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ANALYSIS OF CORROSION DAMAGE CONDITION OF STEEL MEMBERS USING LOW FREQUENCY EDDY CURRENT TESTING
Abstract
In this research, a method which is able to analyze corrosion damage condition such as local thickness reduction or formation of rust layer of steel members of steel members at same time has been investigated. Considering eddy current testing which uses especially low frequency eddy current such as 1~1000Hz, corrosion damage condition analysis method which performs inspection from only one side of steel members has investigated to become easy non-destructive testing method. Firstly, differences of dynamic magnetic field characteristics due to damage form was clarified using numerical simulation, and further data analyzing method to obtain the differences was investigated. Then sweep wave and frequency step wave were designed as induced wave and analysis using both detected voltage and wavelet coefficient was performed. Based on results of all process, a corrosion damage condition analysis method was proposed. Finally, in the applicability of proposed method was clarified by experiment.
... Various non-destructive testing methods such as ultrasonic testing, X-ray diffraction, and eddy current testing have been developed and employed for structural health monitoring of steel structures [3][4][5][6]. Among these, the eddy current testing has been selected in the present study because it is a non-contact method that does not require the use of coupling agent nor surface preparation such as clearing off the top rust layer, thus proving itself time and cost efficient, particularly for field inspection of large structures with damages such as corrosion and cracking that are widespread [6,7]. ...
... Various non-destructive testing methods such as ultrasonic testing, X-ray diffraction, and eddy current testing have been developed and employed for structural health monitoring of steel structures [3][4][5][6]. Among these, the eddy current testing has been selected in the present study because it is a non-contact method that does not require the use of coupling agent nor surface preparation such as clearing off the top rust layer, thus proving itself time and cost efficient, particularly for field inspection of large structures with damages such as corrosion and cracking that are widespread [6,7]. ...
... Conventionally, the extent of corrosion is determined by the remaining thickness of a structural member [6], while the cracks are detected by the change in eddy current signal due to the distortion of eddy current around them [7]. Nevertheless, it is to be noted that both corrosion and cracking cause change in stress distribution in the structural members, which can be detected by eddy current testing based on the Villari effect [8]. ...
Structural health monitoring of steel structures is crucial for inspection of corrosion and cracking in structural members, compromising their safety and serviceability. In the present study, the prospective of evaluation of change in stress state of structural member due to corrosion and cracking through eddy current based stress measurement is investigated. For this, three-dimensional numerical simulations are carried out in the FE software COMSOL Multiphysics 5.2a for a steel plate subjected to change in relative permeability, representative of change in stress state, whereby the eddy current indices are characterized, including the effects of additional influential parameters namely, lift-off, excitation frequency, and probe size. Phase Diagram is then proposed as a concise method to evaluate the variation of relative permeability and lift-off concurrently in a single graph for an excitation frequency and probe size. It further facilitates the selection of suitable excitation frequency and probe size to conduct the eddy current based stress measurement.
High-strength bolted joints are widely used in steel bridges since they have advantages to construction, maintenance, and replacement. On the other hand, bolt axial force may decrease after tightening due to tightening method, tightening order, relaxation, traffic load, or earthquake. The remaining axial force of high-strength bolts is a factor that includes uncertainty therefore, it is important to evaluate the remaining bolt axial force. In this study, in order to evaluate the remaining axial force of high-strength bolts non-destructively and simply, a new remaining axial force evaluation method based on eddy current measurement was constructed. In the eddy current measurement, the stress change can be obtained by using the relationship between the magnetic permeability and the stress. Therefore, authors proposed the method considering the stress state of the bolt head and simply install the eddy current probe on the bolt head. In addition, considering that the bolt head is deformed by the axial force, the relationship between the output signal and the gap between the probe and the measurement target, which is an important evaluation index in eddy current measurement, has clarified, and the influence of the axial force evaluation has also investigated.
This paper presents determination of the stress state of steel bridges, due to stress redistribution because of corrosion or cracking, as a new important parameter in structural health monitoring for evaluating their condition. For this, eddy current testing is used to investigate the effect of change in stress in a steel plate through three-dimensional numerical simulations carried out in the FE software, COMSOL Multiphysics 5.2a. A reflection probe, comprising of an outer exciting coil and an inner detecting coil, is used to characterize the eddy current indices - real, imaginary, and absolute voltages, and phase with respect to the change in relative permeability, which is dependent on stress, lift-off, exciting frequency, and probe size. A new concise method of simultaneously representing the effects of relative permeability and lift-off at an excitation frequency, termed as Phase diagram, is then proposed, which is further used for selection of suitable probe size and excitation frequency for eddy current based stress measurement. It is found that larger probes and lower excitation frequencies are preferable, because of better sensitivity of larger probes to significant lift-offs and uniform trend of change of eddy current indices at lower frequency, requisite of calibration.
This article presents corrosion profile estimation as a method to determine the extent of corrosion in steel members having corroded exterior, with or without protective cover, when the only side accessible to the eddy current probe is highly uneven. A high frequency eddy current method is proposed to estimate the corrosion profile of a steel plate directly from the probe lift-offs regardless of varying plate thickness. The three-dimensional, numerical simulations were carried out in the FEM software, COMSOL Multiphysics 5.2a to verify the suitability of this method. A parametric study is conducted to determine the effect of coil height, diameter, their ratio and tilt angle of reflection probe on the estimation accuracy of a triangular notched profile, representative of corrosion, whereby a geometrically optimized probe is obtained. It is then employed to determine the corrosion profile of a corroded steel plate with and without protective cover. The estimated corrosion profiles show good agreement with the actual profile measured by laser displacement meter, confirming the sufficiency of proposed method.
The authors have proposed a corrosion damage evaluation method for corroded steel members to estimate rust layer and residual thickness. In this method, detected voltage signal obtained by a low frequency eddy current of 500 Hz or less was used as an index. In this research, in order to further advance the method and to simplify the measurement system, a more sophisticated method utilizing a phase characteristic which is considered to be able to simultaneously estimation of thickness information together with the detected voltage has been investigated. Firstly, to clarify the low frequency eddy current characteristics assuming the difference in damaged condition by numerical simulation was performed. Then evaluation method focusing on the phase between the detected voltage and the input current and the detected voltage as an index to capture the differences has investigated. Based on results of all process, a corrosion damage evaluation method focused on phase characteristics which can obtain information of rust layer and residual thickness simultaneously was proposed. Finally, the applicability of proposed method was clarified by experiment.
The present study deals with the effect of uniform and non-uniform local corrosion damages at the bearing stiffener and nearby web on the bearing capacity of plate girder. Five plate girder ends simulated with different uniform and non-uniform types of corrosion damage, at the base of the bearing stiffener are used in the experimental program. The experimental results are modeled and verified on a Finite Element (FE) software ABAQUS, considering shell-solid coupling elements formulation. The study is further extended to various damage cases i.e., stiffener damage, stiffer plus web damages etc. considering different damage heights and residual thicknesses. The study concludes that minimum thickness within any damage height may be used to simulate the corrosion damage in a FE analysis. At the end, some empirical relationships are also proposed to estimate the bearing strength of the plate girder affected by the local corrosion damage at plate girder end.
Local wall thinning in pipelines affects the structural integrity of industries like nuclear power plants (NPPs). In the present study, a pulsed eddy current (PEC) differential probe with two excitation coils and two Hall-sen-sors was fabricated to measure the wall thinning in insulated pipelines. A stainless steel test sample was pre-pared with a thickness that varied from 1 mm to 5 mm and was laminated by plastic insulation to simulate the pipelines in NPPs. The excitation coils in the probe were driven by a rectangular current pulse, the difference of signals from two Hall-sensors was measured as the resultant PEC signal. The peak value of the detected signal is used to describe the wall thinning. The peak value increased as the thickness of the test sample increased. The results were measured at different insulation thicknesses on the sample. Results show that the differential PEC probe has the potential to detect wall thinning in an insulated NPP pipelines.
Measurement of lift-off can be used to assess the thermal insulation thickness and it has the potential to reduce the lift-off effect in pulsed eddy current testing. In this paper, first, the relative variation of magnetic flux is proposed as a feature for the measurement of lift-off. And then, how to directly obtain the key parameters of the feature from the testing signals is provided. At last, the validity of the feature is verified by simulations and experiments, respectively. The results show that the feature is suitable when the lift-off is tens of millimeters and the plate is ferromagnetic.
We had proposed and developed an electromagnetic thickness sensor which can measure the thickness of a conductor plate or tube coated with rust. However, our measurements were made under rust-free conditions in the previous paper. Therefore, this paper presents the measurement of simulated rusty objects. The theoretical analysis related to rust-free measurement is expanded accordingly. The effects of rust on measurement are discussed using this expanded analytical result. Experimental results for steel plates coated with Teflon film in place of rust verify that our method can measure the thickness of rusty objects and confirm that the upper limit of the measurement is about 2.5mm when the Teflon film thickness is 0.5mm.
Ultrasonic guided wave inspection is expanding rapidly to many different areas of manufacturing and in-service inspection. The purpose of this paper is to provide a vision of ultrasonic guided wave inspection potential as we move forward into the new millennium. An increased understanding of the basic physics and wave mechanics associated with guided wave inspection has led to an increase in practical nondestructive evaluation and inspection problems. Some fundamental concepts and a number of different applications that are currently being considered will be presented in the paper along with a brief description of the sensor and software technology that will make ultrasonic guided wave inspection commonplace in the next century.
In order to study the actual states and tendencies of corrosion of steel structures in marine environments, some analyses were performed on the results of the investigations conducted in the various ports and the following results were obtained. Corrosion rate near low water varied from structure to structure. The corrosion rate of steel pipe piles was a little larger than that of steel sheet piles. Intensive corrosion under mean low water level, so called concentrated corrosion, was found in many of the structures; the concentrated corrosion had a tendency to occur at steel pipe piles with high upper exposed end of steel members, in sea water of poor quality and in sea water with inflow of fresh water.
This study discusses the effect of local corrosion damages nearby the bearing region including the bearing stiffener and adjacent web damage on the ultimate shear and post-buckling strength of steel plate girder bridges. A computer package ABAQUS is utilized for the verification and extension of the numerical study. Various corrosion damage cases, specifically the bearing stiffener are studied considering different damage thickness and damage heights. The study concludes that local damage on bearing stiffener damage is more fatal than the local web damage for the shear resistance of the plate girder. Further, some empirical relationships are also derived to assess the residual shear capacity of the locally corroded plate girder at the bearing region.
Non-destructive evaluation of materials and structures is still a key issue in some industrial scenarios as the production process and the quality inspection. In the case of metallic materials, economic and implementation reasons push for the use of eddy current testing techniques. In the last years, the effort of the research activity is been focused on the development of eddy current measurement procedures capable of providing as much information as possible about the presence, the location and the geometrical characteristics of defects. To this aim, newer signals characterized by a wide spectral content able to penetrate in the different layers of the material under test are substituting the older sinusoidal excitation. Among these, multi-frequency and chirp represent two optimal candidates within the class of frequency domain-based signals. The former is characterized by the simultaneous presence of many sinusoidal tones, while the latter exhibits a constant envelope and an instantaneous frequency that increases or decreases with time. In literature many interesting papers dealing with both excitation types are reported but an experimental performance comparison on a number of real defects is missing. Moreover the comparisons are usually executed on single measurements collected in presence of a defect in the location corresponding to the highest defect signal. Even if this strategy allows the analysis of the defect signature in time and in frequency domain, from both experimental and practical point of view, this approach is extremely sensitive to noise and it could be also difficult to be applied in on-line or in-situ inspections. In this paper, the proposed comparison aims at highlighting the suitability of each considered excitation method with respect to the extraction of defects geometrical features. It is proposed to combine the various excitation signals with image processing: indeed by developing a proper 2D image procedure from 1D eddy-current data it is possible to improve the defect detection capability when difficult cases are experienced (such as annealed and small cracks) and to extract more accurate information about the defect’s geometric characteristics. After the image processing application, the multi-tone and the chirp approaches are quantitatively compared by using an ad-hoc figure of merit.
The nondestructive method to detect subsurface defects is limited because conventional eddy current are concentrated near to the surfaces adjacent to the excitation coil. The PEC technique enables detection of cracks buried deeper under the surface with relatively small current density. In the present study, an attempt has been made to investigate detection of subsurface cracks using a specially designed double-D differential probe. The tested sample is a SS304 with a thickness of 5 mm; small EDM notches were machined in the test sample at different depths from the surface to simulate the sub surface cracks in a pipe. The designed PEC probe has two excitation coils and two detecting Hall-sensors. The difference between two sensors is the resultant PEC signal. The cracks under the surface were detected using peak amplitude of the detected pulse; in addition, for a clear understanding of the crack depth, the Fourier transform is applied. In time domain, the peak amplitude of the detected pulse is decreased, and in the frequency domain, the magnitude of the lower frequency component has been increased with an increase in the crack depth. The experimental results have indicated that the proposed differential probe has the potential to detect the sub surface cracks in a stainless steel structure.
A through-wall eddy current system produces similar signals for defects of equal size, independent of the location of the defects in the sample being tested. The remote field eddy current technique is also reported to generate similar signals for equal defects, independent of the location of the defects in the wall of the tube which is being inspected. A theoretical study of an ideal conventional through-wall eddy current system is presented, and the results are compared with the results of a remote field eddy current system. It is shown that while the remote field system does not always obey the one-dimensional skin effect equation, it detects external material loss with nearly equal sensitivity to internal material loss. Experimental results are presented which show that the complex plane displays of similar ID and OD defects are similar in shape, size and orientation.
Multi-frequency eddy current test (MFECT) is an important NDT technology, and it can be applied to realize multi-parameter detection and interference elimination effectively. Aiming at the problem that conventional single-frequency eddy current test can only identify defects but can't classify defects effectively, a new MFECT technology which adopts LFM signal exciting and spectrum analysis method was proposed. The probe-coil in an ac bridge was excited by a LFM signal, and output signals of the ac bridge were amplified and sampled to detect defect's property by spectrum method. These defects could be detected according to spectrum total energy variation. With the purpose of defect classification, a new feature, termed as spectrum barycenter excursion was proposed. Compared with traditional MFECT, it has the merits of small peak factor and low system cost. The agreement between theory and experiment shows that the present method is correct.
Many field surveys depict that steel plate girder ends tend to corrode due to the leakage of water from construction joints and/or detention of rain water near bearing regions. In many cases, buckling failures of plate girders were observed due to the excessive loss of material at the bottom portion of bearing stiffeners. In this study, the effect on the buckling strength of steel plate girder due to the corrosion damage at bearing stiffeners was investigated. Full-scale experimental tests were performed on two plate girder ends to determine the bearing capacity. Bearing capacity and failure modes were also verified by numerical analyses. Initial imperfections and welding residual stresses were also taken into account in analytical models. The simulation of the model was extended to the various corrosion damages, considering different damaged heights and thicknesses of the bearing stiffener near weld seam. All results were plotted in terms of remaining bearing capacity versus damage thickness ratio keeping the damage height of the stiffener constant. In this study, a damage parameter Reduced Thickness Ratio was used to assess the ultimate capacity and buckling of steel plate girder end with various corrosion levels.
Summary form only given. The finite-element method for numerical
field solutions is philosophically different from the finite difference
approach. Rather than solve the limiting differential equations, the
idea is to apply integral conservation laws over small elements of the
solution volume. The author reviews finite-element principles for
electric and magnetic field solutions and emphasizes advantages for
vacuum electron devices. Because solutions on conformal triangular
meshes give accurate field values on curved electrodes, they are ideally
suited to field-emission guns. For space-charge-limited emission, novel
numerical methods give significantly improved accuracy compared to
conventional finite difference approaches. Design procedures for power
tubes and vacuum micro-electronic devices are illustrated with solutions
from TRAK, a ray tracing program for quasi-steady-state gun design.
Development of the code was supported in part by an SBIR Grant through
Northstar Research Corporation. The goal of the program was to study
high-power vacuum tubes for voltage control in Heavy Ion Fusion
induction linacs
Eddy current tests in sinusoidal mode are of great interest to detect flaws in metallic structures. A limitation of this classical method concerns the detection of deep defects in very conductive materials. Such a drawback can be reduced by operating in the transient mode, using a pulsed eddy current technique. In fact, the pulsed character of the electric excitation allows a high peak value of the density of eddy currents in the material; also the broadband signal contains optimized frequencies to probe the sample over a more extended depth. The analysis of the resulting signals is then different from those issue of the sinusoidal mode. The purpose of this paper is to apply pulsed eddy current techniques to specimens containing cracks of different size and depth. First, an original signal analysis was developed to extract the characteristics of well known defects. Then, this approach has been validated by probing different samples with real flaws.
ECT-based thickness loss estimation for corroded steel plate
- H Tamura
- S Tominaga
- E Sasaki
- G V Minesawa
Tamura, H., Tominaga, S., Sasaki, E. and Minesawa, G.
V.: ECT-based thickness loss estimation for corroded steel
plate, The 20th International Workshop on Electromagnetic Nondestructive Evaluation 2015, Sendai, Japan, OS3-1,
2015.