Researches On Frequency Domain Characteristics Of The Laser-induced Ultrasonic Waves In FGMs

In this paper, propagation characteristics of laser-induced ultrasonic waves in functionally gradient materials are studied, the effects of elastic properties distribution of functionally graded materials on propagation characteristics of ultrasonic waves are systematic discussed, the relationship of phase velocity dispersion curves and functionally graded materials’elastic constants, density, thickness and porosity are also established, and the problem of ultrasonic waves generated by laser in solids are theoretically discussed.Laser ultrasonic force source excitation model in materials is built. Based on reciprocity relations and eigenfunction expansion method, ultrasonic guided waves in materials under laser loading are studied, vertical displacements with the different distances from receiving point to the center of force source on the iron plate surface are obtained, the effects of different receiving distances on ultrasonic guided waveforms are analyzed.Ultrasonic propagation theoretical model in functionally graded materials with finite thickness is built. Taylor expansion method is used to solve the situation that the distribution function is an arbitrary function. Using eigenfunction expansion method, the acoustic fields can be expressed by linear combinations of orthonormal functions, phase velocities of Lamb waves can be directly obtained by solving eigenvalue problem. The effects of elastic properties distribution on propagation characteristics of Lamb waves in functionally graded materials are discussed.Ultrasonic propagation theoretical model in functionally graded materials with semi-infinite thickness is built. Due to the manufacture craft during the production of functionally graded materials, pore may exist in samples, which will have an effect on materials’mechanical properties. Therefore, ultrasonic propagation characteristics in porous functionally gradient materials are studied. With the help of eigenfunction expansion method, the ultrasonic waves’dispersion curves in functionally gradient materials with different porosity, sample’s thickness and material properties’distribution are calculated. The theoretical result shows that phase velocity dispersion curves of ultrasonic waves can simultaneously reflect materials’elastic constants, density, thickness and porosity.An experiment setup based on PVDF has been built to detect the laser surface acoustic waves on the upper surface layer and base layer of iron-based alumina functionally graded materials. With the help of two-dimensional Fourier transform method, surface acoustic waves are analyzed and phase velocity dispersion curves of surface acoustic waves in functionally graded materials are obtained. The experimental result shows that porosity and elastic constants and density of the sample will influence phase velocities of surface acoustic waves together, phase velocity dispersion curves of surface acoustic waves can reflect elastic properties distribution of functionally graded materials.Results of this paper may provide theoretical and experimental basis for inversing elastic properties which are varied along the thickness direction of functionally graded materials.