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Technical Discussions
Elmar van den Elzen
Engineering,
Lismar Engineering B.V., Netherlands, Joined Aug 2000, 15

Elmar van den Elzen

Engineering,
Lismar Engineering B.V.,
Netherlands,
Joined Aug 2000
15
00:06 Feb-12-2004
cracks with low EC-response

Dear colleagues,

Recently we tested a hardened ferromagnetic steel with a small percentage of finely distributed free graphite. De test pieces had deep cracks more than several mm deep. The cracks could be easily detected with dye penetrant, but the EC-response was very low and compared to a crack depth of around 0.1mm in normal hardened steel. Does anyone know if the low response can be due to graphite forming electrical bridges in the crack faces? Or is this unlikely? Does anyone in general have a (quantified) idea about the influence of electrical bridges in the crack faces? And perhaps any ideas how to reduce this influence?

Elmar van den Elzen


    
 
 Reply 
 
Godfrey Hands
Consultant,
PRI Nadcap, United Kingdom, Joined Nov 1998, 298

Godfrey Hands

Consultant,
PRI Nadcap,
United Kingdom,
Joined Nov 1998
298
05:49 Feb-12-2004
Re: cracks with low EC-response
----------- Start Original Message -----------
: Dear colleagues,
: Recently we tested a hardened ferromagnetic steel with a small percentage of finely distributed free graphite. De test pieces had deep cracks more than several mm deep. The cracks could be easily detected with dye penetrant, but the EC-response was very low and compared to a crack depth of around 0.1mm in normal hardened steel. Does anyone know if the low response can be due to graphite forming electrical bridges in the crack faces? Or is this unlikely? Does anyone in general have a (quantified) idea about the influence of electrical bridges in the crack faces? And perhaps any ideas how to reduce this influence?
: Elmar van den Elzen
------------ End Original Message ------------


Elmar,
Sounds as if the conductivity of the graphite was having an effect on the test.
Was the Eddy Current "Noise" level significantly higher than on the hardened steel block?

Was this with a static or a dynamic test, i.e. was the probemoving very slowly and you looking at phase and amplitude changes of the unmodulated signal or was it with a modulated signal with the probe moving quickly over the surface?
What phase angle did you get from the Calbration Defect and what phase angle from the cracks that were difficult to detect ?

Regards,

Godfrey Hands



    
 
 Reply 
 
John Hansem
Director, - Eddy Current Technology
ETher NDE Ltd, United Kingdom, Joined Oct 1999, 73

John Hansem

Director, - Eddy Current Technology
ETher NDE Ltd,
United Kingdom,
Joined Oct 1999
73
06:06 Feb-12-2004
Re: cracks with low EC-response
A bit more information on how exactly you were testing would help us all.

Probe type, configuration and size?

Instrument

Frequency

Crack mechanism (eg fatigue, stress corrosion, hydrogen induced etv)?

Rgds

John Hansen


    
 
 Reply 
 
Elmar van den Elzen
Engineering,
Lismar Engineering B.V., Netherlands, Joined Aug 2000, 15

Elmar van den Elzen

Engineering,
Lismar Engineering B.V.,
Netherlands,
Joined Aug 2000
15
04:26 Feb-16-2004
Re: cracks with low EC-response
Dear All,

In reply to the questions asked by John Hansen and Godfrey Hands.

Probe type and instrument: In house made equipment. Probe consists of 4mm diameter ferite core coils, in a differential configuration. EC-frequency is 100kHz. Phase angle not defined as such, but approximately the component in-phase with the drive signal is evaluated.

Crack: We have received reports that possibly the response of our equipment on one type of steel is low. The cracks in question are not one single type of cracks, the cracks in question are initiated by incidental overloading and some might grow further as a fatigue crack.

There is no out of the usual signal or 'noise' of the unaffected parts of the material. The only thing is that cracks are more than several mm deep, but give signals that compare to cracks or slots of a few tenth of a mm.

One hypothetical reason we can think of is that the lamellar graphite inside the material increases the leakage of eddy currents through the crack. Has such a thing been observed before? Is such a thing likely? Would there be any test to prove this? For example if we can somehow etch away possible graphite inside the crack? By the way, the test pieces are too large to open the crack by bending.

Elmar van den Elzen

----------- Start Original Message -----------
: A bit more information on how exactly you were testing would help us all.
: Probe type, configuration and size?
: Instrument
: Frequency
: Crack mechanism (eg fatigue, stress corrosion, hydrogen induced etv)?
: Rgds
: John Hansen
------------ End Original Message ------------




    
 
 Reply 
 
John Hansen
Director, - Eddy Current Technology
ETher NDE Ltd, United Kingdom, Joined Oct 1999, 73

John Hansen

Director, - Eddy Current Technology
ETher NDE Ltd,
United Kingdom,
Joined Oct 1999
73
03:08 Feb-17-2004
Re: cracks with low EC-response
Elmar

Thanks for that.

I'm not convinced that this is bridging as you describe. Certainly in my experience this is not a known problem.

My suggestion is that it may be your equipment.

Why don't you senmd me a sample with cracks an I will give you a report using our equipment or if they are too big our local distributor could contact youi.

Rgds

John Hansen


    
 
 Reply 
 

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