Abstract
The nondestructive inspection of concrete structures is indispensable for ensuring the safety and reliability of aging infrastructures. Ultrasonic waves having a frequency of tens of kHz are frequently used to reduce the scattering attenuation due to coarse aggregates. Such low frequencies enable the measurement of the thickness of concrete structures and detection of layer-type defects, such as delamination, whereas it causes a lack of sensitivity to crack-type defects. In this paper, to realize the ultrasonic phased array (PA) imaging of crack-type defects, we fabricated a low-frequency (LF) array transducer with a center frequency of hundreds of kHz. To avoid the crosstalk between piezoelectric elements and dampen the vibration of each element, we adopted soft lead zirconate titanate (soft PZT) with a low mechanical quality factor. Subsequently, we optimized the geometry of each piezoelectric element using a finite element method to generate a short pulse. After validating the design in a fundamental experiment using a single-element transducer, we fabricated a 32-element array transducer with a center frequency of 350 kHz. To show the imaging capability of the LF array transducer, we applied it to a concrete specimen with a delamination. As a result, the PA with the LF array transducer clearly visualized the delamination, which could not be visualized using the PA with a 2.5 MHz array transducer. Furthermore, we applied it to a more challenging defect, a slit, which is sometimes used to simulate crack-type defects. As a result, the PA with the LF array transducer clearly visualized a slit of 1 mm width and 40 mm height in a concrete specimen. Thus, we demonstrated the usefulness of the LF array transducer for inspecting crack-type defects.
Highlights
The aging of concrete infrastructures, such as bridges, highways, and tunnels, is a vital problem worldwide
For the 2 mm wide transducer, a single pulse was observed in the upper waveform of Figure 5b, which is suitable for phased array (PA) imaging because of its excellent temporal resolution
For the 2 mmA7075 wide transducer, a single pulse was observed in the upper waveform of Figure 5b, which is suitable for PA imaging because of its excellent temporal resolution
Summary
The aging of concrete infrastructures, such as bridges, highways, and tunnels, is a vital problem worldwide. The excitation of piezoelectric elements of an array transducer generates thickness vibration and lateral vibration The former generates an ultrasonic wave in a sample and is used for PA imaging. For the 2 mm wide transducer, a single pulse was observed in the upper waveform of Figure 5b, which is suitable for PA imaging because of its excellent temporal resolution This shows that the soft PZT transducer without a backing layer is useful for dampening the LF vibration of the piezoelectric element. For the 2 mmA7075 wide transducer, a single pulse was observed in the upper waveform of Figure 5b, which is suitable for PA imaging because of its excellent temporal resolution This shows that theLaser soft PZT transducer without a backing layer is useful for dampening.
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