Application Cases

Experiment Name: Application of Power Amplifiers in the Research of Ultrasonic Guided Wave Nondestructive Testing for Pipelines

Research Direction: Research on Ultrasonic Guided Wave Nondestructive Testing for Pipelines

Experimental Content:
The scattering of guided waves in curved pipes was studied using the normal mode expansion method. First, the biorthogonal relationship of normal modes in curved pipes was derived. Based on this, the displacement and stress fields at the interface between the straight and curved sections were expanded using normal modes from both parts. Then, based on the continuity principle of displacement and stress fields, the scattering problem was treated as an eigenvalue problem of the transfer matrix, with the solution providing mode conversion at the interface. A case study on the low-frequency longitudinal mode incident on a curved pipe revealed dominant mode conversions, specifically the reflection of L(0,1) and mode conversion from L(0,1) to F(1,1). Finite element simulations and experiments were also conducted. The curved reflection of L(0,1) and mode conversion to F(1,1) were clearly observed, consistent with theoretical predictions. Research on mode conversion of guided waves in curved pipes lays the foundation for the engineering application of pipeline nondestructive testing.

Test Objective:
Investigate the mode conversion characteristics of guided waves in curved pipes.

Test Equipment:
Signal generator, power amplifier (Aigtek ATA-3080), signal processing system, self-designed sensors, self-designed signal reception and conditioning device.

Amplifier Model: ATA-3080 Power Amplifier

Experimental Procedure:
The test pipe is identical to the one used in the "Numerical Verification of Biorthogonal Relationship" section. The stainless steel pipe is bent 90° at its midpoint through thermal bending. A 5-cycle, 30 kHz tone burst is generated by an arbitrary function generator and then amplified by a high-voltage power amplifier (Aigtek ATA-3080). The amplified signal is sent to the transmitting transducer to excite longitudinal guided waves in the pipe. The weak guided wave signals are sensed by the receiving transducer, pre-amplified, and high-pass filtered before being acquired by the data acquisition system. Both transmitting and receiving transducers are placed at the same end of the pipe. Magnetostrictive patch transducers are employed. Four pre-magnetized iron-cobalt alloy strips, each 70 mm in length, 5 mm in width, and 0.15 mm in thickness, are evenly spaced circumferentially and longitudinally bonded to the pipe using epoxy resin. A 40-turn electromagnetic coil is wound around the patch to transmit and receive signals.

Test Results:
The curved reflection of L(0,1) is clearly observed between two consecutive end reflections, as the bend is located at the midpoint of the pipe. Mode-converted F(1,1) is also observed and can be verified by its propagation time. The time difference between the L(0,1) end reflection (waveform 1 in Figure 10) and its subsequent F(1,1) waveform (waveform 2 in Figure 10) is approximately 0.33 ms. For a round trip, the incident L(0,1) passes through the bend twice (forward and backward), so mode conversion from L(0,1) to F(1,1) occurs twice. Waveform 2 represents the scattered F(1,1) mode when L(0,1) propagates back. According to the dispersion curve (see Figure 2), the theoretical time difference between waveforms 1 and 2 is 0.3 ms, which aligns well with the experimental results.

The amplitude ratio of the first L(0,1) curved reflection to the first L end reflection in the figure is approximately 0.2. In this pulse-echo experimental configuration, the first curved reflection in the figure is actually the second curved reflection, as the first is obscured by the initial pulse and cannot be distinguished. Therefore, the first L(0,1) curved reflection also consists of two L(0,1) curved reflections. Thus, approximately 10% of the incident L(0,1) mode is reflected by the bend. In summary, significant reflected L(0,1) and mode-converted F(1,1) modes are scattered at the bend, and the experimental result that about 10% of the incident L(0,1) mode is reflected by the bend aligns well with numerical simulations, validating the theoretical predictions.

Figure: ATA-3080C Power Amplifier Specifications and Parameters