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- since 1996 -

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Technical Discussions
M.Kovacik
M.Kovacik
08:20 Feb-12-2004
ET

Dear friends

Has anybody idea how to eliminate or supress permeability variation signal when testing ferromagnetic surface with ET pen probe. We tried various differential probes - no expexted results. Is there for example any possibility to saturate the material around the probe?

thanks for any advice


    
 
 Reply 
 
Ilham Mukriz
R & D
Malaysian Institute for Nuclear Technology Researc, Malaysia, Joined Feb 2004, 2

Ilham Mukriz

R & D
Malaysian Institute for Nuclear Technology Researc,
Malaysia,
Joined Feb 2004
2
09:27 Feb-13-2004
Re: ET
Hi there,

The large variations in permeability make conventional eddy current testing for defects in ferromagnetic materials very difficult if not impossible.
It is possible to magnetically saturate some materials by DC magnetization or by using permanent magnets.Permeability variation are completely suppressed by saturation
If the material is slightly magnetic, it can be heated above its Curie temperature to make them nonferromagnetic.

Hopefully this information will be helpful for you.

----------- Start Original Message -----------
: Dear friends
: Has anybody idea how to eliminate or supress permeability variation signal when testing ferromagnetic surface with ET pen probe. We tried various differential probes - no expexted results. Is there for example any possibility to saturate the material around the probe?
: thanks for any advice
------------ End Original Message ------------




    
 
 Reply 
 
Joe Buckley
Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT, United Kingdom, Joined Oct 1999, 520

Joe Buckley

Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT,
United Kingdom,
Joined Oct 1999
520
01:24 Feb-13-2004
Re: ET
Have you tried using an orthogonal coil differential probe e.g. Zetec '+ point' or Hocking 'Weldscan' types - These have two coils which are acting in orthonal directions on the same point, so Permeability should be cancelled out. They were originally designed for weld-Inspection to cope with the permeability cahnges in the heat affected Zone, but in practice they work very well in many other applications.

Joe


----------- Start Original Message -----------
: Hi there,
: The large variations in permeability make conventional eddy current testing for defects in ferromagnetic materials very difficult if not impossible.
: It is possible to magnetically saturate some materials by DC magnetization or by using permanent magnets.Permeability variation are completely suppressed by saturation
: If the material is slightly magnetic, it can be heated above its Curie temperature to make them nonferromagnetic.
: Hopefully this information will be helpful for you.
: : Dear friends
: :Has anybody idea how to eliminate or supress permeability variation signal when testing ferromagnetic surface with ET pen probe. We tried various differential probes - no expexted results. Is there for example any possibility to saturate the material around the probe?
: : thanks for any advice
------------ End Original Message ------------




    
 
 Reply 
 
Mike
Mike
07:01 Feb-13-2004
Re: ET
----------- Start Original Message -----------
: Hi there,
: The large variations in permeability make conventional eddy current testing for defects in ferromagnetic materials very difficult if not impossible.


You're living in the dark ages. Eddy current inspection of ferromagnetic materials is done on a regular basis.


    
 
 Reply 
 
D
D
07:43 Feb-13-2004
Re: ET
Zetec type Plus-Point or Hocking Weld-Scan type cross wound coils are a good starting point. If this fails then you could try magnetic saturation or mag-bias, although this is not much use on ferritic steels. The next step (and what I would suggest) is to use a Transmit / Recieve type coil arrangement.

This uses two seperate coils in the probe and gives a far better signal to noise ratio (about ten times better). The down-side of this is that the probe has directional properties, and must be scanned at least twice for full coverage.

The problem of low signal strength is a common result of cross-talk between the two defect faces reducing the voltage (although not altering the phase angle much). Typically I would expect a calibration signal to give 10V say, and a defect of similar dimensions to give 1 volt. There is no way around this problem other than turning up the gain. Data should always be analysed with regard to phase angle, as this is the critical factor, and which is not altered much as aresult of conductivity across the defect.
Don




    
 
 Reply 
 
Ilham Mukriz
R & D
Malaysian Institute for Nuclear Technology Researc, Malaysia, Joined Feb 2004, 2

Ilham Mukriz

R & D
Malaysian Institute for Nuclear Technology Researc,
Malaysia,
Joined Feb 2004
2
03:36 Feb-16-2004
Re: ET
My mistake. Maybe I should Underlined and Bold the word 'CONVENTIONAL'.

----------- Start Original Message -----------
: : Hi there,
: : The large variations in permeability make conventional eddy current testing for defects in ferromagnetic materials very difficult if not impossible.
:
: You're living in the dark ages. Eddy current inspection of ferromagnetic materials is done on a regular basis.
------------ End Original Message ------------




    
 
 Reply 
 
Mike
Mike
00:22 Feb-24-2004
Re: ET
So, in your opinion, what would be 'unconventional' about the eddy current examination of ferromagnetic materials?

----------- Start Original Message -----------
: My mistake. Maybe I should Underlined and Bold the word 'CONVENTIONAL'.
: : : Hi there,
: : : The large variations in permeability make conventional eddy current testing for defects in ferromagnetic materials very difficult if not impossible.
: :
: : You're living in the dark ages. Eddy current inspection of ferromagnetic materials is done on a regular basis.
------------ End Original Message ------------




    
 
 Reply 
 

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