Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general. 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.
03:37 Sep-11-1996 Robert A. Day Engineering Milky Way Jewels, USA, Joined Nov 1998 40
Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general. 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.
Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general. Dear 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 Rolfs 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 Rolfs question.
00:24 Sep-12-1996 Rolf D. Director, Editor, Publisher, Internet, PHP MySQL NDT.net, Germany, Joined Nov 1998 598
Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general.
thanks for your answer with so much details.
With the attached picture I want to make my questions more clear.
Rocky explained a very complicate subject, I didn't wanted to release a so detailed discussion. However If you like you could continue and try to keep the general aspect on sound propagation for this workshop theme. Further I could past this messages to a future workshop on flaw detection, this could be a valuable start.
Besides the air coupling capability I wanted to get a more detailed information about the smallest flaw size for a simple shaped defect (FBH). Especially the area where the curve starts to drop and further how the curve continue.
I can imagine that the curve drops within x dB per wavelength. What will be x?
I remember from my time I did really ultrasonic (no virtual like now) that we recognized conclusions in plastic pipes mostly in a range of 1/2 to 1 wavelength, so this confirms what Bill stated. Refer also to the attached article where transducer selection for a given task is discussed.
Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general. Dear Rolf,
Thanks for your clarifying remarks about the subject of flaw size
detection. For those interested to continue this route of discussion, I
have added my detailed response to Robert Day's remarks in the following
Thanks to Robert and Rolf for making such discussions possible.
Regards, Bill Grandia
REMARKS ON MINIMAL FLAW SIZE DETECTABILITY, ESPECIALLY FOR AIR-COUPLED ULTRASONICS
(In response to the discussion of Robert A. Day of September 11, 1996)
From all the reactions received, I am becoming aware that the principles of non-contact (Airscan) ultrasonics are not quite
understood by persons experienced in conventional fluid coupled ultrasonics.
In the reply from Robert, it states that at a sound velocity of 2.5 mm/msec at 500 kHz the wavelength is 5 mm (this would apply
for something like graphite epoxy material). This does NOT imply that the smallest detectable flaw size would be in the order of
Here is a misconception, which will be further explained in this writing. Prior to the explanation, I would like to react on the quoted "analog sample" of a pulse-echo inspection at 15 MHz, to detect a 0.003" deep notch with a length of 1/4 inch. This is an extreme example, which should not be used to compare with the method of a through-transmitted non-contact ultrasonic inspection for the following reasons. The detection of a small notch (0.003") requires a pulse-echo system, using either a spherical, cylindrical, or better yet, a compound focused transducer. When such a transducer is used, let us say a wide-band 15 MHz, F#4 (which means that the focal length is 4 x the element diameter, which is the 1 inch), the diameter of the focal waist area, based on a -3 dB drop, is 0.10 mm, while the depth of field becomes 11.5 mm. These dimensions are of course for water. Of course, inside the steel, these dimensions change as a function of the difference of the L-wave sound velocity in water and the propagation speed of shear waves in steel. Also the shape of the focal spot becomes somewhat distorted or flattened in favor of the detectability for the shallow notch detection. Now, to detect a 0.003" notch, we need a high frequency contents in the transmitting signal. Most commercially available ultrasonic instruments feature a transmitting pulse rise-time of approximately 10 nanoseconds. The fastest pulse width recovery is usually larger then 10 nanoseconds. So, we may reason that the fastest pulse width available is around 20 nanoseconds, representing a transmitter pulse frequency of 25 MHz. Of course, the water and steel are acting like a low-pass filter and a considerable amount of the high frequency contents becomes attenuated. Since we need to detect the 0.003" deep notch, the use of a narrow band 25 MHz transducer is less desirable. We may select a wide-band 15 MHz center frequency transducer, having enough bandwidth to reach into the higher frequency regime. For this reason, the bandpass of the receiving amplifier should be set for high pass filtering above lets say 10 MHz to take advantage of the high frequency resolving power (Not all ultrasonic instruments have such a feature). At the same time, low pass filtering under about 30 MHz is desirable to limit the overall bandpass as would be beneficial to suppress the noise floor. Again, most instruments do not have this option. From this discussion, it becomes apparent that the 0.003" notch becomes detectable, especially since the notch width is much larger than focal waist width. I would like to emphasize again that we are discussing a high frequency wide band condition. For this reason, it is not quite fair to use the above discussed technique as a comparison to a narrow band 400 or 500 kHz Airscan through-transmission method. Despite the fact that Airscan uses a narrow band approach, the technique is very much capable of detecting small flaws ( in the order of 1 mm). The wave length in air at 400 kHz is 0,85 mm. The focal waist diameter for a 1 inch (25 mm) diameter 400 kHz transducer, having a focal distance in air of 1.5 inch or 38 mm equal to 1.2 mm with a depth of field equal to 25 mm in air. According to Robert, the minimum detectable flaw size is about 5 mm because of his reasoning that the wavelength in the material is about 5 mm, which is the misconception. First of all, in our Workshop paper we have included an Airscan C-scan image (fig.7), where we show that we can definitely detect a missing bond of a single wall in a honey-comb core structure. In this case we are detecting a flaw with a length dimension of approximately 1 mm. Looking at the picture must make every body a believer. All we need now is to explain why such a fine lateral resolution is possible. For simplicity sake, lets discuss the focused non-contact inspection on a flat Graphite/Epoxy laminate. The transmitting transducer is usually aligned perpendicularly to the surface of the part, such that the focal spot coincides with the entrance interface "air to laminate". Opposed is a similar receiving transducer located at the sound-exit side. After the transmitting sound beam (having a narrow focal width) hits the surface, there will be sound propagating in many different wave modes in various directions within the test part. Wave modes are possible like longitudinal- , shear-, bulk- and guided plate waves. Each mode will have different propagation speeds. However the longitudinal wave mode is the fastest one. This means that at the exit side of the panel the L-wave component will be arriving first. In case of a simple laminate, this is the mode we are interested in (this is not the case for honey-combs later explained) The shortest travel occurring through a virtual cylindrical sound port, having a diameter of the focal spot waist width can be detected and gated in its appropriate time domain. The amplitude of the L-wave component within the gate is monitored and further processed via a data acquisition/ imaging routine. Any flaw-like sound obstruction such as small voids, distributed porous contents, delaminations, resin starved conditions, etc., can now be detected. For this reason the minimum detectable flaw size is in the order of the focal spot diameter (in air), or even smaller if we take a sound drop of - 6 dB into account. It was also suggested by Robert to check with the Ames Center of NDE in regards to the program that allows approximate modeling to be done in complex shapes and providing a reasonable prediction of arrival and amplitude of flaw signals along with the other signals to be expected. Unfortunately, Dr. Chimenty fromAmes has not contacted us for exchange of information, and therefor my knowledge about their progress is only limited to a few published papers. However our group at QMI has been working successfully with Airscan in excess of 10 years. During this period of time, we and a large group of Aerospace customers have been using our technology with great success. Especially strong progress has been made since the recent years after our instrument model "SONDA-007" was released. Judging from honey-comb C-Scan (fig 7 of our workshop paper), the high lateral resolution capability can not be denied. I also would like to present some inside into the evaluation technique for the inspection of organic honey comb materials (such as solar panels) and even metallic honey comb structures. When we align a set of transducers in a perpendicular manner with the honey comb sheet, we cannot expect that a longitudinal wave travels through a thin core wall, having a wall- thickness much smaller then the wavelength. In this case, the wave mode through the core wall is an Ao (antisymmetrical) wave, which has a slow phase velocity and therefore a longer wave length. (pretty much like the ondulating motion in a rope when one end is fastened to the door knob, while the other end is moved by hand with a brief up-and-down motion.) This wave type experiences a low signal attenuation. On the surface of the part, the L -wave at the focal spot becomes refracted and a mode conversion occurs of which the Ao component is the one that propagates towards the exit side. At this point the reverse mode conversion occurs. Using this technique, it is very necessary to locate the monitoring gate such that the through transmitted at a location very closely to the core wall is captured. We do not wish to get a through transmitted signal in the gate from a sound beam hittingthe center of the core (drum /membrane effect), since as a consequence the sound will travel through all the adjacent core walls, hereby impairing the detectability of a single core wall. It is obvious that the received signal amplitudes are extremely small. To overcome this problem, we use a strong tone-burst transmitting approach, while the receiver is preceded by a remote super-low-noise preamplifier. Good signal to noise ratios are obtained in real time without the necessity of digital or real time averaging or without the use of pulse correlation or other means of noise reduction. It is also possible to inspect solid panels by using a single sided plate wave mode, hereby aligning the tranducers under a critical angle to allow an Ao guided plate wave to travel through the material. The value of the transducer entrance angle is related to the wall thickness and phase velocity as can be seen from a dispersion diagram. Or instead theangles can be simply aligned to offer the maximum signal amplitude. We have detected a delamination with a diameter of 5/16 inch in a PEEK material plate having the transducer focal spots at a distance span of about 12 inches, hereby gating for the very first "signal-component-only" to arrive and captured within the gate. Hopefully, this writing will take away some of the confusions.
09:40 Sep-19-1996 Yosi Bar-Cohen R & D, Jet Propulsion Lab (JPL), USA, Joined Nov 1998 26
Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general. Ultrasonic methods of detecting flaws are using the perturbation effect of flaws on the propagating ultrasonic waves. The delectability of the wave depends on the flaw characteristics, i.e., size and the material content.
Material content - This characteristics is related to inclusion flaws. A simplistic rule suggests that the larger the acoustic impedance mismatch with the host material the greater the flaw detectability.
Flaw size - Flaws will scatter ultrasounds that are impinging on them. A flaw that is larger than the wavelength will form a backscattering directivety, i.e., reflection. Flaw size equal or greater than the wavelength will cause a reflected amplitude that is equal to the impinging wave amplitude. When the size of the flaw is smaller than the wavelength, the scattered wave amplitude drops exponentially as a function of the flaw-size/wavelength ratio. As a general rule: the minimum detectable flaw size is considered the value of half the wavelength.
As an example, for a steel part that is tested at 0.5 MHz, the velocity is about 6x10^6 mm/sec and the wavelength is 12 mm. Therefore, the smallest detectable flaw is 6 mm.
Overall this is only a guiding criteria. The actual detectable flaw depends on numerous factors that include instrumentation capability and inspector attentiveness & qualification. This factors are used to determine the probability of detection (POD) of the flaw.