Finite Element Simulation Of Laser-Generated Ultrasound In The Transverse Isotropic Composited Material

In this paper,by using the finite element method(FEM),a numerical model of laser-generated ultrasound on a transverse isotropic composite material is established,and the propagation and waveform characteristics of laser-generated ultrasonic waves in the transverse isotropic composite material are studied numerically.Based on the theory of plane strain and the elastic equations of dynamic equilibrium,a numerical model of laser-generated ultrasound in frequency domain is established by using FEM.To verify the correctness of the finite element model in the frequency domain,a finite element model in the time domain is established for elastic material.The calculated results in the frequency domain are in good agreement with those obtained in the time domain.And then,the waveform characteristics of laser-generated Lamb wave on a thin viscoelastic plate are analyzed in the frequency domain.The results show laser-generated Lamb waves mainly include the lower frequency components of the symmetric mode S₀ and the anti-symmetric mode A₀.In addition,the amplitudes of Lamb waves are attenuated in the viscoelastic material,and the attenuation coefficients to different modes of the viscoelastic wave are different.A numerical model of laser-generated ultrasonic waves on the transverse isotropic elastic material is established in the frequency domain by using the FEM.The propagation and waveform characteristics of ultrasonic waves in the anisotropic and isotropic planes of the transverse isotropic elastic material are studied.Moreover,the effect of the different material thickness to ultrasound mode is analyzed in details. The results indicate the Lamb waves propagating in the direction normal to the fiber direction show clearer dispersive characteristics and the frequencies are higher than those propagating in the direction parallel to the fiber direction.However,the velocities of the ultrasonic waves propagating parallel to the fiber direction are faster than those propagating perpendicular to the fiber direction.Moreover, laser-generated Lamb waves are obtained on the thin plate.When the plate thickness increases so that the higher modes can propagate in the plate,a skimming surface longitudinal wave and a Rayleigh wave are formed at the plate surface.Taking account of the viscosity characteristic of the composite material,a finite element model for simulating laser-generated Rayleigh waves on the viscoelastic material is developed in the frequency domain. Based on the numerical calculations,the differences of Rayleigh waves in the viscoelastic material and the elastic material are compared,and the effects of the material viscosity to the characteristic of Rayleigh wave are analyzed.In addition,the effect of the different viscosity modulus to the characteristics of Rayleigh waves is studied in detail.The results show Rayleigh waves are attenuated and dispersed gradually in the viscoelastic material due to energy dissipation with the increase of source-receiver distance and viscosity modulus.Furthermore,the attenuation coefficients to different modes of the viscoelastic wave are different.This work will provide a useful guidance for the use of laser ultrasonic on the characterization of transverse isotropic composite material and complement the system info of laser ultrasonic.