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Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general.
Posted by: W. Grandia , E-mail: wgrandia@aol.com, on September 12, 1996 at 02:17:45:In Reply to: Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general. posted by : Robert A. Day on September 11, 1996 at 23:37:40:
: 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 DiederichsDear Mr. Day,
I have to disagree with your reply . We certainly do not inspect flaws in air. We are using air as a couplant medium, and tests are usually performed in through-transmission mode.
We related Rolf’s question to the lateral resolution - which should be in agreement with the Rayleigh criterium - and we were considering either rectangular or circular flaw shapes. Your example of an infinitely long notch deviates considerably from our considerations. Small lines, numerous tiny flaws are known to generate different effects, which are outside the scope of Rolf’s question.
Sincerely,
W. Grandia