Synthetic Aperture Imaging of Acoustic Linearity and Nonlinearity

Abstract

In the nondestructive testing field, ultrasonic imaging techniques have been used to detect flaws. Since the advent of array probes and automatic transducer moving systems, synthetic aperture imaging (SAI) has been widely implemented. Especially, synthetic aperture focusing technique (SAFT) and total focusing method (TFM) have improved the resolution of ultrasonic images, which contributes to an increase in the probability of detection (PoD). However, these conventional SAI techniques have still two weaknesses: the trade-off in resolution enhancement according to direction and the insensitivity to closing interfaces which are related to acoustic nonlinearity. In order to enhance both the resolutions simultaneously and visualize the closing interface, this thesis proposes hybrid SAI and synthetic aperture imaging of acoustic nonlinearity (SAIAN), respectively. The hybrid SAI method is proposed to compose the results reconstructed by the SAFT and TFM for high axial and lateral resolutions. For this, the image reconstruction algorithms for the SAI and hybrid SAI were developed and verified through finite-element method (FEM) simulations as well as experiments. Especially, the algorithms considered the sensitivity of ultrasonic transducers and the directivity and attenuation of the excited ultrasounds to maximize image quality. The simulation results showed the complementary superiority of the two conventional SAI techniques, SAFT and TFM, according to whether the defects were distributed vertically or horizontally. This feature was similarly observed during the experiments using an array probe system and the specimens having the artificial defects identical to those in the simulations. In order to highlight the reflectors in both high resolutions regardless of the distributed direction, the hybrid SAI through the summation or multiplication of the images reconstructed by SAFT and TFM was developed; its effectiveness was verified by simulations and experiments. The hybrid SAI reduced the noise significantly and improved the accuracy in defect sizing. To detect closing interfaces, contact acoustic nonlinearity (CAN) at the closing interfaces was measured by simulations and experiments. The CAN phenomenon is due to clapping at solid-solid interfaces under high contact pressure or the tip of a micro-crack by an incident wave. Its theoretical derivation and numerical analysis were conducted to investigate the CAN effect according to the characteristics of the incident wave. From the FEM simulations and experiments for the specimens having the closing interfaces, the harmonic generation caused by the CAN phenomenon was observed; the CAN parameter was obtained by frequency analysis. The results support that the CAN measurement is effective to detect the closing interfaces and increase the accuracy in interface sizing. As the convergence of SAI and CAN measurement, the SAIAN transform is proposed to realize the SAI of the closing interfaces with high resolution. This transform converts a tone-burst signal into a pulse-like acoustic nonlinearity parameter signal. Through the SAIAN, the pulse-like CAN parameter signals are extracted from the tone-burst signals measured by full matrix capture. The converted data are directly used for the SAI. In the results of simulations and experiments, the closing interfaces were visualized with high resolution; however, they were not shown in the images of linear ultrasonic characteristics. In this thesis, the hybrid SAI by combining the results of SAFT and TFM was developed to take the advantages of both SAFT and TFM in terms of resolution enhancement. The SAI was applied to visualize the CAN via the proposed SAIAN transform in order to reconstruct the closing interfaces with high resolution. Applying the hybrid SAI and SAIAN can improve the PoD of micro-cracks and accuracy in crack sizing, which enhances the level of structure health monitoring and the quality control in manufacturing process. Furthermore, the SAIAN is now ready for its application to the detection of real micro-cracks occurred in service. Although the simulations and experiments were carried out in this study to verify the effectiveness of the SAIAN for closing interface visualization, further optimization is necessary to visualize the actual micro-cracks in diverse circumstances. For this reason, the proposed SAIAN should be applied to visualize the closing cracks in the real parts provided by specialized agencies such as Nuclear Regulatory Commission (NRC), Korea Hydro & Nuclear Power (KHNP) Co. Ltd., Korea Atomic Energy Research Institute (KAERI), Korea Institute of Nuclear Safety (KINS), and Korea Research Institute of Standards and Science (KRISS). Through trials and researches for the field applications, the completeness and feasibility of the SAIAN can be maximized.