Synthetic Aperture Focusing Technique in Frequency Domain for Inspecting Timber Structure Using Ultrasonic Array System

A frequency-domain synthetic aperture focusing technique for orthotropic media (FD-ORTHO-SAFT) is developed for the in-situ inspection of wood products. The developed algorithm uses the fast Fourier transform to manipulate an ultrasonic array signal in a frequency-wavenumber domain for fast computation speed. In order to take into account the orthotropic property of the media, the group velocity curve information of the media is applied to the developed algorithm. The FD-ORTHOSAFT algorithm is verified with the simulated ultrasonic array signal by the mathematical model and the finite-difference time-domain method. The performances of the algorithm with and without the noise are tested with the simulated signal by the mathematical model and the finite-difference time-domain method, respectively. The FD-ORTHO-SAFT algorithm is compared with the conventional time-domain synthetic aperture focusing technique for orthotropic media (TD-ORTHO-SAFT) in terms of the array performance index, performance map, and the computation time. The results of both algorithms are analyzed using the potential damage curve as well as the operating time. Also, the feasibility of the FD-ORTHO-SAFT is tested using measured ultrasonic array signals obtained by the commercial ultrasonic array device A1040 MIRA from wood specimens. The FD-ORTHO-SAFT algorithm shows a higher detectability than the synthetic aperture focusing technique algorithm for isotropic media and a faster computation speed than the conventional TD-ORTHOSAFT. The results show that the FD-ORTHO-SAFT has a better performance for wood products than the time-domain method at the center of the array and has strong suitability for the in-situ inspection of wood products as well as short computation time

3D Internal Visualization of Concrete Structure Using Multifaceted Data for Ultrasonic Array Pulse-Echo Tomography

This research proposes a 3D internal visualization using ultrasonic pulse-echo tomography technique to evaluate accurately the state of concrete structures for their efficient maintenance within a limited budget. Synthetic aperture focusing technique (SAFT) is used as a post-processing algorithm to manipulate the data measured by the ultrasonic pulse-echo technique. Multifaceted measurements improve the weakness of the existing ultrasonic pulse-echo tomography technique that cannot identify the area beyond a reflector as well as the area located far away from measuring surfaces. The application of apodization factor, pulse peak delay calibration and elimination of trivial response not only complements the weaknesses of the SAFT algorithm but also improves the accuracy of the SAFT algorithm. The results show that the proposed method reduces the unnecessary surface noise and improves the expressiveness of the reflector’s boundaries on the resulting images. It is expected that the proposed 3D internal visualization technique will provide a useful non-destructive evaluation tool in combination with another structure evaluation method

Pulse Peak Delay-Total Focusing Method for Ultrasonic Tomography on Concrete Structure

An ultrasonic array device like the A1040 MIRA is used to non-destructively visualize the inside of concrete structures. A data set acquired by the ultrasonic array device is so unfocused that an image reconstruction algorithm is required to transform the data set into an understandable image. The image reconstruction algorithm like total focusing method exploits the time-of-flight of an ultrasonic pulse when focusing the image. While a high frequency ultrasonic pulse barely affects the accuracy of results, a low frequency ultrasonic pulse with a long wavelength causes an overall sagging of the resulting image around half wavelength of the pulse, which results in a poor quality of results. In this research, a modified total focusing method called pulse peak delay-total focusing method is proposed to calibrate the sagging in the resulting images due to the long wavelength of the pulse. The simulation of an ultrasonic array signal is implemented to validate the proposed method. The experimental results are compared with the simulation results to validate the proposed method. The simulation using the proposed method shows good agreement with experimental results. Analysis of results using potential damage curve and array performance indicator shows that the proposed method allows the higher accuracy, as well as the increased resolution of resulting images