Theoretical And Experimental Study Of The Laser-induced Ultrasonic Waves At Fluid-solid Interface And Curved Surface

In this paper, based on the thermoelastic mechanism of laser ultrasonic, the laserinduced ultrasonic waves on the liquid-solid interface and curved (cylinder and hollowcylinder) surfaces have been studied theoretically and experimentally.Based on the mechanism of laser ultrasonic, the excitation processes of laserultrasonic in liquid and solid are analyzed, and then the coupling problem of ultrasonicwaves at liquid-solid interface is studied with emphasis, and the boundary equation isgiven. Combined the thermal conduction equation with Navier-Stokes equation, thetheoretical model for laser-induced liquid-solid interface waves is developed.Using the finite element method, a finite element model of the laser excitation ofliquid-solid interface waves is developed. The finite element forms of thermal conductionequation, Navier-Stokes equation in solid and interface coupling equation are deduced; therelation of element size and time step is solved to give their determine criterion; theinfluence of numerical dispersion on the correction of simulation results is discussed; afinite element procedure is complied.By using the finite element method, the interface waves induced by pulse laser actionat different liquid-solid interfaces, including air-aluminum, water-aluminum plane interfaceand air-aluminum cylindrical interface were simulated, and the propagation characteristicsof interface waves were analyzed.Two kinds of optical deflect detection methods are developed. Based on thosemethods, detection sensor for the leaky waves detection at fluid-solid interface was alsodeveloped. By experimental measurements, we obtained good waveforms of the leakyRayleigh waves and the leaky Lamb waves at different fluid-solid interface. Theexperimental signals are in good agreement with theoretical results.Appling the finite element method, the laser-induced ultrasonic waves in cylinder andhollow cylinder were studied. The propagation characteristics of cylindrical Rayleigh waveand circumferential wave are analyzed. The results show that cylindrical Rayleigh wave isdispersive, and with the diameter of cylinders decreasing, the dispersion of cylindricalRayleigh wave becomes more serious; there is a new mode in the circumferential wave,which relate to the curvature of hollow cylinder. Those works will provide the reference for the application of laser ultrasonic in the nondestructive testing of cylinder and hollowcylinder.Because there are few studies on the laser-induced interface waves at liquid-solidinterface, ultrasonic waves in cylinder and hollow cylinders, and their applications innondestructive testing, we have studied those problems comprehensively in thisdissertation. This work will provide a useful guidance for the use of laser ultrasonic on thecharacterization of material and complement the system info of laser ultrasonic.