Laser Generation of High-Amplitude Stress Waves in Materials

Originally published in Journal of Applied Physics, 50 (3), 1497-1502, (1979).

Authored by B. P. Fairand and A. H. Clauer.

Stress-wave environments generated at a confined surface by a pulsed laser were investigated. Experimental measurements and theoretical calculations demonstrated that confinement of the surface with a transparent overlay provided an effective method of generating high-amplitude laser-induced stress waves in the target material. Peak pressures approaching 10 GPa were possible at laser power densities of several times 109 W/cm2 for laser pulse duration ranging from several nanoseconds to several tens of nanoseconds. These pressures were generated in an air environment at standard conditions, thus enhancing their practical utilization for processing of materials and measurements of material properties. At laser power densities greater than 109 W/cm2, the laser-induced stress-wave environment was controlled by properties of the ionized plasma created near the metal surface. Some enhancement in the amplitude and duration of laserinduced stress-wave environments was observed at laser power densities less than 109 W/cm2 if low thermal conductivity and low heat of vaporization materials were used. Calculations show that peak pressures greater than 10 GPa were possible by superimposing stress waves either through reflection off a high acoustic impedance barrier or through the interaction of stress waves which were generated at different surfaces of a material.
PACS numbers: 79.20.Ds, 62.50.+p, 42.60.-v

I. INTRODUCTION
The generation of high-amplitude stress waves with short bursts of laser radiation was first investigated a few years after the first laser became operational.1-4 These early studies predicted that high amplitude stress waves could be generated in materials by impinging the laser beam on an unconfined surface of the body and vaporizing a small amount of surface material. Later work which involved direct measurements of pressure showed this was not the case and peak pressures typically were less than 1 GPa.5 Subsequently, methods to enhance the pressure environments over the free-surface conditions by modifications in the target surface conditions proved to be successful.6-11 Our interest in this area was stimulated by the need to generate pressures greater than 1 GPa in order to produce significant changes in the in-depth microstructures and mechanical properties of metal alloys.12-14 A comprehensive understanding of the stress-wave environments needed to produce these changes is required for their effective application in altering the properties of materials. This paper presents our studies of laserinduced stress-wave environments with particular attention given to methods of enhancing the magnitude of the stress waves over free-surface conditions. The effects on stress-wave environments from placing transparent confining media on the target surface and addition of absorbent films to the surface are treated. Our results are based on experimental measurements of pressure and theoretical calculations using a one-dimensional radiation hydrodynamic computer code.

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