How do magnetic flux lines behave around external defects in steel structures?

[Aaron1978]

Hi All,

Can anyone help with the following problem (see attached image

)? The left diagram shows a magnet clamped to a piece of steel (in the example the steel is the wall of a pipeline). The lines of flux are shown to pass through the pipe wall. I am happy with this diagram and what it shows.

The diagram on the right shows the same piece of metal with the same magnet clamped to it, only this time there is some metal missing on the outside of the metal (called an external defect in the diagram). This causes (according to the diagram) the magnetic flux lines to follow the contour of the defect, which allows the defect to be detected by the sensor. Is this a true representation of how the lines of flux would appear and how detection by the sensor would occur?

This technique forms the basis of detecting metal loss in steel structures. See http://en.wikipedia.org/wiki/Magnetic_flux_leakage"
if you're interested.

Appreciate the help,

Bewst Regards,

Aaron

 

[ice109]

did you draw these pictures?

 

[Aaron1978]

No. The pictures are taken from a textbook on MFL tools. I'm not satisfied with the explanation of how the magnetic field is displayed when there is a defect present.

Ulimately I'd like someone to show me what the magnet lines of flux would look like in the following diagram -

Thanks,

Aaron

 

[ice109]

No. The pictures are taken from a textbook on MFL tools. I'm not satisfied with the explanation of how the magnetic field is displayed when there is a defect present.

Ulimately I'd like someone to show me what the magnet lines of flux would look like in the following diagram - View attachment 10384

Thanks,

Aaron

obviously by analogy they look exactly the other one with a chunk missing. Though I'm inclined to think that the field lines would protrude from the missing part.

 

[Aaron1978]

Magnetic Flux "Leakage"

Hello again,

I'd just like to try and revive this thread. Does anyone have anymore ideas?

Regards,

Aaron

 

[nissam]

magnetic flux density

hii dear...
i also tried to plot the flux lines to find the magnetic flux density in the pipe.i used the fem to perform the analysis.but magnetic flux density seems to be decreasing from the magnet section towards the centre of the plate.i would like to discuss with you this issue.mail me at nissam.sidheeq@gmail.com

Regards
Nissam Sidheeq

 

[Meir Achuz]

Ultimately I'd like someone to show me what the magnet lines of flux would look like in the following diagram - View attachment 10384

Thanks,

Aaron

They would look like the streamlines of flowing water, and be compressed between the defect and the bottom of the iron bar. The increased B at this area would increase the reluctance of the magnetic circuit.