Application Cases

Experiment Name: Application of Voltage Amplifier in Research on Ultrasonic Wave Propagation Attenuation in Concrete

Research Direction: Ultrasonic Testing

Test Objective:
Ultrasonic testing technology is commonly used in biomedical engineering (BME) and industrial non-destructive testing (NDT). It enables the detection of hidden lesions or defects without the measured  object. Scholars worldwide have conducted fruitful research in the field of ultrasonic imaging algorithms, developing techniques such as ultrasonic computed tomography (CT), phased array focusing, synthetic aperture focusing (SAFT), and composite plane wave imaging. However, in the field of concrete ultrasonic NDT, the application of these algorithms faces specific challenges. These include the coupling problem between ultrasonic transducers and the rough concrete surface, the upper frequency limit of waves penetrating concrete, wave scattering within concrete, and the directivity of ultrasonic beams inside concrete. These challenges primarily arise from the heterogeneous nature of concrete materials and the large scale of concrete structures. BME ultrasound targets biological tissues, where scattering is far less severe than in concrete aggregates, and the  detection depth  is only tens of centimeters. Therefore, high-frequency waves (several MHz) can be used, achieving millimeter-scale imaging resolution. In traditional industrial NDT, the objects are often metals. Except for polycrystalline metals, scattering in general metallic media is also much smaller than in concrete. Currently, research on the impact of concrete material properties on the application of ultrasonic NDT techniques is limited. Based on the above issues, this paper investigates the attenuation behavior of ultrasonic waves propagating in concrete and the statistical randomness of ultrasonic signals.

Testing Equipment: ATA-2041 Voltage Amplifier, Arbitrary Waveform Generator, Data Acquisition System, Laptop Computer.

Figure: Physical Diagram of Experimental Instruments

Experimental Procedure:
All transducers were driven by a 400 Vpp voltage and sampled at a rate of 2 MHz. Transducers SA1~SA21 could be divided into three paths: SA1~SA7 for Path 1, SA8~SA14 for Path 2, and SA15~SA21 for Path 3. Each transducer in the array was excited sequentially, while other transducers within the same path received the ultrasonic signals. For example, in Path 1, when SA1 was excited to emit an ultrasonic pulse, SA2~SA7 acted as receiving sensors.

The ultrasonic pulse signals used in the experiment were calculated according to a formula, with center frequencies f set to five values: 50 kHz, 70 kHz, 90 kHz, 110 kHz, and 140 kHz. For each frequency point, SA1~SA21 were excited sequentially. Each excitation acquired  data from 8 channels: one channel for the excitation signal, one for noise, and six for ultrasonic signals propagating through the medium. A time series column was also added to the final data file. Thus, each excitation's data file contained 9 data columns, each with 80,000 sampling points (corresponding to a 40 ms sampling time at a 2 MHz sampling frequency). Each excitation was repeated 20 times to reduce noise. Assuming each sampling point occupies 8 bits of storage, the total data volume for each excitation frequency would be 21 (transducers) × 20 (repetitions) × 9 (columns) × 8×10⁴ (points) × 8 bits = 2.5×10⁹ bits, equivalent to 302.8 MB. This data volume could be easily handled by the NI-6366 data acquisition system and accompanying LabVIEW software.

Since each propagation path had 7 evenly spaced transducers with a longitudinal spacing of 200 mm, ultrasonic signals at distances of 200 mm, 400 mm, 600 mm, 800 mm, 1000 mm, and 1200 mm from the source could be measured for each excitation frequency. This resulted in a total of 30 frequency-distance combinations. Based on these 30 data sets, statistical analysis could be performed on signals with the same frequency but different propagation distances, as well as signals with the same propagation distance but different excitation frequencies, thereby revealing the attenuation and random characteristics of ultrasonic waves propagating in concrete.

Experimental Results:

1. Attenuation Reference

Figure: Pulse Responses Measured by Two Facing Transducers in Concrete (a)~(e) Time-domain signals, (f)~(j) Frequency-domain signals

SA21 and SA22 were a pair of closely placed transducers embedded in concrete. Their mechanical boundary conditions were expected to be similar to other transducers. Therefore, the signal with SA21 exciting and SA22 receiving was used as the reference for attenuation calculation. Considering that the pulse response of the transducers varies with frequency, the measurement results based on the transducer response under different waveforms and frequencies are shown in the figure above. It can be observed that the received signals are distorted  and deformed in the time domain, and shifted and spread in the frequency domain. This is caused by multiple reflections of waves within the transducer, the electromechanical characteristics of the PZT element, and the inertia of the transducer materials.

2. Ultrasonic Attenuation Measurement Results

Figure: Relationship of Ultrasonic Attenuation Coefficient (a) with Propagation Distance and (b) with Frequency in Concrete

The attenuation coefficient increases with increasing wave propagation distance, as shown in Group 1 (a) of the figure above. This is physically consistent: the farther the wave propagates, the greater the loss of sound pressure and acoustic energy. Both the attenuation coefficient and the attenuation rate coefficient exhibit a quadratic polynomial trend with frequency. Furthermore, within the measured frequency range, there exists a local minimum for attenuation and attenuation rate, corresponding to a frequency around 100 kHz, as shown in Group (b) of both figures. This trend is primarily caused by material attenuation. In Group 2 (a), the attenuation rate decreases with increasing propagation distance, a trend mainly caused by diffraction attenuation .

The diffraction attenuation rate is related to the propagation distance. For test points  in concrete materials with propagation distances less than 800 mm, the diffraction attenuation rate accounts for more than 50% of the total attenuation rate. Therefore, if the attenuation rate coefficient is to be used as an indicator for evaluating material properties, the influence of the diffraction attenuation rate within the total attenuation rate should be considered, and the distance-independent material attenuation should be used to characterize the material properties.

Figure: ATA-2041 High-Voltage Amplifier Specifications and Parameters