Study On The Measurement Of Dispersion Curves And The Inversion Of Elastic Constants Of Coating Materials Based On Acoustic Microscopy

With the continuous development of material science, various functional materialscontinue to emerge, but they are more or less influenced by the preparation process. Alot of new materials are geometrically limited, mostly in the form of thin film orcoatings. Furthermore, because of the special preparation process of compositematerials, they often exhibit anisotropic characteristics. Therefore, the traditionaldestructive mechanical testing methods will be unable to meet the testing requirements.In the non-destructive testing of the acoustic velocity measurements, V(z) curves,which are caused by the wave interference between directly reflected waves and leakyguided waves, contain many micro-structure and material information. Acousticmicroscope, as a wave velocity measurement tool, can be applied to detect the crystalstructure, elastic modulus, residual stress, mechanical properties of internal defects andso on. Thus, it makes acoustic microscope a more and more widely used technique inthe mechanical properties evaluation and quantitatively nondestructive testing method.This article is aimed on the development of the elastic parameter measurement andthe inversion method, using acoustic microscope system both for isotropic semi-infinite,sheet, coating materials, and for planar anisotropic semi-infinite materials. Themechanical properties of isotropic material parameters were multidimensionallyinversed. The research is carried out separately in the aspects of theoretical analysis,numerical calculations, transducer fabrications, experimental studies, and parameterinversions. The main contents include:(1) Based on the classical diffraction theory, using the method of angular spectrumanalysis, we propose a new V(z) curve formation method in the condition of lenslessfocusing acoustic field. The method, by calculating the lensless focusing sound field ofthe focal plane and its angular spectrum, deduces the mechanism of V(z) curveformation. The oscillation period of V(z) curves can be accurately calculated in theabsence of lens.(2) A series of lensless focusing ultrasonic transducers are developed based onPVDF/P[VDF-TrFE] piezoelectric materials, both for line focus and point focus ones.The lensless transducers are also successfully applied on the acoustic microscopesystem. The center frequency is up to60MHz, and the operating frequency is up to100MHz.(3) Through the analysis of V(z) curve characteristics, a modal identificationalgorithm based on two-dimensional Fourier transform is developed, which is calledV(f,z) analysis method. Especially in the dispersion curves measurements for thin plate,accurate detection of more than10Lamb wave modes is realized; in the surface wavemeasurements for semi-infinite anisotropic (100) silicon wafer, the measurement of surface wave propagation along different directions is succeeded. The experimentalresults and the theoretical results are in good agreement.(4) A new objective function based on the determinant of dispersion equationcoefficient matrix is proposed. It overcomes the modal identification constraints whichare the conventional inversion procedures based on the velocity differences betweenexperimental and theoretical calculated dispersion curves. A particle swarm-based-simulated annealing (PS-B-SA) optimization algorithm is established to achieve amulti-dimensional joint inversion. Finally, the PS-B-SA algorithm accurately predictsthe elastic constants of the materials...