Study On Finite Element Analysis And Signal Testing Methods Of Laser-generated Ultrasonic

Laser-generated ultrasonic detection overcomes the application limitation of conventional ultrasonic methods in severe environments for its noncontact property and has the advantages of high resolution in time domain, brand range in frequency domain, small effect area and easiness to scan. This thesis, aiming at the complex waves and high-level noise in laser-generated ultrasonic detection on composite materials, studies the wave characteristics and signal testing methods in materials through finite element simulation and experiments. The main contents are as follows:The coupling process of temperature and displacement field under thermoelastic mechanism is studied based on the laws of thermodynamics in solid materials. The laser-generated ultrasonic Lamb waves, surface waves and epicenter body waves are numerically obtained in metal material using the axisymmetric finite element model. The process of ultrasonic propagation and the relationship between the waveforms and material thickness as well as detection position change.Ignoring the ununiformity of the microstructure, the laser-generated ultrasonic characteristics of waveforms in the unidirectional fiber board are studied. Comparisons are made between results in transverse isotropic material and isotropic material, which show that in the fiber board, because of the anisotropy of material parameters, the ultrasonic generation and propagation parallel to the fiber direction differ obviously from that perpendicular to the fiber direction, mainly reflected in the signal speed, amplitude and mode dispersion.The inner and surface defects are simulated and their effect on ultrasonic epicenter and surface waves are studied. The echo equality with different defect positions and depths are discussed. The defect echo waveforms in the unidirectional fiber material are obtained along different direction and the advantage of scanning with laser line source is analyzed.The empirical mode decomposition (EMD) is applied to laser-generated ultrasonic signal testing. Based on the energy density of intrinsic mode functions (IMF), the principle of choosing the reconstruction start in the time-scale filtering process is discussed. The cause of aliasing between useful signals and noises in several IMFs are analyzed. In order to achieve the aliasing removal and thus improve the denoising ability, a time-windowing method with kurtosis test strategy is proposed. The method is then applied to a polluted finite element simulated signal. The result shows a remarkable improvement in signal equality with the method and the advantage is expected to be given full play in high noise levels.Based on the laser ultrasonic optical experimental platform, a signal conditioning and amplifying circuit is designed. A laser ultrasonic software system for A-scan and B-scan testing is developed using Lab VIEW to detect the parameters and defects in composite materials. The effect and self-adaption ability of the proposed signal denoising method is verified through experimental signal processing. The speed of epicenter body wave and Rayleigh wave propagating along the fiber direction in the unidirectional fiber board is calculated. The modulus of elasticity in the isotropic plane is estimated. Scanning on a fiber board with a flaw in the surface, the flaw location is determined by B-scan curves.