Bridge – FHWA InfoTechnology

Bridge - Magnetic Flux Leakage (MFL)

Target of Investigation

The MFL method can be used to detect the location and extent of corrosion in post-tensioned and precast prestressed strands in concrete girders, and breakage of wires and strands in post-tensioning tendons and prestressing strands. MFL is also commonly used to test the cables of suspension or cable stayed bridges. MFL units can be clamped onto a cable as part of a climbing module or rolled across a surface.

Description

The MFL method uses a magnetic field to detect flaws in ferromagnetic materials such as steel in bridge components like prestressed girders, post-tensioned girders and decks, and suspension cables. The technique involves using an electromagnet or a permanent magnet yoke to magnetize the object being tested. The injected magnetic fluxes flow through the ferromagnetic object because it has a higher permeability than air. Any defect or discontinuity in the test object causes leakage of magnetic fluxes from the object, detectable by magnetic field sensors. It is very similar to magnetic particle testing, however instead of using magnetic particles to create a visual indication on the surface, MFL uses sensors and data processing to provide real-time results during inspections.

Physical Principle

MFL equipment magnetizes embedded reinforcement bar (like steel, prestressed element/strand, post-tensioned tendon, etc.) inside a concrete structure along its length via an external magnet. The potential leakage of the magnetic field from the magnetized reinforcement is evaluated by one or more magnetic sensors within the MFL equipment.⁽¹⁾The output of the sensors is normally recorded in the form of the magnetic signal amplitude as a function of the position along the length of the reinforcement being tested (figure 1). Without flaws present, the internal magnetic flux lines are uniform and travel through the material. Any change in the magnetic field within reinforcement can be linked to the extent of section loss from corrosion or fracture in the reinforcement.

Magnetic Flux Leakage. The illustration contains two parts. The top portion is a two-dimensional schematic side view of a concrete structure, with an embedded horizontal steel bar. Above the structure are two sensors, each of which monitors magnetic field created by magnetic flux leakage equipment. The magnetic fields are depicted by red arrows dipping into the concrete from the north pole on the right , flowing down to the embedded steel bar, then rising to the south pole on the left. A sensor is between the two poles. The portion of the steel bar encountered by the magnetic field on the left is not flawed, and the arrows between the poles are not distorted. The portion of the bar encountered by the magnetic field on the right is flawed, depicted by a narrowed portion of the bar (which simulates section loss in the steel reinforcement). The arrows between the poles are distorted in the vicinity of the flaw which indicates leakage of magnetic field signaling potential section loss. The bottom portion is the recorded waveform in the form of the magnetic signal amplitude as a function of the position along the length of the reinforcement being tested. The waveform is horizontal at the location where there is no flaw in the reinforcement and has a sharp peak and valley at the location where there is flaw (section loss) in the reinforcement. — Figure 1. Illustration. Magnetic Flux Leakage.

Data Acquisition

Only a limited number of private service entities possess the required equipment to perform MFL tests on concrete girders or decks. One or more magnetic sensors are used in the MFL equipment. MFL equipment may be operated manually by moving the equipment on the top of a bridge deck or close to the concrete surface and along the length of the prestressing strands or post-tensioning tendon located inside the concrete. It is also possible to use a robotic mechanism to perform the MFL testing with little to no interruption of traffic flowing under bridge girders (figure 1). For MFL testing on the underside of prestressed adjacent box girders, platforms are necessary for operating the equipment.⁽¹⁾

Use of the MFL Equipment on Prestressed Concrete I-Girders. Magnetic flux leakage robotic equipment is mounted on the underside of a bridge girder. The viewpoint of the photo is looking up from below the bridge — Figure 2. Photo. Use of the MFL Equipment on Prestressed Concrete I-Girders.

Data Processing

Postprocessing of the MFL data is required when detecting small section losses, e.g., less than approximately 10 percent or when there are interfering materials and elements near the prestressing strands or post-tensioning tendons inside concrete. Such postprocessing approaches include comparative and correlation analyses based on existing results from known laboratory and field conditions.

Data Interpretation

The operator monitors the magnetic field amplitude as a function of the test position on a computer screen in real time to identify changes in the magnetic field. In the absence of interfering ferromagnetic materials in the surrounding areas of prestressing strands or post-tensioning tendons, a real-time visual examination of the MFL data or graphs can offer sufficient knowledge about the lack or presence of corrosion or fracture in the strands. Any changes in the magnetic field can be directly related to section loss in the prestressing strands or in the post-tensioning tendons due to corrosion or fracture.⁽²⁾ When there are interfering ferromagnetic materials near the prestressing strands or post-tensioning tendons, more advanced analysis and interpretation of the MFL signal are necessary. Figure 2 shows a MFL graph for a prestressing strand in an in-service bridge that includes a strand fracture as well as transverse steel bars (stirrups).

MFL Records and Calculated Change. The figure contains three images. The top two images are the output of magnetic flux leakage scans on a prestressed girder conducted at two dates about eight months apart. The top graphs is marked as Channel A, while the middle graps as Channel B. The graphs have X axis describing the longitudinal distance on the girder in feet, and Y axis describing the measured amplitude in Volts. The X axis starts at 0 feet and ends at 37 feet. Y axis covers a range from -7 to 7 Volts. The output takes the form of a curve. The pronounced peaks in the curve represent the changes in the magnetic field due to stirrups. The bottom image represents a subtraction of Channel B curve from the Channel A curve. It is marked as Channel A-B and Subtracted Data. The output also takes the form of a curve. The curve is generally flat. There are a couple of locations on the curve marked Probable Defects, where the curve has more pronounced amplitude. — Figure 3. Composite Graph. MFL Record and Calculated Change.

Advantages

Ability to make a qualitative estimate of section loss.
Rapid testing of a large number of surface areas.
Automated testing of concrete girders allows for testing with little to no interruption to traffic.

Limitations

Difficult data interpretation when interfering ferromagnetic materials are present.
Only reliable for post-tensioning tendons and prestressing strands within approximately 6 inches of the concrete surface.
Inexact measurements of flaw sizes (section losses).
Uses specialized equipment not readily available commercially.
Requires well-trained and experienced operators.

References

Ghorbanpoor, A., Borchelt, R., Edwards, M., and Abdel Salam, E., Magnetic-Based NDE of Prestressed and Post-Tensioned Concrete Members – The MFL System, FHWA-RD-00-026, Federal Highway Administration, US DOT, May 2000.
McGogney, C., “Magnetic Flux Leakage for Bridge Inspection,” Nondestructive Testing Methods for Civil Infrastructure, pp. 31-44, 1995.