Ling Yuan

Experimental and Numerical Study of Mode Transformation and Energy Attenuation of Wedge Waves Generated by Laser Ultrasound Technique

This research focuses on measuring the mode transformation and energy attenuation of wedge waves by the laser ultrasound technique. First, the finite-element method was used to simulate the laser-induced wedge waves and different orders of wedge-wave modes with nondispersion could be clearly observed. Then the pulsed laser excitation and optical deflection beam method for detection was developed to investigate the wedge waves experimentally. Plenty of wedge waves at different locations were recorded by scanning the excitation laser along the wedge tip, and different orders of wedge waves were observed. By fixing the distance between the excitation and detection position and scanning the samples along the direction normal to the wedge tip, the mode transformation process was achieved. Finally, it was found that the energy of acoustic waves decreases exponentially to a steady energy of the Rayleigh wave within the main wavelength range of these modes.

Experimental and numerical studies for nondestructive evaluation of human enamel using laser ultrasonic technique

In this paper, a nondestructive laser ultrasonic technique is used to generate and detect broadband surface acoustic waves (SAWs) on human teeth with different demineralization treatment. A scanning laser line-source technique is used to generate a series of SAW signals for obtaining the dispersion spectrum through a two-dimensional fast Fourier translation method. The experimental dispersion curves of SAWs are studied for evaluating the elastic properties of the sound tooth and carious tooth. The propagation and dispersion of SAWs in human teeth are also been studied using the finite element method. Results from numerical simulation and experiment are compared and discussed, and the elastic properties of teeth with different conditions are evaluated by combining the simulation and experimental results.

Experimental study of functionally graded materials of Fe/Al2O3 compound coatings on the steel substrate by using the laser ultrasound method

In this paper, the propagation characteristics of surface acoustic wave (SAW) and longitudinal wave in the iron-based alumina functionally graded materials (FGMs) were studied by using laser based ultrasonic technology, and then the ultrasound signals were analyzed to evaluate the properties of FGM. SAW and longitudinal wave were excited on FGM by using Nd:YAG pulsed laser and then detected by Polyvinylidene Fluoride (PVDF) transducer and laser interferometer, respectively. By analyzing the dispersion of SAW in the FGM the surface properties of materials were evaluated, and the internal nature and layered structures of FGM were determined by analyzing longitudinal wave and echo signals. The experimental results validate the correlation properties of FGM and demonstrate the feasibility and validity of the method by using SAW and longitudinal wave.

Experimental and numerical investigations of wedge waves and its dispersion behaviors propagating along wedges

In this paper, both numerical simulation employing finite element method (FEM) and experimental work by using laser ultrasound technique was done to investigate the wedge wave modes in wedges with different apex angles. We focused not only on the wedge waves propagating along the wedge tip, but also the dispersion characteristics of the wave modes. Firstly, finite-element method was used to simulate the laser induced wedge waves and different orders of wedge wave modes could be clearly observed for different apex-angle wedges with non-dispersion. Then pulsed laser excitation and optical deflection beam method for detection was built to investigate the wedge waves experimentally. The wedge waves at different positions were recorded by scanning the excitation laser source along the wedge tip and the dispersions and phase velocities of the wedge waves were obtained by using 2D Fourier transformation method. The numbers of wedge wave modes were observed on wedges with different apex angles is in good agreement with Lagasses empirical formula and our numerical simulation.

Theoretical analysis of acoustic waves propagating in materials with continuous variations of near-surface elastic constants

The near-surface elastic properties (NSEPs) of materials take on continuous distribution after surface modification such as surface polishing. In this paper, the influence of the continuous variations of NSEPs on the ultrasonic waves propagating in finite-thick and semi-infinite-thick plates is investigated. An orthogonal polynomial extended method is presented to study the acoustic waves propagating in metals with continuous distribution of NSEPs. By this method, the near-surface layer of the materials does not need to be discrete sublayers, which can avoid the presence of pseudoacoustic waves. The velocities of different ultrasonic wave modes in finite-thick and semi-infinite-thick plates with continuous distributions of NSEPs are obtained. In addition, the influence of the near-surface layer’s thickness and the distribution of the NSEPs on the relative velocity dispersion of Lamb waves and surface acoustic waves are discussed, providing further theoretical foundation for the inversion of near-surface p...

Theoretical and experimental study of wedge wave mode transformation and energy attenuation

Wedge Waves are guided acoustic waves propagating along the tip of wedge. In this paper, both theoretical and experimental work are done to research the wedge wave modes transformation and energy attenuation. Firstly, finite-element method (FEM) was used to simulate the laser induced wedge waves and different orders of wedge wave modes and the mode transformation process were clearly observed. Then pulsed laser excitation and optical deflection beam method for detection was built to investigate these characteristics experimentally. Plenty of wedge waves at different locations were recorded by scanning the excitation laser along the wedge tip, and different orders of wedge waves were observed. By fixing the distance between the excitation and detection position and scanning the samples along the direction normal to the wedge tip, the modes transformation process was obtained. Finally, theoretical solution by the method of potentials is used to explain the principle of mode transformation and energy attenua...

The numerical simulation of laser-generated ultrasound to detect the debonding in the honeycomb sandwich board

Among complex materials, honeycomb sandwich structure has a lot of advantages. However, it is usually found poor bonding or even debonding. Some convenient nondestructive methods should be found to measure the defect efficiently. Laser-generated ultrasound detection is a new nondestructive method with a bright future. Compared with traditional methods, the advantages of non-contact and high resolution in spatial and temporal made it applicable not only in measuring defects with high precision, but also in the characterization of various complex shapes. In this paper, to detect the debonding of the honeycomb sandwich board, the laser-generated ultrasound technology was used. By the use of the finite element method (FEM), a 2D model dealing with laser-generated ultrasound in the honeycomb sandwich board was presented. Take into account of the debonding problem, by adopting the scanned laser source, the propagation of Lamb wave and longitudinal wave were studied. The debonding in the honeycomb sandwich board was assessed though the characteristic of the Lamb wave and longitudinal wave in time domain and frequency domain.