Numerical simulation of laser-generated ultrasound by the finite element method

Abstract

The results obtained from the finite element model of laser-generated ultrasound are presented in terms of temperature and displacement. According to thermoelastic theory, considering the temperature dependence of the thermophysical parameters of the material, the transient temperature field can be precisely calculated by using the finite element method; then, laser-generated surface acoustic wave forms are calculated in Al plates of various thicknesses. The elastic waves excited by a pulsed laser in a thin plate are typical Lamb waves, and the numerical results demonstrate that the surface vibration is mainly determined by the lower frequency components of the symmetric mode s0 and antisymmetric mode a0 of the lowest order in very thin plate materials. It is also indicated that, when the sample thickness increases, both the higher frequency components of the lower Lamb wave modes and the higher order Lamb wave modes should be considered. In a relatively thicker plate, the numerical model can still capture the significant details of the transient features taking place in the plates. The surface skimming longitudinal and shear waves are relatively stronger in the near field, distorting the observed wave form of the Rayleigh wave. These effects must be considered when the quantitative near field is applied to determining elastic parameters or residual stresses.

Vibration energy analysis of a plate for defect imaging with a scanning laser source technique

“…45,50 For small laser pulse energy, below the ablation regime, sudden thermal expansion of the surface of a material generates elastic waves. 45,48,49,[51][52][53][54][55][56] Here, considering the two generation mechanisms, elastic wave energy is discussed for point normal loading and dipole loading, as shown in Fig. 4.…”

“…Elastic waves are generated in a solid medium by a laser through one of two mechanisms: ablation or the thermoelastic effect. [42][43][44][45][46][47][48][49][50][51][52][53] For large pulse energy, a sudden laser energy input induces plasma emission from the surface of a material, and a normal force is instantaneously loaded as a reaction force of the plasma emission. 45,50 For small laser pulse energy, below the ablation regime, sudden thermal expansion of the surface of a material generates elastic waves.…”

Ultrafast magnetoelastic probing of surface acoustic transients

“…This velocity compares favorably with the Rayleigh SAW velocity of the glass substrates, 3100 m/s for soda lime glass. The upper branch, propagating at a velocity of 5590 ± 15 m/s, is the in-plane longitudinal acoustic wave, which has been termed previously as a surface skimming longitudinal wave (SSLW) [10], or the surface skimming bulk wave (SSBW) [11], an elastic excitation that has been used extensively for nondestructive material evaluation [12,13]. Again, the velocity is very close to the longitudinal sound velocity in glass (literature value 5400 m/s) due to the predominant concentration of elastic energy in the substrate.…”

Skin Thermal Effect by FE Simulation and Experiment of Laser Ultrasonics

“…Laser ultrasonics uses a short laser pulse as a remote ultrasound input to excite thermal expansion, and then induces ultrasonic waves on surfaces, which contains the information of elastic properties. This technology has been widely used in industry to detect the surface condition of metallic materials and obtain relative mechanical properties of coatings [1][2][3][4][5].…”

“…Laser ultrasonics uses a short laser pulse as a remote ultrasound input to excite thermal expansion, and then induces ultrasonic waves on surfaces, which contains the information of elastic properties. This technology has been widely used in industry to detect the surface condition of metallic materials and obtain relative mechanical properties of coatings [1][2][3][4][5].Laser ultrasonics has potential to quantify mechanical properties of skin for diagnosis and accurate assessment of skin diseases [6][7]. In skin laser ultrasonics, the study of thermal interaction between a laser pulse and skin becomes a very essential part for the following two reasons:…”