Study On Selective Generation Of Ultrasonic Modes By Moving Continuous-wave Laser

In this paper,the generation of ultrasound on different structure surfaces irradiated by the moving continuous-wave laser is studied.The bulk waves,Lamb waves and surface waves are generated in the volume medium,thin plate medium and linear thin film/nonlinear substrate system respectively.The selective excitation of ultrasonic modes is successfully realized.Meanwhile,the negative group velocity effect of S₁ mode and the propagation characteristics of surface wave in film/substrate system are separately studied by continuous-wave laser moving method,the switching of S₁ mode wave propagation direction and the excitation of non-dispersive surface waves are realized respectively.The selective generation of ultrasonic bulk wave modes by moving continuous-wave laser is studied theoretically and experimentally.Based on the wave equation and thermoelastic theory,a theoretical model of bulk wave induced by the moving continuous-wave laser irradiation on aluminum surface is established.Through the numerical research,it is found that the wavefront angleθof ultrasound inside the material,the moving speed of continuous-wave laser V and the bulk wave propagation velocity c_us satisfy a certain geometric relationship.In addition,not only can the wave modes be selectively excited,especially the excitation of single transverse wave,and under a specific laser moving speed circumstance,when the wavefront angle of ultrasound matches with the main propagation angle of this mode,the ultrasonic signals with amplitude enhanced obviously are generated,thus greatly improving the signal-to-noise ratio.Meanwhile,the experimental setup is built up to excite bulk waves by a moving continuous-wave laser,which could realize the excitation of single transverse wave.The selective generation of Lamb wave modes by moving continuous-wave laser is studied theoretically and experimentally.The excitations of single A₀ and S₀ mode are simulated by using an equivalent moving force source model.The energy distribution along thickness direction of the two lowest order modes are severally given under specific frequency thickness product,which provides a basis for the selective excitation of appropriate modes in subsequent experiments.The experimental system is established to generate Lamb waves by a moving continuous-wave laser,and the dispersion curves of Lamb wave can be obtained directly from the power spectrum of the time-domain signal recorded at each laser’s moving speed.Through the experimental research,it is found that the desired single mode of Lamb wave can be generated effectively by a moving continuous-wave laser,while other modes are suppressed,as long as the moving speed matches the phase velocity of the mode.Compared with the Lamb waves excited by a pulsed laser with at least two lowest order modes A₀and S₀existence at any given frequency,the method is more suitable to excite a single Lamb wave mode.The excitation of zero group velocity and negative group velocity effects of Lamb wave S₁mode in the polystyrene sheet by using a moving laser source is studied theoretically and experimentally.Based on the dispersion characteristics of Lamb wave and the negative group velocity existence condition of each higher order mode,the Lamb wave dispersion curve in polystyrene sheet is simulated by using eigenfrequency analysis method,and the frequency and phase velocity of S₁ mode at zero-group velocity point are given,which provide a basis in subsequent experiments.Experimentally,the whole process of S₁ mode propagation direction from forward to reversal at the frequency thickness product of 1.13MHz·mm is successfully observed by setting three different signal receiving positions and using the fixed phase velocityω/k method.The theoretical model of the non-dispersive surface wave generated in a linear film/nonlinear substrate system induced by a moving continuous wave laser source is established.The use of the moving laser source facilitates the thermoelastic generation of large amplitude waves,which is beneficial to the nonlinear excitation of surface wave.The numerical simulation results indicate that non-dispersive acoustic surface waves with waveform unchanged can be generated via the moving laser source approach for specific values of moving speed and thickness of the coated film,realizing the complete offset between the nonlinear effect and dispersion effect of ultrasound.The research results could provide theoretical and experimental basis for selective excitation of ultrasonic modes in laser ultrasonic.