Application Cases

Experiment Name: Research on Health Monitoring of Steel Structure Joint Connections Based on Piezoelectric Smart Sensing Technology

Research Direction: Nondestructive Testing

Test Objective:
This study focuses on two common types of damage in the joint connection areas of steel structures during installation and service: bolt loosening and crack propagation. A health monitoring method for steel structure joint connections based on piezoelectric smart sensing technology is proposed, aiming to provide some guarantee for the safe operation of steel structures. The specific research content includes: monitoring of bolt loosening in steel tower flange joints based on time reversal technology; and monitoring of crack propagation in steel grid structure joint connection areas based on piezoelectric impedance and BP neural networks.

Testing Equipment: ATA-2022B high-voltage amplifier, independent multi-channel data acquisition card, torque wrench, piezoelectric ceramic patches, laptop computer.

Figure: Flowchart for Monitoring Bolt Loosening in Steel Tower Flange Joints Based on Time Reversal Technology

Experimental Procedure:

Figure: Experimental Setup for Monitoring Bolt Loosening in Steel Tower Flange Joints Based on Time Reversal Technology

The experimental setup for monitoring bolt loosening in steel tower flange joints based on time reversal technology is shown in the figure above. Taking bolt No. 1 as an example, the specific experimental process is as follows:

1. Equipment connection and signal transmission. According to the experimental setup shown in the figure above, complete the equipment connections. Turn on the power, warm up the instruments, and set the relevant parameters. After the indicator lights show that all devices are connected normally, proceed with signal transmission.

2. Signal acquisition. Apply condition one (initial tightening) to the bolt at the flange joint. The computer LabVIEW program controls the data acquisition card to emit a Gaussian pulse signal. The signal reaches the power amplifier via the connecting wire and is amplified 50 times. The amplified signal reaches PZT1 on the bolt cap via the connecting wire, generating a stress wave. The stress wave passes through the bolt and the flange connection surface and is then received by PZT2. The LabVIEW program uses time reversal technology to reverse and retransmit the signal. Finally, the stress wave signal forms a signal focus at PZT1. PZT1 converts the signal into an electrical signal through the inverse piezoelectric effect, which is transmitted to the signal acquisition card via the connecting wire. The signal acquisition card converts the electrical signal into a digital signal and saves it to the computer. In the same manner, complete signal acquisition for bolt No. 1 under different conditions.

In this experiment, a frequency peak of PZT near 100 kHz was selected, meaning the center frequency was set to 100 kHz. The sampling frequency in the experiment was 1 MHz, and the sampling duration was 1 second. Specific parameters are shown in Table 3.3. According to the relevant parameters of the pulse signal, the pulse signal generated using the Gaussian modulated sine mode is shown in the figure below.

Figure: Gaussian Pulse Signal Diagram

Experimental Results:
Damage mechanism of the steel tower flange joint: After the PZT patch on the flange joint is excited by a Gaussian signal, it generates a stress wave that propagates along the flange joint. When the connection at the flange joint becomes loose, the contact area between the flange joints relatively decreases. This affects the propagation and attenuation of stress wave energy between the sensors. The change in energy leads to different focusing peak values. The tightness state of the flange joint can be determined by the change in the focusing signal peak.

Damage identification results of the steel tower flange joint: The focusing signal peaks measured by the steel tower flange joint structure based on time reversal technology under the action of seven different torque values are shown in the figure below. It can be observed from the image that under different conditions (i.e., under the action of different torque values), the stress wave signals form different degrees of focusing, verifying the effectiveness of the time reversal technology.

Figure: Stress Wave Focusing Signal Diagram of Bolt No. 1 under Different Torque Values

To more intuitively reflect the change trend of the stress wave focusing signal under different conditions, the focusing peak of the stress wave for each condition of bolt No. 1 was extracted. Origin software was used to plot the curve showing the relationship between the focusing peak of the stress wave signal and the torque value for bolt No. 1 under different torque values, as shown in the figure below. The curve shows that: 1) The focusing peak of the stress wave differs for each condition. There is a nonlinear relationship between the torque value and the focusing peak of the stress wave. The magnitude of the stress wave signal focusing peak increases with increasing torque value. This is consistent with the previous derivation result: the greater the focusing peak of the stress wave signal received by the PZT patch, the greater the effective contact area of the flange joint, and the tighter the flange joint connection. This further proves that the focusing peak of the stress wave signal obtained based on time reversal technology can effectively monitor the connection state of the steel tower flange joint. 2) Throughout the curve graph, the focusing peak of the stress wave increases rapidly in the torque value range of 0–45 N·m, and the growth rate tends to level off in the later stage. This is because as the torque value continuously increases, the state approaches the healthy state. Therefore, the image changes more slowly, and the growth rate is slower.

Figure: Variation Diagram of Focusing Signal Peak of Bolt No. 1 under Different Torque Values

Aigtek ATA-2022B High-Voltage Amplifier:

Figure: Specifications of the ATA-2022B High-Voltage Amplifier