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Technical Discussions
Tom Bloodworth
Consultant, R&D
Bloodworth Consulting Limited, United Kingdom, Joined Aug 2002, 12

Tom Bloodworth

Consultant, R&D
Bloodworth Consulting Limited,
United Kingdom,
Joined Aug 2002
12
09:39 Aug-09-2002
Permeability Variations in Steel

For many years I have known that a big limitation to eddy-current testing of steels is the noise created by changes in the relative permeability (for ET, the initial permeability - the low-field limit) of the material. This can be due to work history, thermal history etc.

Although this is "well-known", I have never seen any figures for the variation that one might find e.g. across a steel plate or at a weld. I just know a rule of thumb that for most steels mu_i is of the order 100.

At present I am trying to establish a standard for test and evaluation of metal detectors used for landmine detection. I am investigating the feasibility of using ordinary chrome steel balls as standard testing targets. One of the arguments against using such a material is that batch-to-batch mu_i variation could mean that the targets do not give a "standard" response.

Does anyone know of any data available that would give me an idea of what % variation I might expect to see in the mu_i of such steel balls of the same, or similar material. Just to know whether it would be 10% or 100% of the nominal value would be useful. If not for steel balls, have such data been collected for other materials and product forms?


    
 
 
rodney fordham
Consultant
United Kingdom, Joined Feb 2002, 12

rodney fordham

Consultant
United Kingdom,
Joined Feb 2002
12
02:39 Aug-09-2002
Re: Permeability Variations in Steel
: For many years I have known that a big limitation to eddy-current testing of steels is the noise created by changes in the relative permeability (for ET, the initial permeability - the low-field limit) of the material. This can be due to work history, thermal history etc.
.
: Although this is "well-known", I have never seen any figures for the variation that one might find e.g. across a steel plate or at a weld. I just know a rule of thumb that for most steels mu_i is of the order 100.
.
: At present I am trying to establish a standard for test and evaluation of metal detectors used for landmine detection. I am investigating the feasibility of using ordinary chrome steel balls as standard testing targets. One of the arguments against using such a material is that batch-to-batch mu_i variation could mean that the targets do not give a "standard" response.
.
: Does anyone know of any data available that would give me an idea of what % variation I might expect to see in the mu_i of suchsteel balls of the same, or similar material. Just to know whether it would be 10% or 100% of the nominal value would be useful. If not for steel balls, have such data been collected for other materials and product forms?
.
the properties of steel balls from one source is not likely to vary by much. in any case testing is a very simple matter and should be investigated in any case.



    
 
 
Joe Buckley, Sonatest Plc
Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT, United Kingdom, Joined Oct 1999, 519

Joe Buckley, Sonatest Plc

Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT,
United Kingdom,
Joined Oct 1999
519
03:17 Aug-09-2002
Re: Permeability Variations in Steel

Tom,
I don't have much experience of direct permeability measurements, but quite a lot on comparative Eddy current reponse (e.g in a sorting station) Which I would think is analogous to what one would detect with a typical metal detector circuit.

Variations within a 'consistent batch' (e.g. Ball bearings being sorted for hardness) are small typically a 2-3% of the total response. Variations between different batches of nominally the same material may be a bit bigger - say 5-10% at most.
variations due to heat treatment may be huge, in extreme cases of soft vs. fully hardened can be 50% or more.

So I would hypothesise that in using this as a reference for metal detection you would need to have some kind of standardisation of eddy current response to elininate the possibility of gross error, but given that, ensuring adequate consistancy would not be a big problem. (obviously I'm one of several vendors who woul be happy to help here)

Of course reality may be different, but it sounds worth a try.

Good luck

Joe

: For many years I have known that a big limitation to eddy-current testing of steels is the noise created by changes in the relative permeability (for ET, the initial permeability - the low-field limit) of the material. This can be due to work history, thermal history etc.
.
: Although this is "well-known", I have never seen any figures for the variation that one might find e.g. across a steel plate or at a weld. I just know a rule of thumb that for most steels mu_i is of the order 100.
.
: At present I am trying to establish a standard for test and evaluation of metal detectors used for landmine detection. I am investigating the feasibility of using ordinary chrome steel balls as standard testing targets. One of the arguments against using such a material is that batch-to-batch mu_i variation could mean that the targets do not give a "standard" response.
.
: Does anyone know of any data available that would give me an idea of what % variation I might expect to see in the mu_i of such steel balls of the same, or similar material. Just to know whether it would be 10% or 100% of the nominal value would be useful. If not for steel balls, have such data been collected for other materials and product forms?
.



    
 
 
David J. Reid
David J. Reid
02:53 Sep-16-2002
Re: Permeability Variations in Steel
Tom,

Your biggest limitation in doing eddy current testing on steels is not permeability but the non-ferrous plating you often find on steel. Small mintue differances in the thickness of the plating over the part will cause noise. Resiudal magnetism in the steel part can also pose noise problems in some cases.

Another thing you would have to keep in mind is that with eddy current and penetrant testing, cracks in the plating will also give the same response / indication as cracks in the steel base metal. For this, it would require magneic particle to sort out the good, the bad and the ugly.

Hope this helps.

David

: For many years I have known that a big limitation to eddy-current testing of steels is the noise created by changes in the relative permeability (for ET, the initial permeability - the low-field limit) of the material. This can be due to work history, thermal history etc.
.
: Although this is "well-known", I have never seen any figures for the variation that one might find e.g.across a steel plate or at a weld. I just know a rule of thumb that for most steels mu_i is of the order 100.
.
: At present I am trying to establish a standard for test and evaluation of metal detectors used for landmine detection. I am investigating the feasibility of using ordinary chrome steel balls as standard testing targets. One of the arguments against using such a material is that batch-to-batch mu_i variation could mean that the targets do not give a "standard" response.
.
: Does anyone know of any data available that would give me an idea of what % variation I might expect to see in the mu_i of such steel balls of the same, or similar material. Just to know whether it would be 10% or 100% of the nominal value would be useful. If not for steel balls, have such data been collected for other materials and product forms?
.



    
 
 

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