Investigation of surface acoustic waves in laser shock peened metals

Abstract

Laser shock peening is a well-known method for extending the fatigue life of metal components by introducing near-surface compressive residual stress. The surface acoustic waves (SAWs) are dispersive when the near-surface properties of materials are changed. So the near-surface properties (such as the thickness of hardened layers, elastic properties, residual stresses, etc.) can be analyzed by the phase velocity dispersion. To study the propagation of SAWs in metal samples after peening, a more reasonable experimental method of broadband excitation and reception is introduced. The ultrasonic signals are excited by laser and received by polyvinylindene fluoride (PVDF) transducer. The SAW signals in aluminum alloy materials with different impact times by laser shock peening are detected. Signal spectrum and phase velocity dispersion curves of SAWs are analyzed. Moreover, reasons for dispersion are discussed.

Surface Roughness of Laser-Shock-Peened Aluminum Alloys and Its Effect on the Dispersion of Surface Acoustic Waves

“…The setup for generating and detecting SAWs is similar to that used in ref. 7. A Q-switched Nd:YAG laser with a wavelength of 1064 nm and a pulse duration of 10 ns full width at half maximum was used to excite SAWs.…”

“…However, laser shock peening is a noncontact method and the sample surface is usually covered with black paint, which can protect the surface from being damaged; as a result, the laser-shock-peened surface roughness is assumed much smaller than that obtained from conventional shot peening. Hence, in our previous paper, 7) we assumed that the surface roughness was sufficiently small to be neglected when discussing the SAW dispersion in a laser-shockpeened specimen. Recently, air-coupled ultrasound has proven to be an efficient noncontact surface roughness evaluation method, 8) and the numerical analysis of Rayleigh wave dispersion 9,10) and Lamb wave attenuation 11,12) due to surface roughness have been reported.…”