Application Cases

Experiment Name: Rebar Debonding Damage Identification Experiment

Research Direction: Nondestructive Testing

Test Objective:
Using piezoelectric guided wave testing technology to investigate the corrosion problem of steel bars in reinforced concrete structures, a basic theory and method for steel bar corrosion detection based on piezoelectric ultrasonic guided waves and continuous wavelet transform technology is proposed. Experimental and simulation methods are employed to study the issue of steel bar corrosion debonding. The results show that the proposed basic theory and method for steel bar corrosion detection based on piezoelectric ultrasonic guided waves and continuous wavelet transform can identify the location and extent of corrosion damage in steel bars, featuring convenience, speed, high accuracy, and wide detection range, thus meeting the requirements for corrosion detection of steel bars in reinforced concrete structures.

Testing Equipment: ATA-2041 voltage amplifier, function generator, digital oscilloscope, computer, piezoelectric ceramic patches, test specimens.

Experimental Procedure:
The experiment uses a PZT-4 piezoelectric ceramic patch with integrated transmitting and receiving functions, having a diameter d = 20 mm and thickness b = 2 mm. To ensure the accuracy of experimental data and corrosion damage identification, in addition to determining the PZT attachment method and model, a  supporting  corrosion debonding damage identification system was established, as shown in the figure below.

Figure: Corrosion Damage Identification System

The experimental procedure is as follows: First, a five-peak wave signal is excited by the function generator in the form of an electrical signal. The excited original  signal is then amplified by the amplifier. The amplified current is transmitted through the exciting-end PZT to the steel bar in the form of mechanical displacement. The signal generates mechanical waves due to oscillation, which then propagate within the steel bar as ultrasonic guided waves. Subsequently, the signal is received by the receiving-end PZT on the steel bar, which converts the mechanical displacement back into an electrical signal displayed on the digital oscilloscope via wires. Finally, the data is processed on the computer.

Figure: Experimental Setup Arrangement

First, a filtering program is written using MATLAB mathematical programming software to filter the extracted experimental signals. Based on the center frequency of the excitation signal used in the experiment, the high and low frequency limits of the filtering program are set to retain the signal waveform within the useful frequency band. The following figure shows a schematic diagram of the sensing signal for a healthy specimen.

Figure: Sensing Signal Diagram for Healthy Specimen

According to the figure above, the sensing signal propagates from one end of the steel bar to the other, and then the reflection from the receiving end travels back to the receiving end via the excitation end. The propagation path length between the first wave and the refracted wave is twice the length of the steel bar, i.e., 2 m. The propagation time difference between the first wave and the refracted wave is 0.399 × 10⁻⁴ s. From the dispersion curve of the bare steel bar, the propagation velocity of the 15 kHz frequency signal is 5136.5 m/s. Due to the influence of various factors such as instrument precision , operation, and environment in the experiment, certain errors may exist. Therefore, if the error between the experimental value and the theoretical value is within 10%, the dispersion curve is considered correct. In summary, the propagation time for the 15 kHz frequency signal over a 2 m path according to the dispersion curve is 3.89 × 10⁻⁴ s, and the error compared to the experiment is 2.5%, far less than 10%. Based on this, other conditions  of the bare steel bar were verified, as shown in the following figure, which is a sensing signal diagram for a corroded steel bar specimen.

Figure: Sensing Signal Diagram for Corroded Specimen

Experimental Results:
According to the figure above, when corrosion debonding (PVC soft adhesive) damage occurs at the  middle position  of the steel bar, the signal received by the receiving end shows partial debonding damage waveforms between the first wave and the echo. Additionally, the propagation time of the echo is longer compared to that of the undamaged bare steel bar. Analysis indicates that the echo in the corrosion debonding specimen exhibits a hysteresis  influenced by the thickness and length of the corrosion, with the change in corrosion thickness having a more significant impact on the echo.

Aigtek ATA-2041 High-Voltage Amplifier:

Figure: ATA-2041 Voltage Amplifier Specifications and Parameters