Key words Laser shock-wave, laser-driven foils, spallation, ultimate strength. PACS 41.75.Jv Direct laser interaction and laser-driven thin foils were used for investigation spallation phenomena in polymetyl metacrylate (PMMA) targets in case of high strain rate. The aluminium foils with thickness 8 and 15 mkm were used as impactors. Mass and velocity of the laser-driven foils after laser ablation and acceleration were determined by the method of foil deceleration in a gas atmosphere. Basing on experimental data, we determined the spallation plane position and the moment for the spall layer to arrive at an additional electrocontact sensor located beside the rear side of the target. Then the spall strength and strain rate have been calculated numerically using a hydrodynamic code based on a wide-range semi-empirical equation of state for PMMA. As a result of these experiments, we have for the first time shown that the ultimate spall strength of PMMA (about 10 kbar) is achieved in case of strain rate ranging 1.5×10 6 to 6×10 6
In this paper we present Hugoniot data for plastic foams obtained with laser-driven shocks. Relative equation-of-state data for foams were obtained using Al as a reference material. The diagnostics consisted in the detection of shock breakout from double layer Al/foam targets. The foams [poly(4-methyl-1-pentene) with density 130 > rho > 60 mg/cm3] were produced at the Institute of Laser Engineering of Osaka University. The experiment was performed using the Prague PALS iodine laser working at 0.44 microm wavelength and irradiances up to a few 10(14) W/cm2. Pressures as high as 3.6 Mbar (previously unreached for such low-density materials) where generated in the foams. Samples with four different values of initial density were used, in order to explore a wider region of the phase diagram. Shock acceleration when the shock crosses the Al/foam interface was also measured.
New results concerning the process of dynamic fracture of materials~spallation! by laser-induced shock waves are presented. The Nd-glass laser installations SIRIUS and KAMERTON were used for generation of shock waves with pressure up to 1 Mbar in plane Al alloy targets. The wavelengths of laser radiation were 1.06 and 0.53 mm, the target thickness was changed from 180 to 460 mm, and the laser radiation was focused in a spot with a 1-mm diameter on the surface of AMg6M aluminum alloy targets. Experimental results were compared to predictions of a numerical code which employed a real semiempirical wide-range equation of state. Strain rates in experiments were changed from 10 6 to 5 ϫ10 7 s Ϫ1 . Two regimes of spallation were evidenced: the already known dynamic regime and a new quasi-stationary regime. An ultimate dynamic strength of 80 kbar was measured. Finally, experiments on targets with artificial spall layers were performed showing material hardening due to shock-wave compression.
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