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01:25 May-23-1997

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
Re: Small Void/Inclusion detection
: Are standards and techniques available to detect very small voids or inclusions in pure metals using standard C-scan equipment?? The size I am trying to detect is 0.05 to 0.1mm. If this can be accomplished can anyone tell me where I can get flat bottom holes made to use for calibration standards.

: Thanks
: Gerald

It is certainly possible, in many metals, to find voids and inclusions in the 50-100 micron range using high frequency, high resolution pulse/echo C-scan systems working in the 50 to 100 MHz range. The optimum transducer frequency should be selected based on the specific material and detection requirements at hand. This sort of equipment is commercially available from many sources. You do need high frequency capability, so check bandwidth specifications. You should also be aware that in many cases penetration at these frequencies will be effectively limited to a few millimeters due to a combination of attenuation, scattering, and the very short focal zones of typical commercialhigh frequency transducers.

My own laboratory has a set of very-small-hole steel reference standards that were made for us a number of years ago by a machine shop that uses EDM techniques. A number of our customers have done the same thing when evaluating our equipment. You may want to find a local vendor who can do EDM machining and design your own set of standards.

--Tom Nelligan




 
01:29 May-23-1997

Linas Svilainis

R & D,
Kaunas University of Technology,
Lithuania,
Joined Nov 1998
66
Re: Small Void/Inclusion detection : Are standards and techniques available to detect very small voids or inclusions in pure metals using standard C-scan equipment?? The size I am trying to detect is 0.05 to 0.1mm. If this can be accomplished can anyone tell me where I can get flat bottom holes made to use for calibration standards.

Dear Gerald,

I've seemed paper dealing with problem like yours.
They use the phenomenon that reflectors exhibit particular
frequency template, depending on their size (and,
unfortunately, the shape). So, after inspecting just
time domain (A-scan) frequency response it is possible
to tell whether it was the defect of desired(?) size
or not. They went even further. The exciting pulse
was the complex signal, which contained inverse of
the spectra of reflection from defect of targeted size.
In such case just the reflection from defects of targeted
size will give the largest reflection. C-scan (I mean a
MaxPlot C-scan) will exhibit such defects reflections.
One can goeven further. The spectral processing can
be applied to C-scan data in spatial domain, or, if
you can collect the whole volume of 3-D space(time,x,y),
you could apply the correlation algorithms or neural
networks. Might be SAFT'ing in connection with frequency
signature analysis could eliminate the orientation
problem... But...
One has to consider few factors, limiting such application.
One problem is the facilities required to do such task.
Good reference is right here - Tom Nelligan's paper is
presenting the time and storage requirements for ultrasonic
automated systems:
http://www.ultrasonic.de/article/wsho0597/nelligan/nelligan.htm

Another problem is related to the small size of the
defects you are looking at. It would be good to have
the inspection wave of order of the defect you are
looking for. But in such case you'll have to switch to
frequencies of 20-50MHz. In such case the attenuation
will raise rapidly. Also it should be denoted what you
mean by pure metals. If the level of "grain noise" is
high, then your defects could be masked by the structure
of the metal.
Calibration blocks...do not know. We usually use the
holes produced by electrical spark. But the shape
of boottom of such hole could not be predicted. Might
be laser-drilled hole will suite?

Good luck,

Linas


 
09:35 May-26-1997

Godfrey Hands

Engineering,
PRI Nadcap,
United Kingdom,
Joined Nov 1998
281
Re: Small Void/Inclusion detection : Are standards and techniques available to detect very small voids or inclusions in pure metals using standard C-scan equipment?? The size I am trying to detect is 0.05 to 0.1mm. If this can be accomplished can anyone tell me where I can get flat bottom holes made to use for calibration standards.

: Thanks
: Gerald

Dear Gerald,
It is certainly possible to detect this size of inclusion or void in steel
samples, and some replies have already said that at the higher frequencies,
penetration is very limited.
Together with anothet author, I wrote an article "In search of cleaner
steel" based on our experiences of a similar application to yours. I think
it was in Materials Evaluation September 1995, (ME is sure, date is not).
We were able to clearly show 35 micron defects (0.35mm diameter). With
"Lower" frequencies (25 MHz), these were also detectable up to a couple of
centimetres deep in fine grain steel, but up to about 6 or 7mm deep with
80MHz.
Contact me if you need more info.
regards,

Godfrey Hands




 
00:44 May-27-1997
Joachim Bamberg
Re: Small Void/Inclusion detection : Are standards and techniques available to detect very small voids or inclusions in pure metals using standard C-scan equipment?? The size I am trying to detect is 0.05 to 0.1mm. If this can be accomplished can anyone tell me where I can get flat bottom holes made to use for calibration standards.

: Thanks
: Gerald

Hi Gerald!

I think we can help you. We (MTU-München) have a
special drilling machine in our lab for producing flat
bottom holes of size 1.6 to 0.05 mm diameter in super-
alloys. The depth is about seven times diameter. Deeper
holes are possible by setting greater holes at first
and making the desired tip thereafter inside the hole.

My colleague Wolf-Dieter Feist produced a lot of
calibration standards by this way.

25 MHz focused transducers are sufficient for the
detection of these artificial defects
(in the case of low attenuating alloys!).

For more information please contact us.

Grettings
Joachim


 


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