The Study Of The Laser-induced Ultrasonic Waves Propagating In Materials With Continuous Elstic Properties Changes Of Near-surface Layer

Surface treatments such as shot peening, surface heat treatment are usually used to improve the mechanical properties of metal materials in industry. The near-surface elastic properties of solids often differ from those of underlying bulk materials after suffering surface treatments. So the near-surface elastic can serve as criteria to evaluating the effect of surface treatment. In this dissertation, acoustic waves propagating in metals with inhomogeneous near-surface elastic properties have been studied theoretically and experimentally, which can provide the basis for the extract of the near-surface elastic properties by laser-induced acoustic waves.Based on the theory of thermoelastically generated ultrasound, laser-induced acoustic waves propagating in metals with inhomogeneous near-surface elastic properties have been studied by finite element method (FEM). According to thermal analysis, the transient temperature field has been precisely calculated; and then, laser-induced acoustic waves in metals with continuous changes of near-surface elastic properties have been obtained by FEM. The acoustic wave propagation on the materials with different near-surface layer depth and elastic constants have been calculated.The two-dimensional fast Fourier transform (2D-FFT) method has been used to analyze the ultrasonic signals where different wave modes overlaid in the time domain. The different wave modes have been distinguished and the phase velocities of these modes have been obtained. The effects of spatial variation of the material elastic properties and near-surface layer thickness on the acoustic wave modes and velocity have also been discussed.To avoid the present of pseudo-wave caused by the discretization of near-surface layer, an orthogonal polynomial expansion method has been developed to study the acoustic waves propagating in metals with continuous changes of near-surface elastic properties. The proposed extension has taken into account the inhomogeneity of near-surface elastic properties within the thickness of plate by expressing spatially varying properties by polynomials of an appropriate order. The acoustic fields have been expressed by linear combinations of orthonormal functions and the velocity dispersion of acoustic waves can be directly obtained by solving eigenvalue problem. The acoustic waves traveling through finite-thick and semi-infinite thick plates have been studied.An experiment setup based on polyvinylindene fluoride (PVDF) transducer has been built to detect the laser surface acoustic waves (SAWs) in laser-peened aluminum alloys. The phase velocities of surface acoustic waves have been obtained by the phase spectrum method. The effect of different laser-peened condition on the SAW velocity dispersion has been discussed. The experimental results indicate that larger spot diameter and more repeated impacts can cause better effect and bring more significant change of near-surface elastic properties. Futher more, the causes of dispersion have been further discussed.The research results can offer the theoretical and experimental foundation for the non-destructive evaluation of the metals with near-surface inhomogenous elastic properties by laser-induced acoustic waves.