release waves prior to failure. In this chapter, we illustrate this second, more original statement with experimental results obtained under laser driven shock loading. Intense irradiation of an absorbing target by a high power pulsed laser produces the vaporization of a thin layer of material, transformed into a plasma cloud, whose expansion toward the laser source induces by reaction a compressive pulse into the solid target. The main specificity of this technique is the very short time of pressure application (typically a few ns) compared to other shock generators such as plate impacts or explosive loading, where the duration of the pressure load is usually of μs-order. This difference makes laser shocks less destructive than those more conventional techniques, and favours sample recovery for post-shock analyses of residual damage. In a first example, spall fracture observed in laser shock-loaded single crystal quartz provides very clear evidence of the strong effect of crystal anisotropy on stress wave Source: Wave Propagation in Materials for Modern Applications, Book edited by: Andrey Petrin, ISBN 978-953-7619-65-7, pp. 526, January 2010, INTECH, Croatia, downloaded from SCIYO.COM

New advances for laser adhesion test (lasat): From thick to thin coatings

This paper present recent developments in LAser Adhesion Test. This technique sollicitates material interfaces with laser driven shock wave. Results show the ability of the technique to do a quantitative adhesion test for wide range of material and configuration. Edge effect principle and ultra-short shock wave give perspectives for new applications for thick and thin coatings. Fundamental principles are evidenced through experiments on bulk ductile materials before demonstrating their application to coated systems.This paper present recent developments in LAser Adhesion Test. This technique sollicitates material interfaces with laser driven shock wave. Results show the ability of the technique to do a quantitative adhesion test for wide range of material and configuration. Edge effect principle and ultra-short shock wave give perspectives for new applications for thick and thin coatings. Fundamental principles are evidenced through experiments on bulk ductile materials before demonstrating their application to coated systems.