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Technical Discussions
Elmar van den Elzen
Elmar van den Elzen
00:17 May-12-2000
crack sizing with eddy current

Dear colleagues,

Sizing of the depth of surface breaking cracks with eddy current is at least difficult. I would like to make an overview of crack conditions influencing the eddy current response. What are the main factors involved and roughly how much variations can these factors be expected to cause? The local stress seems to be one factor, and what about course of the crack, width of the crack, condition of the internal crack surface etc. etc.
Does anybody have experience with such factors?
Does anybody know existing literature with a proper covering of this subject?
I am especially looking forward to replies relevant to sizing of surface breaking cracks in hardened ferromagnetic steel types.

Elmar van den Elzen



    
 
 Reply 
 
Tim MacInnis
Tim MacInnis
09:40 May-12-2000
Re: crack sizing with eddy current
Eddy Current Crack Sizing


Eddy currents will always take the path of least resistance. When an eddy
current encounters a surface crack that is deep but short in length the
currents will flow around the ends, conversely if the crack is shallow but
long with respect to the coil diameter, the current will flow underneath.
Crack length and depth increase resistance to eddy current flow thereby
changing coil impedance.


Only 5% of the current flows at depths that exceed (3 x Skin Depth) in the
situation where there is a long surface crack most of the current is flowing
under the defect.  From the condition given above only the relative
amplitude and direction are given in an impedance diagram. This is where the
phase angle comes into play in the analysis. The magnetic field and induced
eddy currents have approximately the same phase. Amplitude will drop off
exponentially with distance and eddy current flow increasingly lags in phase
with both depth and axial distance from probe.


For detection and sizing, a starting point is established with a reference
phase direction. The lift-off direction is used and can be easily defined as
the signal resulting from the increased distance between the coil and the
test piece. Starting from the closest point to the defect.


The shallow surface crack will have a large component in the same direction
as lift-off, the approach makes it distinguishable from lift-off. As the
crack depth increases the signal will rotate clockwise due to the increasing
phase angle.


This procedure is good for shallow defects because they are localized and
only intersect one phase and amplitude contour at any given position. The
deeper cracks require the defect to be divided into sections and determine
weighted average values for amplitude and phase at each position.


This conversation assumes all of the eddy currents are blocked by the crack
width, of course, reality is that there is leakage across the crack width
dependent upon the stresses and how tightly closed the crack is.
Sizing is always, and always will, be difficult with and impedance display.
Imaging techniques combined with the procedure above provides repeatable
inspections. Both techniques will require calibration samples containing
known indications in samples manufactured to component specifications or
obtained from actual components. The imaging technique offers more analysis
capabilities when comparing actual test results with results from standards.


Tim MacInnis

-------------
: Dear colleagues,

: Sizing of the depth of surface breaking cracks with eddy current is at least difficult. I would like to make an overview of crack conditions influencing the eddy current response. What are the main factors involved and roughly how much variations can these factors be expected to cause? The local stress seems to be one factor, and what about course of the crack, width of the crack, condition of the internal crack surface etc. etc.
: Does anybody have experience with such factors?
: Does anybody know existing literature with a proper covering of this subject?
: I am especially looking forward to replies relevant to sizing of surface breaking cracks in hardened ferromagnetic steel types.

: Elmar van den Elzen
:




    
 
 Reply 
 

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