The diagnosis and prevention of the deterioration of iron-steel infrastructure has become an important social issue in recent years. The thickness measurement technique (extremely low-frequency eddy current testing (ELECT)) using a magnetic sensor for detecting steel corrosion at extreme frequency ranges has been previously reported. Using the calibration curves based on the correlation between the phase of the detected magnetic signal and the plate thickness, the plate thickness reduction caused by corrosion can be estimated from the detected phase signal. Iron-steel materials have large changes in electromagnetic characteristics; therefore, the reference calibration data for each type of iron-steel are required for plate thickness estimation. In this study, the effect of electromagnetic characteristics on the magnetic thickness measurement was investigated to improve the thickness estimation. Four types of iron-steel plates (SS400, SM400A, SM490A, and SMA400AW) with thicknesses ranging from 1 mm to 18 mm were measured by ELECT, and the phase change at multiple frequencies of each plate were analyzed. The shift in the phase and linearity regions of the calibration curves for each type of steel plate was observed. To analyze this shift phenomenon, the electromagnetic characteristics (permeability μ and conductivity σ) of each type of steel were measured. Compared with the permeability μ and conductivity σ of each steel plate in the applied magnetic field strength range, the product (σμ) for various steel plates decreased in the following order: SM400 > SS400 >SMA400AW > SM490A. The product of μ and σ is related to the skin depth, indicating the electromagnetic wave attenuation and eddy current phase shift in the material. Therefore, each shift in the calibration curve of each type of iron steel is explained by the changes in the parameters σ and μ.
I. INTRODUCTION
A large number of social and industrial infrastructures, such as bridges, roads, chemical plants, and ports, are over 50 years old. Thus, maintaining their safety and longevity has become a challenge, requiring accurate and simple inspection methods. Steel structures have the problem of insufficient strength due to wall thinning by corrosion. Recently, ECT techniques for thickness detection using a wide frequency range have been developed. For thickness measurement, pulse waves1–8 and multiple frequency waves9,10 are used. For thickness analysis, time domain analysis is used to measure the decay of the time response, while frequency domain analysis is used to measure the phase and intensity change at each frequency. Frequency domain analysis methods using pulse waves11 or multiple frequency12 waves have also been reported. For application in the detection of reduced thickness due to steel corrosion, a preliminary study of thickness measurements (extremely low-frequency ECT: ELECT) using a magnetic sensor at extremely low frequencies, and frequency domain analysis with multiple frequency waves, have been reported.13,14 Using ELECT, a well-calibrated curve was obtained for the correlation between the phase of the differential vector of the magnetic signals detected at two different frequencies and the steel plate thickness. From this curve, the steel plate thickness thinned by corrosion was estimated from the detected phase.
Iron-steel materials exhibit large variations in electromagnetic characteristics. Even when the components are the same, the electromagnetic characteristics may change due to heat treatment and processing methods. Many types of steel infrastructure are used. In some older structures, it is unclear which type of steel is used. Therefore, it is necessary to investigate the effect of electromagnetic characteristics on the calibration curve to improve the thickness estimation.
II. EXPERIMENTAL
ELECT is a method in which an extremely low-frequency magnetic field is applied to an object to generate an eddy current, and the magnetic field generated from the eddy current is detected by a magnetic sensor. The details of the measurement system have been previously reported.13 An excitation magnetic field with multiple frequencies (1, 3, 5, 10, 20, 50, 100 Hz) in time series was applied to the steel plate. The detected magnetic signal at each frequency was decomposed into two components, an x-component that was in phase with the excitation magnetic field and a y-component delayed by 90°, using fast Fourier transform analysis. Calibration curves were obtained for the phase of the differential vector of the magnetic signals detected at two selected frequencies and plate thicknesses.
To investigate the effect of electromagnetic characteristics on magnetic thickness measurement, four types of iron-steel plates (SS400, SM400A, SM490A, and SMA400AW) with thicknesses ranging from 1 mm to 18 mm were used. The initial magnetic permeability was measured using test pieces of ring-shaped (outer diameter 100 mm, inner diameter 60 mm, thickness 8 mm) steel which an excitation coil of magnetic field H and a detection coil of magnetic flux density B were attached. The initial magnetic permeability was obtained from the relationship of dB/dt with dH/dt. The conductivity σ was measured by a four-terminal method using rod-shaped (length, 60 mm; width, 3 mm; thickness, 2 mm) test pieces.
III. RESULTS AND DISCUSSION
The detected magnetic vector at each frequency was traced to the magnetic spectrum. Fig. 1 shows an example of the magnetic spectrum of the SM400A steel plate. The magnetic signal at 1 Hz was adjusted to the origin to obtain the magnetic signal from only the eddy current component. The details of the magnetic spectrum analysis method were previously reported.13 An apparent thickness-dependence was observed, and the magnetic spectrum rotated clockwise as the plate thickness increased. The differential magnetic vector between two appropriate frequencies was extracted from the magnetic spectrum. The plate thickness was estimated by the phase of the differential magnetic vector. The calibration curve for thickness estimation was changed by two-frequency selection. For instance, the linearity of the calibration curves of SM400A in the region from 4 mm to 16 mm was obtained by subtracting the 1 Hz vector from the 3 Hz vector (Fig. 2(a)). Among the several frequency sets, the set with good linearity was selected. The range of the linearity of the curve gradually narrowed as the frequency increased, while the sensitivity increased. The calibration curve with the selection of lower frequencies was saturated in the low thickness region (Fig. 2(b)). By increasing the frequency, the linearity was improved in the low thickness region. To obtain a high resolution and suitable detection range for thickness estimation, selecting the appropriate frequency is necessary. This frequency-dependence of the linear range is attributed to the skin effect due to the frequency. A simple model of the amplitude of the AC magnetic field H with frequency f applied parallel to a conductive metal plate surface is considered. The eddy current density J at depth z is expressed as
Frequency-dependence on the plate thickness calibration curve: (a) thickness region from 4 mm to 18 mm, and (b) thickness region from 1 mm to 4 mm.
Frequency-dependence on the plate thickness calibration curve: (a) thickness region from 4 mm to 18 mm, and (b) thickness region from 1 mm to 4 mm.
The first exponential term is the decay term, while the second term is the phase shift term. Both are related not only to the frequency but also the electromagnetic characteristics (permeability μ and conductivity σ). To investigate this effect, the calibration curves of various types of steel plates with different electromagnetic characteristics were compared (Fig. 3). The calibration curve of SM400 shows that it has the highest sensitivity. Although the curves of SS400 and SMA400AW are slightly closer, they decreased in the order of SS400, SMA400AW, and SM490A. To analyze the phase shift and linearity range change phenomenon, the electromagnetic characteristics of each type of steel plate were measured. Because the applied magnetic field of the induction coil is weak in the ELECT, the initial magnetic permeability was measured (Fig. 4). Table I lists the values of the initial magnetic permeability μ at a magnetic field intensity of 50 A/m, conductivity σ, and the product (σμ) for various steel plates. The σμ values for each type of steel decreased in the following order: SM400 > SS400 > SMA400AW > SM490A. The σμ values and the phase at each plate thickness were well correlated at a thickness of 8 mm; however, the correlation weakened slightly as the plate thickness increased (Fig. 5). The μ values in Table I were obtained assuming that the applied magnetic field is uniform; the change in the value of the applied magnetic field distribution along the depth direction of a thick plate was not considered. Therefore, a better correlation can be obtained by considering the applied magnetic-field distribution.
Electromagnetic characteristics of various steel materials.
| Iron-steel materials | SM400 | SMA400AW | SS400 | SM490A |
|---|---|---|---|---|
| σ: Conductivity | 5.85 × 106 | 4.55 × 106 | 4.91 × 106 | 4.08 × 106 |
| (S/m) | ||||
| μ: Initial magnetic | 3.40 × 10−4 | 3.89 × 10−4 | 3.59 × 10−4 | 2.61 × 10−4 |
| permeability | ||||
| (H/m) | ||||
| σ μ (SH/m2) | 1.99 × 103 | 1.55 × 103 | 1.76 × 103 | 1.07 × 103 |
| Iron-steel materials | SM400 | SMA400AW | SS400 | SM490A |
|---|---|---|---|---|
| σ: Conductivity | 5.85 × 106 | 4.55 × 106 | 4.91 × 106 | 4.08 × 106 |
| (S/m) | ||||
| μ: Initial magnetic | 3.40 × 10−4 | 3.89 × 10−4 | 3.59 × 10−4 | 2.61 × 10−4 |
| permeability | ||||
| (H/m) | ||||
| σ μ (SH/m2) | 1.99 × 103 | 1.55 × 103 | 1.76 × 103 | 1.07 × 103 |
Relationship between the phase and electromagnetic parameter (σμ) for each plate thickness.
Relationship between the phase and electromagnetic parameter (σμ) for each plate thickness.
The relationship between the electromagnetic characteristics and the calibration curve was also clarified. It is possible to accurately estimate the thinned plate thickness by selecting a suitable calibration curve after the electromagnetic characteristics of the object are determined. For example, if the reduced thickness of a steel with an original thickness of 9 mm is 4 mm, this reduced thickness can be estimated by the calibration curve with a frequency set of 5 Hz and 3 Hz. If the reduced thickness is in the range of 2–4 mm, it can be estimated by the calibration curve with a frequency set of 20 Hz and 10 Hz. The reduction in plate thickness caused by corrosion can be estimated over a wide thickness range by changing the frequency set obtained from multiple frequency measurements.
IV. CONCLUSION
The iron-steel thickness of various steel materials can be estimated from the calibration curve using the phase parameter of the differential magnetic vector. The difference between the calibration curves of each type of iron steel is explained by the difference in the σμ parameter. By selecting the appropriate frequency set from multiple frequencies, a high sensitivity and wide linear region were obtained. The plate thickness over a wide range from 1 m to 18 mm could be estimated by switching the frequency set. Therefore, the ELECT inspection method is a nondestructive evaluation method, which can be applied to detect the plate thickness reduction caused by corrosion in various steel structures.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
DATA AVAILABILITY
The data that support the findings of this study are available within the article.
