where expertise comes together - since 1996 -

The Largest Open Access Portal of Nondestructive Testing (NDT)

Conference Proceedings, Articles, News, Exhibition, Forum, Network and more

where expertise comes together
- since 1996 -

437 views
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



    
 
 
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
:




    
 
 

Product Spotlight

NovaScope 6000

The all-digital Novascope 6000 is a portable, ultra-high precision thickness gauge for high-speed
...
thickness measurement. Novascope 6000 has unmatched capabilities and unique features including: •Superior Resolution with high contrast, high-speed color RF display •High pulser voltage •Real-time video output •Increased internal/external data storage •Programmable SetUp features •Battery & AC Powered
>

SITEX CPSERIES

Teledyne ICM’s CPSERIES has been designed with a view to revolutionizing the handling and perfor
...
mances of portable X-Ray sets. Despite having managed to halve the weight of similar portable X-Ray generators available on the market (while continuing to provide the same power output), the SITEX CPSERIES generators feature a shutter, a laser pointer, a beryllium window, an aluminum filter and two integrated diaphragms (customized sizes are available upon request). Without compromising the robustness and reliability for which ICM products are renowned, the small size and light weight of the SITEX CPSERIES will radically change the way that you perform your RT inspections. And you will see a positive impact in terms of both quality and return on investment (ROI).
>

Ultrasonic Testing Immersion Tanks with Unmatched Scanning Features

TecScan’s non-destructive testing Ultrasonic Immersion Tanks & scanners are designed for high pe
...
rformance and demanding NDT testing applications. Our Scan3D™ line of High Precision Immersion Tanks are specifically designed for automated ultrasonic testing of complex composites parts used in aerospace and industrial applications.
>

Teletest Focus+

Teletest Focus+ electronics have superior capabilities than rivals on the market. Beyond the usual
...
test features, Focus+ has 24 transmit channels and 24 receive channels, with an additional on-board switching capabilities. The instrument's frequency range is 10–300 kHz.
>

Share...
We use technical and analytics cookies to ensure that we will give you the best experience of our website - More Info
Accept
top
this is debug window