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Tham khảo tài liệu 'wave propagation 2011 part 16', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Wave Propagation and Dynamic Fracture in Laser Shock-Loaded Solid Materials 429 between the measured and calculated velocity profiles except for the final oscillations corresponding to wave reverberation through the spalled layer much too strong in the computations. This discrepancy points out the limitations of the very simple failure criterion not accounting for the damage dynamics. Peak velocity is found to depend strongly on the aspect ratio since it is almost twice higher for the 4 mm-spot than for the 1 mm-spot. This is due to the additional attenuation of the pressure pulse by the lateral release waves when they reach the central zone before the shock breaks up at the free surface see Fig. 7 . A 1D simulation has been performed with the same pressure load and target thickness. It is found to match exactly the 2D simulation for the smaller aspect ratio L D 0.25 which shows that edge effects have not affected wave propagation along the central axis in that case. Fig. 12. Comparison of the velocity profiles measured solid lines and calculated dotted lines at the free surface of 1 mm-thick aluminium samples subjected to laser shocks of 50 GPa applied on spots of diameter 4 mm L D 0.25 or 1 mm L D 1 . The diameter of the spalled zones measured in the free surfaces shown in Fig. 8 is plotted in Fig. 13 for various loading pressures and both aspect ratios compared to the results of 2D simulations. A good consistency is obtained between experiments and calculations. A zero diameter means that no spallation could be observed. In the computations this absence of spall damage is represented by the vertical dashed line. In a 1D geometry one would expect a spalled zone of same diameter as the laser spot. This is observed for loading pressures of 40 GPa with L D 1 D 1 mm and about 60 GPa with L D 0.25 D 4 mm . Above those pressure values the spalled zone is larger than the loaded spot e.g. Fig. 9b because of the effects of the spherical compressive waves propagating .
