Acoustoelastic Effect in Stressed Thin-Film Structures

• Main Authors: Chun-Hung Chen, 陳俊宏
• Other Authors: Ching-Chung Yin
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/78590664757386372291

碩士 === 國立交通大學 === 機械工程系所 === 94 === Phase velocity change of the structural acoustic waves due to residual stresses of the elastic media is known as acoustoelastic (AE) effect. This thesis formulates AE effect in thin-film layered structures with the associated third-order elastic constants by modifying the AE theory for bulk acoustic waves. The single crystalline Ge films deposited on the Z-cut silicon substrate are investigated as numerical examples. Significant biaxial residual stresses induced in the films are caused by misfit between different lattices of the heterostructures. The residual stress has weaker influence on the phase velocities of lower modes of Lamb waves in anisotropic thin-film layered media. On the contrary, the measured phase velocity changes of higher modes of Lamb waves become more significant and are capable of determining the residual stress for practical applications. The experiments used laser-induced ultrasound technique and two dimensional Fourier transform method to measure phase velocities of Lamb waves traveling in metal foils under uniaxial tension. Two kinds of non-contact point-detecting methods, knife-edge technique and confocal Fabry-Perot interferometry, are employed to measure the waveforms. It is of difficulty to generate high-frequency ultrasound using either point-focus or line-focus pulsed laser. Experimental evidence indicates that phase velocity change of higher modes of Lamb waves resulted from the AE effect is not able to be observed in the current set-up.