Application Cases

Experiment Name: Multi-Frequency Balanced Electromagnetic Detection of Surface and Backside Defects in Steel Plates

Research Direction: Automation and Detection Technology

Experimental Content: Based on the electromagnetic field propagation characteristics of electromagnetic balance technology, the detection principles for internal and external defects in steel plates were analyzed. Using the finite element method, the influence of excitation frequency on response signals from internal and external defects was studied, leading to the optimization of the best frequency combination of 20 Hz and 400 Hz. Experimental verification was conducted on steel plates with prefabricated internal/external defects and cracks.

Test Objective: According to the skin effect formula, the penetration depth of the electromagnetic field is inversely proportional to the electrical/magnetic conductivity of the tested steel plate and the excitation frequency. When the excitation frequency is low, the electromagnetic circuit is less affected by the skin effect, and the magnetic field distribution within the steel plate is uniform, facilitating the detection of external surface defects. However, the signal is susceptible to interference from surrounding electromagnetic noise. When the excitation frequency is high, the magnetic field is confined to the steel plate surface due to the skin effect, and the skin layer reaches局部 (局部, local) magnetic saturation, enhancing the detection sensitivity for internal surface defects but making effective detection of external surface defects impossible. Multi-frequency balanced electromagnetic technology combines the advantages of both high and low-frequency excitation, simultaneously improving the detection capability for external surface defects and the sensitivity for internal surface defects.

Testing Equipment: Power Amplifier, Function Generator, Multi-Function Filter, High-Precision Oscilloscope, Pipeline Under Test

Experimental Procedure:

Figure: Experimental Platform

Figure: Electromagnetic Property Test Curve of the Specimen

Figure: Detection Probe

During the experiment, a function generator produced a sinusoidal signal with an amplitude of 1 V. The voltage signal was amplified by a constant-voltage power amplifier (Aigtek Corporation ATA-3080) and then connected to the excitation coil of the probe for  excitation  current loading. The  induced  signal from the probe's detection coil was processed and conditioned by a multi-function filter (NF Corporation 3611) before being connected to a high-precision oscilloscope (Tektronix Corporation MSO 508) for  corresponding  processing as needed. This oscilloscope enabled the extraction and analysis of detection signal characteristics, as well as real-time waveform display and data storage.

Test Results:

Figure: Internal Defect Test Results

Figure: External Defect Test Results

Figure: Comparison of Single-Frequency and Multi-Frequency Balanced Electromagnetic Testing for External Defects

Figure: Schematic Diagram of Internal and External Surface Defect Machining on the Pipeline

In the figure, the variation in internal and external defect depth shows good consistency with the voltage signal, effectively validating the detection capability of the multi-frequency balanced electromagnetic method for internal and external pipeline defects. Additionally, when the pipeline is defect-free, the induced voltage signal is non-zero and fluctuates around zero voltage. This may be attributed, on one hand, to the non-smooth inner surface of the pipeline causing unequal distances between the  left and right magnetic poles and the pipeline surface, leading to an incomplete  symmetrical distribution of the induced current. On the other hand, residual stress from the pipeline manufacturing process or failure to demagnetize the pipeline after previous tests may result in a non-uniform magnetic field distribution within the pipeline. Nevertheless, the overall trend of the voltage signal aligns with theoretical analysis. Furthermore, comparing the detection results reveals that the multi-frequency balanced electromagnetic method significantly enhances the detection depth for internal surface defects in pipelines (reaching 8 mm). This is because the low-frequency components in the multi-frequency excitation signal effectively increase the penetration depth of the alternating magnetic field within the pipe wall, thereby improving the detection depth for internal surface defects.