科研之友
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摘要
Elastic wave anisotropy is deemed to be an intrinsic material property which is fully determined by the matrix of elastic moduli. One of the results of this study is the finding that this is not true for ultrasonic waves in plates whose anisotropy is shown to be a function of frequency. The "dispersion of anisotropy" occurs for plate waves in all anisotropic materials including fibre-reinforced composites. The physical reason for the dispersion of velocity anisotropy is concerned with frequency dependent wave structure which activates different elasticity for various propagation directions. To probe anisotropy of a certain stiffness component (e.g. Young's modulus), the plate wave fields should be kept similar by variation in operation frequency. The phase velocity anisotropy and its frequency dependence provide frequency variation of the beam steering angle for plate waves (dispersion of beam steering). This effect is important to be accounted for in various applications since the plate waves are widely used for non-destructive evaluation of composite materials. A strong frequency-dependent focusing of ultrasonic energy (dispersion of phonon focusing) occurs for a(0)-waves in unidirectional carbon fibre reinforced composite. The phonon focusing prevents omni-directional wave propagation that might pose a problem in the plate wave applications for structural health monitoring. The results presented indicate that the effect of plate wave anisotropy and its implications can be enhanced or diminished by matching the operation frequency.
- 出版日期2011-8
