M. Hallouin

Shock profile induced by short laser pulses

Standard 25‐μm‐thick polyvinilydene fluoride (PVDF) piezoelectric gauges and new 450‐μm‐thick P(VDF 70%, TrFE 30%) piezoelectric copolymer have been used to record shock profiles at the back face of metallic targets irradiated by laser pulses of 2.5 and 0.6 ns duration at a 1.06 μm wavelength. The records are fully explained with simplified space–time diagram analysis. The pressure profile applied at the front face of the target has been determined from these records combined with numerical simulations of wave propagation through the target. A numerical code describing the interaction of laser with matter (FILM) has also been used for computing the applied pressure. Both methods lead to very close results. The peak pressure dependence on incident laser intensity is determined up to 30 GPa at 1012 W/cm2.

Absolute equation of state measurements of iron using laser driven shocks

First absolute equation of state measurements obtained for iron with laser driven shock waves are presented. The shock velocity and the free surface velocity of compressed iron have been simultaneously measured by using a VISAR diagnostic, and step targets. The pressure range 1–8 Mbar has been investigated, which is directly relevant to planetary physics. The experiments have been performed at the Laboratoire pour l’Utilisation des Lasers Intenses of the Ecole Polytechnique.

Stress relaxation and precursor decay in laser shock-loaded iron

Iron samples of different thicknesses have been submitted to laser shocks of short duration. Transmitted wave measurements have been performed by means of polymeric piezoelectric transducers. The records indicate a decrease of the elastic precursor amplitude with increasing propagation distance, as well as a stress relaxation behind the elastic wave front. These features were already observed in shock-loaded iron, but this paper extends the experimental data to a previously unexplored range of thin targets and high strain rates. Postshock examinations of the recovered specimens reveal significant twin formation. Computer simulations of the experiments have been made using a constitutive model from the literature, including a theoretical description of rate-dependent twinning. The ability of that model to predict the experimental results has been tested. A correct overall agreement has been obtained, except for some discrepancies which have been discussed.

Structural transformations in laser shock-loaded quartz

The kinetics of the phase transitions occurring in solid materials under strong compression are an issue of considerable interest, which can be studied using dynamic loading processes. Here, we investigate the phase transformation behavior of quartz under laser-induced shock compression of a very short duration (nanosecond order). Time-resolved measurements, coupled with simple computer simulations, have been used to characterize the pressure pulse induced in the samples. Recovery shots have been performed for postshock spectroscopy, that has revealed permanent structural changes of various types, depending on the experimental conditions (loading pressure, sample thickness, and initial temperature). The results, which differ notably from observations reported under quasi-static compression or longer pulsed loads (microsecond order), suggest a partial amorphization of the shocked samples, as well as the formation of a structure that can be compared to known orthorhombic high-pressure phases. They provide a...

Two‐dimensional study of shock breakout at the rear face of laser irradiated metallic targets

The two‐dimensional propagation dynamics of laser‐driven shock waves in solids is studied through the analysis of the shock breakout at the rear face of the target for a set of materials and laser intensities. The laser shock simulations were carried out by means of a two‐dimensional hydrodynamics code in which the laser‐ablation pressure is replaced by an equivalent pressure pulse. It is shown that the two‐dimensional code is a very useful tool to analyze laser‐shock experiments where two‐dimensional effects arise from a finite laser‐spot size or a heterogeneous energy deposition.

Some applications of laser-induced shocks on the dynamic behavior of materials

High-power pulsed lasers are used widely nowadays as shock generators. They settle as a complementary technique to the conventional shock generators by the high peak pressure and short duration shocks they deliver. They are used to investigate the feasibility of new industrial processes and to get information on the behavior of matter in specific conditions of extremely high strain rate. In this paper, some studies about typical applications of laser shocks are presented : surface densification of porous materials, spallation in ductile and brittle materials, α-e phase change in iron, and an estimate of the relaxation time for this transition. Laser shock experiments provide additional data on the dynamic behavior of materials at the nanosecond time scale.

Interaction of two laser shocks inside iron samples

The interaction of two plane symmetric shocks in a solid sample induces a significant increase of both the pressure and the temperature in the central zone where the incident compressive pulses cross each other. In iron samples, such loading conditions may produce typical structural defects (twins, dislocations) and phase transitions that can be revealed by posttest examination of the recovered targets. We have used two high-power laser beams to irradiate simultaneously both surfaces of thin iron foils. The recovered samples have been sectioned and observed in optical microscopy. A very dense twin distribution in the central zone has confirmed the pressure amplification due to the interaction of the incident shocks. The occurrence of a phase transition has been inferred from the presence of short characteristic twins. Spall fraction has been observed near both irradiated surfaces, and additional damage has been evidenced at the center of the samples. Numerical tools have been adapted to simulate the exper...

Shock-induced irreversible transition from α-quartz to CaCl2-like silica

Previous analyses of quartz samples recovered after being submitted to laser shocks of very short duration (nanosecond order) have shown the presence of CaCl2-like silica [T. de Resseguier, P. Berterretche, M. Hallouin, and J. P. Petitet, J. Appl. Phys. 94, 2123 (2003)]. To date, this transition has never been observed under shocks of longer duration (microsecond order) generated by explosives or plate impacts. While this phase is produced from stishovite under static compression at very high pressure (above 50GPa) and disappears on pressure release, it is observed after low pressure laser shocks (below 5GPa) and it is quenched to ambient conditions. The origins of these differences are still unclear. This paper presents complementary laser shock experiments involving setups to provide additional information on the influence of various shock parameters. The results suggest a direct transition from α-quartz to CaCl2-type silica following a diffusionless mechanism involving high shear strains. They also sho...

Laser driven shock pressure measurements by VF2/VF3 and PVDF gages for pulses of 2.5ns up to 1012 W/cm2

Thick 450 μm piezoelectric VF2/VF3 copolymer and thin 25 μm PVDF gages have been used to measure the induced pressure history at the back face of aluminum and copper targets irradiated by infra-red laser pulses† of 2.5 ns with intensities up to 7.1011 W/cm2. The measured pressures in the gages infer pressures up to 200 kbar on the front face of the target. The whole shock pressure temporal pressure profile applied on the front face of the targets is determined roughly by using the laser matter interaction hydro-code FILM. The modifications to bring to this profile in order to fit the experimental record via the simulation of the propagation of the applied profile into the set-up are very limited. The comparison of the applied peak pressure given by simulation of laser matter interaction and the experiment deduced one is also rather good. These results give the peak pressure versus the incident intensity under these laser irradiation conditions and assess the possibility to use this kind of gages as a meas...

PHASE TRANSFORMATION AND SPALL FRACTURE IN LASER SHOCK‐LOADED IRON

Iron is known to undergo a polymorphic phase transition when shocked to pressures above 13 GPa. The details of the resulting wave structure shed light on the kinetics of the transformation. To explore the early stages of this wave structure evolution, we investigate the propagation of short compressive pulses over small distances in laser shock‐loaded iron foils of thicknesses ranging from 150 to 520 μm. Comparing time‐resolved velocity measurements to the predictions of a non‐equilibrium phase change model provides an estimate of the relaxation time governing the dynamics of the phase transition. This time is found to strongly condition the attenuation of the pressure pulse as well as the occurrence of spallation upon reflection of that pulse from the free surface. Metallographic examination of the recovered samples confirms the results inferred from the velocity profiles, in agreement with the model calculations. Finally, they show the very clear change in spallation response to the characteristic smoot...