Application Cases

Experiment Name: Ultrasonic Guided Wave-Based Baseline-Free Evaluation Method for Uniform Corrosion

Research Direction: Ultrasonic Guided Wave-Based Accelerated Corrosion Testing

Testing Objective:
Ultrasonic guided waves, as a precise and efficient non-destructive testing technology, have been widely employed by many scholars for corrosion monitoring in civil engineering structures. Currently, guided waves with different energy density distribution characteristics are used for the qualitative identification of two damage modes in reinforced concrete members: delamination and pitting corrosion. Fractal theory has been applied to corrosion monitoring, and the amplitude information of defect echoes has been utilized to identify defect depths in the outer and central wires of steel strands. Studies have demonstrated that contact between wires in cables can lead to mode conversion, and changes in the bending mode spectrum amplitude can serve as an indicator for damage assessment. However, considering the inevitable changes in the coupling performance between transducers and structural interfaces during long-term corrosion monitoring, the reliability of the aforementioned guided wave amplitude-based testing results remains questionable. Therefore, researchers have attempted to adopt baseline-free detection methods to evaluate structural performance. For example, the time-reversal method has been used to reconstruct excitation signals, with an indicator reflecting the distortion between the original excitation and reconstructed signals to assess the degree of structural damage. Changes in the time-of-flight have also been employed to identify the degree of delamination in concrete beams. In recent years, model-based damage identification methods have been introduced into guided wave monitoring, offering the notable advantage of accurate quantitative damage identification. However, the random morphology and location of corrosion defects pose challenges, as numerical models simulating high-frequency ultrasonic guided waves often struggle to balance computational efficiency and adaptability to defects. Consequently, the practical application of this method in real corrosion scenarios has yet to be reported.

Testing Equipment: Chassis, arbitrary waveform generator, digital signal acquisition board, acoustic emission probes, radio frequency power amplifier (ATA-8202), preamplifier (ATA-5620).

Experimental Process:
The monitoring target was a galvanized steel wire for bridge cables under accelerated corrosion conditions, with a diameter of 7 mm, a total length of approximately 103 cm, and a corrosion segment length of about 94 cm. The electrolyte used was a 3.5% sodium chloride solution. The steel wire served as the anode in the accelerated corrosion system, while four equally spaced stainless steel plates acted as cathodes. A constant current was supplied by an external DC power source. The data acquisition system included a chassis, an arbitrary waveform generator, and a 60 MS/s, 8-channel digital signal acquisition board. The measurement method adopted a single-end excitation and single-end reception configuration. Identical acoustic emission probes were used for both excitation and reception of longitudinal modal guided wave signals, with a frequency response range of 0.1–1.5 MHz. The excitation signal was amplified using a radio frequency power amplifier (ATA-8202), and the received signal was amplified using a preamplifier (ATA-5620). Probes were fixed at both ends of the steel wire using fixtures, with engine oil serving as the interface coupling agent to enhance the transmission efficiency of vibrational energy. The experimental setup is illustrated in the figure below.

Figure: Experimental Setup

Experimental Results:
The external current was maintained at a constant 0.35 A, and the remaining diameter of the steel wire was monitored every 12 hours. The step change in diameter, controlled by Faraday’s law, was approximately 0.05 mm. The time-frequency analysis results of the received signals during the early stage of corrosion monitoring (0–48 hours), with the excitation frequency fixed at 377 kHz, are illustrated below. The frequency of the correlation peak point in the received signal showed slight deviations as corrosion progressed, but the time-of-flight exhibited a clear monotonic decreasing trend, indicating that the guided wave group velocity is highly sensitive to changes in the wire diameter.

The error was less than 0.157 mm. Moreover, the estimated remaining diameter decreased monotonically with increasing corrosion time, further demonstrating the high resolution of guided wave evaluation for diameter changes. However, the absolute error gradually increased in the later stages of corrosion. The reasons for this are as follows:

1. The corrosion rate is related to the electrode distance. Due to uneven surface corrosion rates, localized corrosion effects became more pronounced alongside uniform corrosion, leading to increased diameter variations in different segments of the wire.

2. The gradual accumulation of corrosion products on the surface affected the propagation characteristics of guided waves in the wire, causing deviations between the wave speed and theoretical values.

3. In the later stages, the accumulated corrosion products significantly influenced the attenuation characteristics of guided waves in the wire, resulting in severe attenuation of the received signal amplitude and a noticeable decline in the signal-to-noise ratio. When the corrosion duration was within 250 hours, the randomness of the evaluation results aligned well with the analysis. However, as corrosion duration increased further, external factors led to increased errors in the evaluation results, manifesting as a greater number of outliers outside the confidence interval.

   
   

Figure: Time-of-Flight Variation Trend (0–48 Hours)

Figure: Guided Wave Testing Results

Aigtek ATA-8000 Series Radio Frequency Power Amplifier:

Figure: ATA-8000 Series Radio Frequency Power Amplifier Specifications and Parameters