Jian X, Dixon S, Guo N, Edwards R S, Potter M
University of Warwick, Department of Physics, Ultrasonic Group, Coventry CV4 7AL, UK.
Ultrasonics. 2006 Dec 22;44 Suppl 1:e1131-4. doi: 10.1016/j.ultras.2006.05.153. Epub 2006 Jun 9.
This paper investigates Rayleigh wave interaction with machined slots on flat aluminium blocks to simulate surface breaking cracks. Using a finite element method, Rayleigh wave scattering by narrow slots of varied depth ranging from 0.5 mm to 20 mm is calculated. Pulsed wideband Rayleigh waves with a centre frequency of 590 kHz and -6 dB bandwidth of 520 kHz is considered. Reflection and transmission coefficients are calculated and compare well with the published literature. We and other workers have reported enhancement of the measured amplitude or particle velocity of an apparent Rayleigh wave close to a surface defect. In this paper, it is found that the predicted enhancement of in-plane components of particle velocities close to a crack is significantly higher than that of the out-of-plane components of particle velocities which appears to be mainly due to the mode-converted surface skimming longitudinal wave from the crack that has mainly in-plane components near the sample surface. The enhancement of the in-plane particle velocity will be observed regardless of the type of in-plane sensitive ultrasonic detector used. The explanation of the discrepancy of the reflection and transmission coefficients obtained by pulsed and narrow band or pseudo continuous Rayleigh waves is discussed. The later-arriving Rayleigh waves from reverberation along the inside of the crack surface are observed, as has been previously reported by other workers, and this may also be used to gauge slot depth.
本文研究瑞利波与平面铝块上加工槽的相互作用，以模拟表面开口裂纹。采用有限元方法，计算了深度从0.5毫米到20毫米不等的窄槽对瑞利波的散射。考虑了中心频率为590千赫兹、-6分贝带宽为520千赫兹的脉冲宽带瑞利波。计算了反射系数和透射系数，其结果与已发表的文献吻合良好。我们和其他研究人员曾报道，靠近表面缺陷处的表观瑞利波的测量振幅或质点速度会增强。本文发现，靠近裂纹处的质点速度面内分量的预测增强明显高于面外分量，这似乎主要是由于裂纹处模式转换的表面掠射纵波，其在样品表面附近主要具有面内分量。无论使用何种面内敏感超声探测器，都能观察到面内质点速度的增强。文中讨论了脉冲瑞利波与窄带或伪连续瑞利波所获得的反射系数和透射系数存在差异的原因。如其他研究人员之前所报道的，观察到了沿裂纹表面内部的混响产生的延迟到达的瑞利波，这也可用于测量槽的深度。
