A nonlinear acoustic technique for detection of contact defects in plates was devel- oped associating guided waves with vibro-modulation techniques. The low frequency vibration varies the interface area within the contact defect; then the amplitude and phase of high fre- quency guided waves reflected from the contact defect will be modulated. Due to the exiting of contact defects, there was obvious low frequency component in the time-frequency distribution of modulated guided wave series. Furthermore the nonlinear responses at frequency of vibra- tion were extracted out and used for defect imaging. It was shown that the nonlinear guided wave imaging method based on vibration modulation can effectively characterize and locate the nonlinear contact defects.
JIAO JingpinHE CunfuWU BinB. W. DrinkwaterS. A. NeildP. D. Wilcox
The fundamental shear horizontal（SH0） wave has several unique features that are attractive for long-range nondestructive testing（NDT）. By a careful design of the geometric configuration, electromagnetic acoustic transducers（EMATs） have the capability to generate a wide range of guided wave modes, such as Lamb waves and shear-horizontal（SH） waves in plates. However, the performance of EMATs is influenced by their parameters. To evaluate the performance of periodic permanent magnet（PPM） EMATs, a distributed-line-source model is developed to calculate the angular acoustic field cross-section in the far-field. Numerical analysis is conducted to investigate the performance of such EMATs with different geometric parameters, such as period and number of magnet arrays, and inner and outer coil widths. Such parameters have a great influence on the directivity of the generated SH0 waves that arises mainly in the amplitude and width of both main and side lobes. According to the numerical analysis, these parameters are optimized to obtain better directivity. Optimized PPM EMATs are designed and used for NDT of strip plates. Experimental results show that the lateral boundary of the strip plate has no perceivable influence on SHO-wave propagation, thus validating their used in NDT. The proposed model predicts the radiation pattern ofPPM EMATs, and can be used for their parameter optimization.
MA QingzengJIAO JingpinHU PingZHONG XiWU BinHE Cunfu
