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Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general.

Posted by: Robert A. Day , E-mail: rockyd@netcom.com, on September 11, 1996 at 23:37:40:

In Reply to: Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general. posted by : Bill Grandia on September 08, 1996 at 00:32:42:

Rolf -

I have to disagree with Bill Grandia's estimate of
minimum detectable flaw size because we rarely detect
flaws in air. Usually in the material under test. Using
the diffraction limit of the transducer [at best Lambda
the wavelength] is not a bad approximation to the
longest dimension of the reflector that may be detected
the other dimensions maybe substantially smaller.

An example would be detection of 0.003 inch notch in
steel tubing at 15 MHz. The wavelength in steel is 0.016
inch so we are detecting less than a fifth of a
wavelength. The notch is in this case 0.25 inch long so
for all practical purposes it is infinite. Since this
is an angle beam detection the diffraction limit tells
us little about the beam because of refraction caused
astigmatism but clearly we are in one dimension doing
much better than a wavelength.

We pay a heavy penalty however to detect at these levels
because we use a focused probe that has a beam width in
steel of about 0.03 inches. To achieve 20% beam overlap
we would need to scan at a pitch of 0.024 inch. The
speed of the scan would have to be really fast to get
reasonable throughput. In the example above the scanning
was done to get 3 hits on a 0.25 notch so the pitch was
about 0.08 inches. It was still a relatively slow test.

Actual flaw detection levels are very much a function of
the test parameters, frequency, focus, diameter, and
geometry. Many things can be done to improve detection.
In the steel tubing example we could go to aspherical
transducers that would achieve a better focus in the
steel and presumably detect smaller flaws. Actually in
this example signal-to-noise was still very good at
0.003 so improvement wasn't needed. Since flaw detection
concerns often happen when doing curved surfaces the use
of asphericals may be valuable.

In a low velocity material, like one we would do air
couple sound on, would have a velocity of 2.5
mm/microsec. So at 500 kH we would have a wavelength of
5 mm. We certainly would expect to be able to see
something this size in homogeneous material, like steel. The only way to significantly improve on this is to use near field imaging which only works if the material is very thin, not much thicker than the resolution we want to get. This isn't practical for most NDT situations but was demonstrated to a 200th of a wavelength by Eric Ashe many years ago. This principle is the basis of
Scanning Tunneling Microscope for which Bennig and Rohre
got the Nobel.

I haven't even touched on the probability of detection
issue which has to do with the random errors that creep
into any measurement process. Or more precisely to the
definition of what do you mean by detection. Assuming we
want some probability and confidence that a flaw is
detect, what numbers do we use and is it possible to
obtain that performance from a given ultrasonic test.

On possible helper in all this is computer modeling.
Most users do not currently have access to this type of
capability but I expect that will change soon. The Ames
Center for NDE has a program that allows approximate
modeling to be done on complex shapes and provides
reasonable prediction of time of arrival and amplitude
of flaw signals along with the other signals to be
expected. This is a great aid to analyzing some of the
problems associated with how small a flaw can I see?
You should visit their web sight at
http://www.cnde.iastate.edu/ to get more information.
Better modeling tools are in the works and will help
answer complex questions better than we can answer them
now.


Rocky


: : For an applied frequency of 0.5 MHz what will be the smallest flaw
: : that air coupled ultrasound could find?

: : Furthermore I want to enter into a general discussion about smallest detectable flaw sizes.
: : Is there a certain relation between flaw size and frequency?

: : - lambda/flaw dia ratio?

: : - Is this relation linear?

: Yes, we estimate the minimal detectable flaw size to be in the order of the size of the wavelength.
: The focal spot diameter in air for a 25mm diameter element with a focal distance of 35 mm is 1.2 mm (-3dB points) at 400 kHz, with a focal depth of 25 mm (-3dB). In through-transmission with the focal spots coinciding with each surface, the lateral resolution therefore is about 1.2 mm.

: :
: : Preconditions: ideal reflection (shape and impedance).

: :
: : Rolf Diederichs



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