Shen Zhong-Hua

Theoretical analysis and numerical simulation of the impulse delivering from laser-produced plasma to solid target

A plasma is produced in air by using a high-intensity Q-switch Nd:YAG pulsed laser to irradiate a solid target, and the impulses delivering from the plasma to the target are measured at different laser power densities. Analysing the formation process of laser plasma and the laser supported detonation wave (LSDW) and using fluid mechanics theory and Pirris methods, an approximately theoretical solution of the impulse delivering from the plasma to the target under our experimental condition is found. Furthermore, according to the formation time of plasma and the variation of pressure in plasma in a non-equilibrium state, a physical model of the interaction between the pulse laser and the solid target is developed. The plasma evolutions with time during and after the laser pulse irradiating the target are simulated numerically by using a three-dimensional difference scheme. And the numerical solutions of the impulse delivering from the plasma to the target are obtained. A comparison among the theoretical, numerical and experimental results and their analyses are performed. The experimental results are explained reasonably. The consistency between numerical results and experimental results implies that the numerical calculation model used in this paper can well describe the mechanical action of the laser on the target.

A study of surface breaking crack evaluating by using Scanning Laser Source-Dual Laser Source technique

A hybrid non-destructive surface breaking crack evaluation technique, combining a Scanning Laser Source (SLS) technique and Dual Laser Source technique is presented and studied by FEM method and experimentation. By using this technique, surface breaking crack information, such as location, depth and orientation can be measured in one measurement cycle. First, the SLS technique is employed to locate the crack, by detecting the characteristic changes in the amplitude content. Then the Dual Laser Source technique is utilised to quantify the crack depth and orientation. Numerical simulating results show good agreement with the experimental results. Furthermore, the diffracted and reflected ultrasonic wave modes are studied as a function of crack depth and orientation separately, in order to reveal the relationship between these ultrasonic wave components and the crack depth and orientation.

Influence of air pressure on mechanical effect of laser plasma shock wave

The influence of air pressure on mechanical effect of laser plasma shock wave in a vacuum chamber produced by a Nd:YAG laser has been studied. The laser pulses with pulse width of 10ns and pulse energy of about 320mJ at 1.06μm wavelength is focused on the aluminium target mounted on a ballistic pendulum, and the air pressure in the chamber changes from 2.8 × 103 to 1.01 × 105Pa. The experimental results show that the impulse coupling coefficient changes as the air pressure and the distance of the target from focus change. The mechanical effects of the plasma shock wave on the target are analysed at different distances from focus and the air pressure.

On the generation of laser-induced plasma acoustic waves

In this paper, the transition of laser plasma waves into laser plasma acoustic waves has been discussed first, and it is suggested that the weak limitation of laser-induced-plasma shock wave is an acoustic wave. Based on this idea, a definition of laser plasma acoustic wave is given, and this kind of acoustic waves for some materials has been obtained experimentally.

Theoretical imaging study of early caries by a laser induced Rayleigh wave

Vivo human teeth are scanned by computed tomography through three-dimensional (3D) reconstruction technology. Geometric models of teeth are built. Based on that, a physical model of the laser induced acoustic waves propagating in teeth is established, and the finite element method is used to solve this physical model. As the velocity of the Rayleigh wave is sensitive to the elastic modulus of the teeth, the parameters, such as the position, demineralization degree, depth, and morphology of the caries, can be evaluated by the velocity field of the Rayleigh wave, which propagate in teeth. Furthermore, by the frequency domain characters of surface acoustic waves, the depth of the caries region can be evaluated. Therefore, the 3D evaluation method is established to develop the nondestructive and quantitative detection of the early stages of caries.

Influence of low pressure on laser inducing leaky Lamb wave and Scholte wave at air–solid interface

A setup with Q-switched Nd:YAG laser inducing acoustic wave at air–solid interface and air-coupled optical deflection sensor was developed to research the influences of low air pressure on laser inducing leaky Lamb and Scholte waves at air–solid interface. The solid plate is settled in an airtight vessel abounded with standard air and its pressure can be adjusted by a vacuum pump. By experiments, the waveforms of laser induced interface acoustic waves, leaky Lamb and Scholte waves, under air pressures from 0.02 to 0.08 MPa with 0.01 MPa interval were first measured comprehensively. From the waveforms, we find that with increasing air pressure, the leaky energy of Lamb waves increase linearly and the amplitude of Scholte waves increase exponentially, but the velocity of Scholte waves decreases obviously.

The relation between composition in laser absorption region and ambient pressure

In this paper, the compositions in a laser absorption region can be determined from the experiment of laser impulse coupling. When the ambient pressure varies from 9325 to 33325Pa, the compositions are vapour and plasma; while from 35325 to 101325Pa, they are ambient air and plasma. By analysing the relation between the degree of compression and the ambient pressure, the compositions can be determined and the variation of plasma can be explained.