No abstract

A finite element method for simulation of lens-less line focus acoustic microscopy is proposed in this paper to nondestructively evaluate the leaky surface wave (LSW) velocity. The defocusing measurement model is established, in which the geometrical focusing radius will be 20 mm. The piezoelectric polyvinylidene fluoride film is selected as the active element. The excitation is a standard black Harris wavelet signal with a centre frequency of 5 MHz. Simulations of measurements on typical bulk materials (Al) are carried out. Then, the time-resolved wave signal series are acquired when the defocusing distance varies continuously. The LSW velocity will be easily determined by the examination of the slope of the LSW's arrival time versus the defocusing position. The LSWs' propagating path will be analyzed geometrically in time-domain. Meanwhile, the LSWs' velocities are also extracted by applying the specially developed digital signal processing algorithm to the defocusing experimental data, which is called V (f, z) analysis method based on two-dimensional fast Fourier transform. Finally, the relationship between the time-resolved method and the V (f, z) technique is discussed, in which the interpretation of the formation of surface waves and the description of its analysing methods will be given.