Spall fracture and twinning in laser shock-loaded single-crystal magnesium

As a major failure process in materials subjected to dynamic loading, spall fracture is one of the most widely studied issues in shock physics. To investigate its dependence on the microstructure, including both initial and shock-induced features, laser shock experiments were performed on single crystal magnesium. Shock loading was applied in directions parallel and perpendicular to the c-axis of the crystals. Both the spall strength and the fracture surface morphology are found to depend on the direction of the shock application with respect to crystal orientations. The results complement data obtained previously over ranges of lower strain rates. A detailed analysis of the residual microstructure and crack patterns in the recovered samples shows strong correlations between damage localization and twins, both pre-existing and shock-induced. Thus, cracks match specific twinning directions, which is discussed on the basis of deformation mechanisms reported under quasi-static loading conditions, either prismatic slip or twinning depending on local orientations

On the shock-based determination of the adhesive strength at a substrate-coating interface

International audienceEvaluating the bonding strength at the interface between two layers is an issue of considerable practical interest for a wide variety of engineering applications involving coatings, such as thermal protective ceramics coated on engine blades. Spallation under laser driven shock loading is one of the experimental means to test interface debonding. However, numerical simulations are usually needed to infer a quantitative value of the bonding strength from such tests, where the coating free surface velocity is usually the only measurable variable. In this paper, the analysis of the propagation and interactions of compression and release waves leading to spall fracture in a shock-loaded material is detailed, then it is extended to a substrate-coating system. Different cases are considered, depending on the acoustic impedances of the substrate and coating materials and on the duration of the loading pressure pulse with respect to the wave transit time through the coating thickness. In each case, the interfacial strength can be analytically estimated from the velocity variations without resorting to numerical models

Spallation of glass materials under laser induced shocks

L'étude de l'écaillage des matériaux de type verre est rendue difficile par leur grande fragilité, qui entraîne la destruction quasi-totale des échantillons testés sous chocs conventionnels. La très courte durée des chocs générés par irradiation laser limite cette destruction, et permet une étude originale, basée sur l'examen post-mortem des cibles endommagées. L'écaillage se traduit par la formation d'une zone fracturée, dont l'épaisseur dépend des paramètres expérimentaux. Une description macroscopique de cette fragmentation est proposée. Un modèle phénoménologique simple, implémenté dans un code de calcul monodimensionnel, permet de prédire approximativement l'étendue de la zone fracturée dans une cible de verre sodo-calcique soumise à un choc-laser de courte durée.The investigation of spallation in brittle materials like glass is usually limited by the severe destruction of the specimens. In this paper, an original study of this phenomenon, based on the post-shock examination of damaged targets, is presented. A high-power pulsed laser is used to generate very short compressive pulses in glass plates. A fractured layer is observed near the free surface of the recovered samples. The influences of the experimental parameters upon the thickness of this layer are evaluated. Then, a simple macroscopic description for spallation in glass is proposed. The resulting model, implemented in a one-dimensional simulation code, can predict the extent of the damage induced in a soda-lime glass target by a laser-driven shock

Response of iron to two symmetric laser shocks

–Laser-driven shocks provide a means of studying the dynamic behaviour of solid materials at very high strain rates. Here, we investigate the effects of a new pulsed load, generated by the crossing of two symmetric laser shocks. Both surfaces of thin iron foils are irradiated simultaneously by two high-power laser beams, producing two compressive pulses of duration about 3 ns and amplitude about 10 to 60 GPa. When they cross each other in the central region of the sample, considerable increases of the pressure and the temperature are induced, leading to twin formation and phase transition. Then, the interactions of all the incident and reflected release waves which propagate inside the sample result in various types of spall damage, depending strongly on the sample thickness and on the shock pressure. All those effects have been observed in the recovered targets, and explained by a phenomenological analysis of wave propagation. The influences of various experimental parameters have been investigated. Finally, one-dimensional computations have been performed to test the ability of a simple constitutive model, including twin formation, phase transformation and spallation, to predict the observed results. A rough agreement between computations and experiments, better at lower shock pressure, bas been obtained

Wave propagation and permanent densification in a porous steel submitted to laser-driven shocks

Instrumented laser shock experiments have been performed to investigate the response of a sintered porous steel to uniaxial compressive loading at very high strain rates. The water-confinement technique has been used to increase both amplitude and duration of the laser shocks. Two steels of different initial porosities have been studied. Wave profiles have been measured with thick piezoelectric transducers stuck at the back of the steel targets. Residual compaction has been estimated by post-shock microscopic examination of the recovered samples. A simple constitutive material model, based on a macroscopic description involving the equation of state of the compact steel and a traditional P-α compaction model, has been adapted and introduced in a one-dimensional hydrocode to simulate the tests. A correct overall agreement has been obtained between measured and computed wave profiles, and the residual porosity distribution predicted by the model has been compared to that observed along the direction of wave propagation

Behaviour of metals at ultra-high strain rate by using femtosecond laser shockwaves

The mechanical behavior of materials under extreme conditions can be investigated by using laser driven shocks. Actually, femtosecond (fs) technologies allow to reach strong pressures over a very fast duration. This work is dedicated to characterize metals behavior in this ultra-short mode, (aluminum, tantalum), leading to an extreme dynamic solicitation in the target (>107s−1). The study includes the validation of experimental results obtained on the LULI 100TW facility by comparison with numerical model. Three modeling steps are considered. First, we characterize the pressure loading resulting from the fs laser-matter interaction, different from what happens in the classical nanosecond regime. Then, the shock wave propagation is observed through the target and particularly its pressure decay, strong in this regime. The elastic-plastic influence on the shock attenuation is discussed, particularly for tantalum which has a high elastic limit. Dynamic damage appears with spallation. Experimentally, spallation is characterized by VISAR measurements and post-test observations. Shots with different thicknesses have been carried out to determine the damage properties in function of strain rate. We show in this work that a simple instantaneous rupture criterion is not sufficient to reproduce the damage induced in the sample. Only the Kanel model, which includes damage kinetics, is able to reproduce experimental data (VISAR measurements, spall thickness). A generalization of this model to any strain rate can be performed by confronting these results to other shock generators data (ns laser driven shocks, plate impacts). One remarkable result is that every Kanel parameters follows a power law with strain rate in dynamic regime (105 to 108s−1) for both aluminum and tantalum

Measurement of the Effects of High-Pressure Laser Shocks on Metallic Targets

Piezoelectric measurements have been performed behind metallic samples submitted to short laser shocks of intensities up to 7 TW/cm2, using recently developed polymeric transducers. Some records exhibit a recompression indicating spall damage, confirmed by post-test microscopic observations. Computations involving a laser-matter interaction model and a wave-propagation code lead to a correct agreement on the gauge records. They indicate peak pressures of 7 to 150 GPa in the targets. A phenomenological spall model of the literature provides an approximated description of the relaxation associated with spallation, and a fair estimate of the damage level and location in the target.Des mesures piézoélectriques ont été réalisées à l'arrière de cibles métalliques soumises à des chocs laser brefs d'intensité pouvant atteindre 7 TW/cm2, au moyen de capteurs polymériques. Certains signaux comportent une recompression traduisant un écaillage des cibles, confirmé par un examen post-test. La simulation des expériences avec un modèle d'interaction laser-matière et un code de propagation est en accord correct avec les enregistrements. Elle indique des pressions maximales de 7 à 150 GPa dans les cibles. Un modèle phénoménologique d'écaillage fournit une description approximative de la relaxation associée à la rupture, et de la distribution de l'endommagement dans la cible

Laser Shock Waves: Fundamentals and Applications

International audienceHigh power pulsed laser (above 1GW/cm²) interaction with matter yields to very high amplitude pressureloadings with very short durations, initiating into solids a strong short shock wave. Compared to theconventional generators of shock (launchers of projectiles, explosives), these particular characteristicsoffer the possibility to study the behaviour of matter under extreme dynamic conditions, with a possiblerecovery of the shocked samples presenting the effects of the passage of the shock in most cases. Thisarticle introduces the principle of laser shock generation, the characterization of these shocks, principalmechanisms and some effects associated with their propagation in the solids (spallation, Laser AdhesionTest, geological applications)

Laser shock techniques to investigate shock-induced phase transitions in quartz

The very short duration of laser shocks and their low-destructive character provide new means of studying the kinetics of phase transitions. Here, we investigate the phase transformation behavior of quartz under dynamic compression of short duration (nanosecond order) generated by various laser shock techniques. VISAR measurements have been performed to characterize the stress history induced in the targets. Recovered samples have been analyzed by micro-Raman spectroscopy. The influences of several experimental parameters (pulse duration, tensile and shear stresses...) have been investigated. 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 new structure that can be compared to known orthorhombic high pressure phases