LASER-INDUCED GENERATION OF PURE TENSILE STRESSES

While short compressive stresses can readily be produced by laser abla tion, the generation of pure tensile stresses is more difficult. We de monstrate that a 90 degrees prism made of polyethylene can serve to pr oduce short and pure tensile stresses. A compressive wave is generated by ablating a thin layer of strongly absorbing ink on one surface of the prism with a e-switched frequency-doubled Nd:YAG laser. The compre ssive wave driven into the prism is reflected as a tensile were by the polyethylene-air interface at its long surface. The low acoustic impe dance of polyethylene makes it ideal for coupling tensile stresses int o liquids, In water, tensile stresses up to -200 bars with a rise time of the order of 20 ns and a duration of 100 ns are achieved. The tens ile strength of water is determined for pure tensile stresses lasting for 100 ns only. The technique has potential application in studying t he initiation of cavitation in liquids and in comparing the effect of compressive and tensile stress transients on biological media. (C) 199 7 American Institute of Physics.