Development of Low-Frequency Phased Array for Imaging Defects in Concrete Structures

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.

Keywords: concrete; crack-type defect; delamination; low-frequency phased array; ultrasonic imaging.

Figure 1

Structure of conventional array transducer with filling material and backing layer.

Figure 2

Proposed structure without filling materials or backing layer for LF array transducer.

Figure 3

Electrical impedance calculated by 2D FE analysis with various element widths and a fixed thickness of 4 mm. Element width: (a) 8 mm, (b) 6 mm, (c) 4 mm, (d) 3 mm, and (e) 2 mm.

Figure 4

Transducer composed of a single piezoelectric (C9) element with 4 mm thickness and 2 mm width and the spectra of the electrical impedance. (a) Schematic illustration; (b) photograph of the fabricated transducer; (c) electrical impedance spectra.

Figure 5

Transmitted waves of the 2 mm and 9 mm wide transducers. (a) Schematic of the experimental setup; (b) transmitted waves measured at the bottom of an aluminum-alloy sample using a laser Doppler vibrometer.

Figure 6

Fabrication of 32-element LF array transducer with the center frequency of 350 kHz. (a) Schematic showing the structure of LF array transducer; (b) photographs of the fabricated LF array transducer.

Figure 7

Electrical impedance spectra obtained by experiment and 2D FE simulation. The black curves represent the electrical impedance spectra of all of the fabricated LF array elements. The red curve represents the electrical impedance spectrum (Figure 3e) predicted by 2D FE simulation.

Figure 8

Concrete specimen with a delamination simulated with a Styrofoam plate (2 mm thickness, 80 mm width).

Figure 9

Experimental configurations for visualizing delamination in concrete specimen and imaging results obtained with LF and 2.5 MHz array transducers. (a,b) Experimental configurations for the LF and 2.5 MHz array transducers, respectively; (c,d) imaging results obtained with the LF and 2.5 MHz array transducers, respectively.

Figure 10

Concrete specimen with a slit (1 mm width, 40 mm height).

Figure 11

Experimental configurations for visualizing slit in concrete specimen and imaging results obtained with LF and 2.5 MHz array transducers. (a,b) Experimental configurations for the LF and 2.5 MHz array transducers, respectively; (c,d) imaging results obtained with the LF and 2.5 MHz array transducers, respectively.