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General Discussions
13:46 Dec-21-2011
coool guy
2943 views
Dead Zone?
One person told that there is no dead zone when doing immersion testing? If i am right, each and every ultrasonic transducer has got a dead zone. Please correct me if i am wrong.

There will be a dead zone due to the reverberations of the ring down of the crystal.
 
 
14:47 Dec-21-2011

Tom Nelligan

Engineering, ,
Olympus NDT,
USA,
Joined Nov 1998
309 Stand of Olympus
Re: Dead Zone?
In Reply to coool guy on 13:46 Dec-21-2011 (Opening).

Yes, there will be a dead zone in typical pulse/echo immersion tests. Its size is determined by the width of the interface echo from the boundary between the water and your test piece. For a given frequency/bandwidth/instrument setup, that front surface reflection will typically be shorter than the dead zone caused by excitation pulse ringdown, but it is definitely there.

 
 
17:45 Dec-21-2011

Nigel Armstrong

Engineering, - Specialist services,
United Kingdom,
Joined Oct 2000
863
Re: Dead Zone?
In Reply to coool guy on 13:46 Dec-21-2011 (Opening).

Hello

The Dead Zone (DZ) always exists for single crystal probes as it is a product of the ringing time of the electrically excited transducer, but the whole point of immersion testing is to nulify DZ, Nead Zone and surface coupling effects throgh the water path. Ringing time and DZ depend on probe damping amongst other factors such as transducer thickness.

If the immersion test is set up properly then all of this will not impact on the quality of your test - that is the beauty of immersion testing along with regulated coupling conditions.

 
 
09:23 Dec-22-2011
coool guy
Re: Dead Zone?
In Reply to Nigel Armstrong on 17:45 Dec-21-2011 .

Thank you Tom and Nigel,

Mr.Nigel,

AN example. I am using IRIS set up and using a 25Mhz transducer. As you have stated , each and every single crystal transducers has got dead zone. But a Level 3 was challenging that there is no dead zone in high frequencu transducers like 25 Mhz. SO to confirm that , this post was made. Thanbk you for the xplanations.

 
 
13:04 Dec-22-2011

S V Swamy
Engineering, - Material Testing Inspection & Quality Control,
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
650
Re: Dead Zone?
In Reply to coool guy on 09:23 Dec-22-2011 .

Please read the replies carefully again. The dead zone is caused by the initial pulse of the transducer and it is not totally absent even in immersion testing. However, the dead zone caused by the front surface reflection is much less (as correctly pointed out by Tom Nelligan) and Nigel Armstrong compared to contact testing. And there are contact testing probes that have heavily damped crystals (called half wave or quarter wave probes), that have a much less dead zone compared to the normal high power probes. High frequency probes are generally made of thin slices of piezo electric materials and the excitation voltage applied and the deformation induced and the consequent dead zone is less. Contact testing probes with adhesive bonded perspex / acrylic shoes are also available where the dead zone is almost totally removed.

To me, it appears as if there is a simple lack of communication between you and that level III.

Regards

Swamy

 
 
13:22 Dec-22-2011

Ed Ginzel
R & D, -,
Materials Research Institute,
Canada,
Joined Nov 1998
743 Stand of Materials Research Institute
Re: Dead Zone?
In Reply to S V Swamy on 13:04 Dec-22-2011 .

To add to your good advice, Mr. Swamy, I would simply add that there is another factor to consider. When the beam is used to generate a transverse mode the interface signal is virtually eliminated in immersion UT because the reflected pulse is directed away from the probe. But perhaps this aspect is irrelevant in Mr. Guy's application where I suspect the incident angle is intended to be 0° for thickness testing. If you could get a true 25MHz single cycle pulse from the probes used, the dead zone due to the interface signal referred to by Mr. Nelligan would be equivalent to half the wavelength in pulse-echo or about 0.25mm. That is very small..but not zero.

 
 
09:37 Dec-24-2011

S V Swamy
Engineering, - Material Testing Inspection & Quality Control,
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
650
Re: Dead Zone?
In Reply to Ed Ginzel on 13:22 Dec-22-2011 .

Thanks Ed for a very clear explanation as usual. It is good to go back to basics and be clear periodically.

Regards

Swamy

 
 
12:16 Dec-24-2011
coool guy
Re: Dead Zone?
In Reply to S V Swamy on 09:37 Dec-24-2011 .

Dear Mr.Swamy,

Greetings. In fact there was no communication gap. The argument was whether a high frequency single element transducer has a dead zone or not, whether in immersion or contact is not an issue. In IRIS inspection, or in any immersion testing, there will be definitely an initial pulse which is the dead zone. But the L3 person was telling that there is no dead zone in high frequency transducers.

If we do a immersion testing with a high frequency transducer for a tube of 3mm thick, there will be an ID echo and an OD echo. Is it possible to find any defects starting from the ID?

 
 
15:40 Dec-24-2011

massimo carminati
Consultant, AUT specialist,
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
485 Stand of IMG ULTRASUONI SRL
Re: Dead Zone?
In Reply to coool guy on 12:16 Dec-24-2011 .

L3 is definetely wrong, unless you consider a "defect" any place where a OD reflection is missing. As Ed stated, even a 25 MHz shockwave probe has a DZ. The DZ width depends also on the instrument pulser; with such high frequencies you must use a very "fast" square wave pulser at least down to 15 ns rise time.

 
 
13:12 Dec-25-2011

S V Swamy
Engineering, - Material Testing Inspection & Quality Control,
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
650
Re: Dead Zone?
In Reply to coool guy on 12:16 Dec-24-2011 .

The answer to your query depends on the resolution of the echoes. Since I have not used an IRIS I can't really comment more specifically. We were using specially designed immersion testing equipment for tube inspection with probes looking for longitudinal and transverse discontinuities in zircaloy tubes of varying thicknesses the minimum being just 0.4 mm. We were also using normal probes for thickness gauging. The tubes were passed two times the second time reversing the end in some cases and in others we were using probes opposing (2 Longitudinal and 2 transverse probes). The artificial discontinuities were made both on the OD and ID and could be clearly picked up in the angle beam testing mode. There was additional information available from an Eddy Current Test too. Since the tubes are meant as clads for nuclear fuel, all this and many more tests were needed...

Regards

Swamy






If we do a immersion testing with a high frequency transducer for a tube of 3mm thick, there will be an ID echo and an OD echo. Is it possible to find any defects starting from the ID?

 
 
19:11 Dec-25-2011

massimo carminati
Consultant, AUT specialist,
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
485 Stand of IMG ULTRASUONI SRL
Re: Dead Zone?
In Reply to S V Swamy on 13:12 Dec-25-2011 .

I think we've listened enough about these 0,4 mm zircaloy tubes! The fact is that there is a Dead Zone; the answer to dool guy question is no! you cannot find any defect starting from ID. Measuring the wall thickness is a totally different argument, Mr.Swamy!

 
 
20:34 Dec-25-2011

Ed Ginzel
R & D, -,
Materials Research Institute,
Canada,
Joined Nov 1998
743 Stand of Materials Research Institute
Re: Dead Zone?
In Reply to massimo carminati on 19:11 Dec-25-2011 .

zoom image[+]

In an attempt to bring this discussion back to the application, I would point out that boiler tubes are typically 3-5mm thick (so in the range of Mr. Cool Guy’s concern). These might even be suitable candidates for an IRIS style test technique. I agree with Massimo that “defect detection” would be a silly expectation if it was inclusions being sought in the tube wall. However, wall thickness reduction below tmin (typically 40% nominal) is a common unacceptable condition being determined in tube wall assessments. There will indeed be an ID and OD echo and these will limit the ability to resolve the two interfaces. To quantify the resolution we can make estimates, but the surface condition will dictate the signal quality and it will only deteriorate from the ideal. I have attached a real image of a relatively thin tube being assessed for thickness.
In the image, the signal identified as #1 is the initial pulse and on the A-scan indicates a large amplifier recovery signal. Here the high voltage pulse along with the probe ringing occurs (time zero). The timebase-scale is converted to distance in metal so the recovery time is equivalent to approximately 1.5mm. The high-voltage shock portion is avoided when we look later in time to the interface where only the pulse occurs. The waterpath indicates that an equivalent to 4.8mm metal is traversed in water from the initial pulse to the metal surface. The water to metal interface causes a reflected signal with a pulse-duration equivalent to nearly 1mm in metal. The pulse in the metal travels to the opposite wall and produces another interface signal at 9.3mm equivalent in metal with a pulse-duration of approximately 0.9mm (note that the phase-reversed signals are to be used). As measured from peak to peak response (negative to positive) the wall thickness is about 4.5mm. (See Tom Nelligan's article at http://www.ndt.net/article/ecndt02/175/175.htm for a more detailed explanation of accurate thickness determination).
As the wall thins the distance between the 2 interfaces diminishes, but it depends which side of the wall the loss occurs that you would configure the referencing. If testing from the inside of a tube and monitoring for internal wall loss you would prefer to use an “interface lock” on the outer surface signal and changes in the wall difference would indicate wall loss as a combined difference between waterpath and metal path. However, because signals 4 and 5 have a measurable ring-time this will limit the separation between them that could be tolerated before the front and back wall signals are not possible to discriminate. For the presented A-scan image this will be about 1.2mm to 1.4mm for nice smooth surfaces. Rough surfaces may cause the interface signals to spread out and increase the thickness required to separate the front and backwall signals.
Higher frequency probes and more damping (fewer cycles of pulse-ring) can improve the thickness resolution of the UT. Note the importance of phase information when accurate thickness assessment is required. It is recommended that rectified signals be avoided if any accuracy is required.

 
 
10:38 Dec-26-2011

massimo carminati
Consultant, AUT specialist,
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
485 Stand of IMG ULTRASUONI SRL
Re: Dead Zone?
In Reply to Ed Ginzel on 20:34 Dec-25-2011 .

Ed focussed exactly on the point from the application point of view, without any other and totally different application example. I would just add that one of the main IRIS application limits is the minimum measurable remaining wall thickness. In my experience, and depending on tube cleaning and inner surface smoothness, in the range from 0,7 mm to 1,2mm. It also depends if the thickness reduction is on outer surface or inner surface. The measurements below these values are to be accurately evaluated for consinstency. There must be a separation between the ID and OD echoes in order to get reliable WT numbers. With a 15 MHz properly focussed shockwave probe, with pulser parameters accurately optimized, this can be as low as 0.5 mm in metal. Let's not forget that the "mirror" rotates at about 30 rounds per second and the trigger is just the pin echo and that we're not in a "clean room".
Conclusion: this 0.7 mm real, on site, minimum measurable WT is just 50% of nominal WT of 1.4 mm tube and around 25% of 3 mm nominal WT. This means that it is not possible to measure major WT reduction, but a skilled operator may determine that there is a major problem, if the tube cleaning is properly done.
Let's not forget the importance of knowing in advance what type of damage we expect to find. (erosion, corrosion, pits, wear etc...)

 
 
05:05 Dec-27-2011

S V Swamy
Engineering, - Material Testing Inspection & Quality Control,
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
650
Re: Dead Zone?
In Reply to massimo carminati on 19:11 Dec-25-2011 .

Dear Mr.Carminati,

I find your comment about the 0.4 mm zircaloy tubes rather offending and unprofessional. I have shared my experience with UT and ET inspection of very critical tubes and I have clearly mentioned that I have no specific experience with IRIS. I was very clear that we were picking up ID discontinuities and were not limiting ourselves to wall thickness measurement.

I would request you and fellow members to maintain some decorum in technical discussions.

Best regards and a happy new year to you and fellow NDTians.

Swamy
Hyderabad
India

 
 
12:25 Dec-27-2011

massimo carminati
Consultant, AUT specialist,
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
485 Stand of IMG ULTRASUONI SRL
Re: Dead Zone?
In Reply to S V Swamy on 05:05 Dec-27-2011 .

I apologize for being "rude" and my intention was not to offend anyone.

I believe that we should think very carefully before bringing examples like this to support other completely different applications. If I remember well, this application was used in a thread were limitations about minimum thickness of manual SMAW welds on steel pipelines to be inspected with AUT or MUT. I was always suggesting to perform RT on very low thickness because the geometrical reflections impact can be very important. After having read that thread, someone called me and asked why I was suggesting RT if someone did AUT on 0,4 mm! I have seen testing machines to inspect very small tubes (the testing machine is called "spaghetti"!) with very low thickness for both transverse and longitudinal defects and thickness, but this does not mean I could use this technique for SMAW welds or for IRIS inspection!
Again, my apologize for being irrespectful or brutal, but we must take into account that the forum is open to any knowledge level and anyone, even not directly involved in NDT, but who BUYs NDT services........
My best wishing for a brilliant 2012 to the NDT community.

 
 
04:09 Dec-29-2011
madrid
Re: Dead Zone?
In Reply to coool guy on 13:46 Dec-21-2011 (Opening).

hey cool guy. Im sure what your level 3 was referring to was; that because immersion testing is not contact testing there wont be a dead zone in or near the part. yes there will be a dead zone because of the probe but because its immersion is just better to say that because we are using water as a delay line. the dead zone and near zone will be basically removed out of the inspection. thats why we move the water path out of the screen with the sweep and leave the entry surface signal at the left of the screen almost as an initial pip.

i think thats what he ment. explain to him what you think he is saying and if anything you can always calculate the dead zone of a probe with an equation.

i hoped this helped

 
 
12:30 Jan-01-2012
cool guy
Re: Dead Zone?
In Reply to madrid on 04:09 Dec-29-2011 .

Hello All,

Thank you for the kind reply. Madrid, can you kindly give me the formula for the calculation?

 
 
05:50 Jan-02-2012
madrid
Re: Dead Zone?
In Reply to cool guy on 12:30 Jan-01-2012 .

the dead zone is equal to the thickness of the transducer

T=Wavelength/2

T=crystal thickness = DZ

Wavelength= V/F

just divide the wavelength by 2 and you get the thickness of your crystal. which is equal to the dead zone.

can anybody correct me if im wrong plz

 
 
14:58 Jan-02-2012

S V Swamy
Engineering, - Material Testing Inspection & Quality Control,
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
650
Re: Dead Zone?
In Reply to madrid on 05:50 Jan-02-2012 .

I thought the dead zone was linked to the pulse width. While the thickness of the transducer affects the natural frequency of the crystal, most of the transducers are driven at higher frequencies. The width of the pulse depends on the amplitude and damping.

The depth of the dead zone depends on the velocity in the material (velocity x time = distance).

Please check and correct me since I am writing from memory.

Regards

Swamy

 
 
15:22 Jan-02-2012

massimo carminati
Consultant, AUT specialist,
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
485 Stand of IMG ULTRASUONI SRL
Re: Dead Zone?
In Reply to S V Swamy on 14:58 Jan-02-2012 .

I agree with Mr. Swamy.

The dead zone of the "application" cannot be calculated theoretically, but need to be measured after all the inspection parameters are properly set (pulser and receiver settings, water path, test sensitivity, threshold setting etc.), better if this measurement is performed while the probe is moving at testing speed, so that you can take into account the influence of entrance surface conditions.

 
 
15:26 Jan-02-2012

Wieslaw Bicz
Engineering, - ,
PBP Optel sp. z o.o.,
Poland,
Joined Feb 2009
158 Stand of PBP Optel sp. z o.o.
Re: Dead Zone?
In Reply to S V Swamy on 14:58 Jan-02-2012 .

Dead zone depends on the electrical parameters of the pulser and receiver and pulse answer of transducer. It can change with cable length, depends on the coupling with the medium, etc. It is not possible to define it with a simple formula. In some cases it is even possible to reduce it, using special software.

Basically higher frequency transducers could have shorter pulse answer, but this statement would be too simple in many cases.

It should be always checked with a transducer, that somebody will use.

 
 
05:36 Jan-04-2012
madrid
Re: Dead Zone?
In Reply to S V Swamy on 14:58 Jan-02-2012 .

my apologies, i had everything mixed up, the correct formula is
Pulse length X velocity/2.

the pulse length is indeed a time based number.

I do know crystal thickness is relative to frequency. i was just looking at it from another perspective which it didnt make any sense at all. i was trying to correlate stuff from constructive and destructive interference all into one... anywho, i like to make theories of my own from time to time and try to see if they are indeed true.

my apologies. thank you for the correction.

cheers

 
 
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