Total Focusing Method or Phased Array Technique: Which Detection Technique Is Better for the Ultrasonic Nondestructive Testing of Concrete?

摘要

Concrete is a composite material composed mainly of water, sand, aggregate, and cement. The characteristics of high attenuation of concrete for ultrasound degrade the performance of ultrasonic testing (UT), especially when the size of the target is much smaller than the aggregate. Thanks to recent improvements in electronic devices and computing technology, the total focusing method (TFM) and the phased array (PA) technique have been applied to facilitate UT for concrete. However, the question of which approach is better for the UT of concrete has not yet been addressed. Thus in this preliminary study, a synthetic PA image, which is similar to an image acquired by the PA system, and a synthetic TFM image, which is similar to an image acquired by the TFM system, were made for the UT of concrete. The qualities of these images were compared with those produced by the traditional B-scan and the synthetic aperture focusing technique (SAFT). Excluding the expense, difficulty, complexity, and efficiency of these techniques, the results of this study show modern (TFM and PA) techniques are always better than traditional (B-scan and SAFT) methods based on 100 kHz ultrasonic experiments with a low signal-to-noise ratio (SNR) condition. The SAFT method can only slightly improve the quality of the B-scan image. In addition, a moderate distance between the source and receiver is suggested for detecting shallow and small targets in concrete in order to avoid near-source backward scattering. However, the PA technique has superior performance in detecting a small target at a specific depth in concrete. On the other hand, while detecting a large target in concrete, the TFM shows better performance than the PA technique. Study results indicate both the PA and the TFM have their own merits and demerits. The PA shows good performance in the status of a low SNR environment, and the TFM works better in a high SNR situation. In conclusion, to detect a small target at a specific depth in concrete, the PA technique is recommended. Otherwise, to detect a large target in concrete, the TFM is a better choice.