Measurement of velocity distribution of Laser-generated Rayleigh wave on welded structure

Abstract

A new method based on laser-generated ultrasound and piezoelectric transducer (PZT) is proposed to measure the velocity distribution on welded metal structure. High-frequency Rayleigh waves are excited by the Nd: YAG pulsed laser and probed by self-made transducer. A serial of ultrasonic pulses can be detected on the surface of the sample by the transducer through the scan of the line source with translation stage. The waveform cross-correlation technique is applied to compute the propagation velocity of Rayleigh waves. Then analogically, a series of wave velocities at different positions are detected, by which the distribution of velocities is obtained. It is found that high frequency wave signals excited by laser line pulse can be probed effectively using the PZT, and results indicate that this method can provide the basis for precision detection with quick scanning and the reliable measurement of velocity distribution.Key words：Laser ultrasonic, Rayleigh wave, PZT, Wave detection INSTRUCTIONSurface acoustic waves (SAWs) which also called Rayleigh waves are of great interest for the potential application in the field of nondestructive testing (NDT) and evaluation (NDE). With the sample absorbing light energy of short laser pulses in particular on its surface which makes temperature rise and results in thermal expansion, the excitation of acoustic transients generates surface acoustic waves of higher frequency. Therefore the technology for detecting laser-introduced surface acoustic waves developed rapidly.The main effect of stress on the propagation of ultrasonic waves in the material is variation of wave velocity of propagation [1, 2]. Another effect, to a less extent, is the variation of the amplitude of the ultrasonic waves, which can be specified in terms of their attenuation [3]. Generally, the relative change of wave velocity due to stress is proportional to latter and the proportional coefficient is a material-dependent parameter known as the acoustoelastic coefficient [4].Among waves that propagate near the surface, the Rayleigh waves are particularly attractive because they propagate without radiation loss (in contrast to the surface skimming L-waves and SV waves) and their energy is concentrated within a layer of about one wavelength thick under the surface. Some studies have shown the sensitivity of the velocity of Rayleigh waves with stresses in different materials. The use of Rayleigh waves has been reported by Jassby and Saltoun [5] for applied stress measurements and by Husson [6,7] for residual stress measurements. Dequennoy et al. [8,9] discussed the effect of initial stresses on the propagation velocity of Rayleigh surface waves on steel rods. The distribution of Rayleigh waves was detected to study the residual stress in metal by Crecraft [2]. More recently, the velocity of Rayleigh waves has been measured by contact transducers, non-contact EMAT transducers, and laser