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Technical Discussions
mohsen
NDT Inspector,
arta industry testing, Iran, Joined Jan 2013, 19

mohsen

NDT Inspector,
arta industry testing,
Iran,
Joined Jan 2013
19
09:22 May-13-2013
near field and dead zone

hi
what's different between near field and dead zone in UT? where is dead zone? is it in surface of the test material or in the probe or..? is it subset of near field or..? can we calculate dead zone? can we see or rely to results of indication in near field?

 
 Reply 
 
S V Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S V Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
09:34 May-13-2013
Re: near field and dead zone
In Reply to mohsen at 09:22 May-13-2013 (Opening).

The dead zone corresponds to the time duration on the screen occupied by the initial pulse. It can thus be easily measured. Any good book on ultrasonic testing will give you detailed information. You can also see www.ndt-ed.org for this.

1
 
 Reply 
 
Dhaval
Engineering,
Global Blade Technology, India, Joined Nov 2012, 13

Dhaval

Engineering,
Global Blade Technology,
India,
Joined Nov 2012
13
10:50 May-13-2013
Re: near field and dead zone
In Reply to mohsen at 09:22 May-13-2013 (Opening).

Hi Mohsen,

Que 1 : What's different between near field and dead zone in UT?
Ans 1 : In Dead Zone you can not find any pulse but near field you can find pulse if there any reflector is there but result is not reliable.

Que 2 : Is it in surface of the test material or in the probe or..?
Ans 2 : Not in probe or nor in surface of the test material. It is related to sound path. Form the probe surface to some distance in the material there is (Sound path) near field. You can find near field by the formula
L=D^2/4λ and λ=V/F
Where
N = Near Field Length or Transition from Near Field to Far Field
D = Diameter of the Transducer
λ = Wavelength of the Transducer
F = Frequency of the Transducer
V = Velocity of Sound in the Material
Dead zone you can find by putting probe on the surface after completing all calibration. You can find initial pulse which is touching to some where at bottom to screen. Convert to as you have taken your range. It is your Dead zone.

Que 3 : Is it subset of near field or..?
Ans 3 : Yes,It is subset of near field.

Que 4 : Can we calculate dead zone?
Ans 4 : I have already answer this in my answer 2.

Que 5 : Can we see or rely to results of indication in near field?
Ans 5 : No we can not rely.

Correct me if my answer is wrong.
Regards,
Dhaval

 
 Reply 
 
Andrew Hurrell
Consultant, Ultrasonic Transducer Production
Precision Acoustics Ltd, United Kingdom, Joined Apr 2000, 25

Andrew Hurrell

Consultant, Ultrasonic Transducer Production
Precision Acoustics Ltd,
United Kingdom,
Joined Apr 2000
25
13:52 May-13-2013
Re: near field and dead zone
In Reply to Dhaval at 10:50 May-13-2013 .

I know this have been covered many times before but I will have to state this again

THE CONCEPT OF DEAD ZONE AND NEARFIELD IS ONLY APPROPRIATE FOR CONTINUOUS WAVES!!!!

Since NDT/NDE uses short pulses these concepts are fundamentally flawed. The complex near-field patter arises from the interaction of the edge and head waves that could be many cycles apart in time. For short pulses there is no such interaction. One wave has passed before the next arrives.

I include a link to some animations to illustrate this.
http://www.acoustics.co.uk/services/acoustic-field-modelling

At the bottom of the pages are 2 animations ( one with continuous waves, and one with a an 8 cycle toneburst). Look at the difference of the axial profiles shown on the graph on the same page. Whilst the CW wavefield as a last axial maxima and profile shape that exactly corresponds with the analytically derived approximation listed by Dhaval, the 8 cycle one does not. Given that most UT is done with a pulser the likelihood that the waveform is even shorter that 8 cycles is a near certainty.

PLEASE, do not cite D^2/(4*lambda) unless you are using continuous waves. It does NOT apply for pulses or short duration waveforms. If you want further technical detail please ask

Best wishes

Andrew

 
 Reply 
 
Wieslaw Bicz
Engineering,
PBP Optel sp. z o.o., Poland, Joined Feb 2009, 276

Wieslaw Bicz

Engineering,
PBP Optel sp. z o.o.,
Poland,
Joined Feb 2009
276
17:35 May-13-2013
Re: near field and dead zone
In Reply to Andrew Hurrell at 13:52 May-13-2013 .

You are correct in the case of the near field and short pulses. Very good explanation of this is to find in Krautkraemer book (in German version on page 94, I can also check the page in Englisch version, but not immediately). The character of this occurrence has to do with pulse length, wave length and transducer size (since it is interference and diffraction dependent effect). But the dead zone has nothing to do with it. It has to do with electronics: if you are using the same transducer for sending and receiving and the voltage of your pulse is high, no circuit is able to recover shortly after the pulse was send. It is necessary to wait some time to be able to receive. This dead time can be from few to some tenths of microseconds.

 
 Reply 
 
ismet hoxha
NDT Inspector,
Italy, Joined Nov 2011, 49

ismet hoxha

NDT Inspector,
Italy,
Joined Nov 2011
49
19:20 May-15-2013
Re: near field and dead zone
In Reply to mohsen at 09:22 May-13-2013 (Opening).

Mohsen, not always the dead zone is "subset" of the near field, an example is the probe, with the near field limited into its wedge, on the screen there is again an initial pulse.
We cannot rely in the near field, but in practice its effects must be taken into account, only when the conditions preclude detection beyond this portion of the beam.
If not supplied with probe documentation (DZ and NF length), the formula N = (D x D x f) / 4 v, could be used for a normal probe, and generally for an approximation, a good preview of the reference block that's the best way to measure them.
all the best

 
 Reply 
 
Eng. Ahamd
NDT Inspector, ASNT NDT Level III (UT, PT & MT) New Vision for Technical Services GM (www.nevcc.com)
New Vision for Technical services , Jordan, Joined Jan 2007, 38

Eng. Ahamd

NDT Inspector, ASNT NDT Level III (UT, PT & MT) New Vision for Technical Services GM (www.nevcc.com)
New Vision for Technical services ,
Jordan,
Joined Jan 2007
38
20:46 Jan-03-2016
Re: near field and dead zone
In Reply to mohsen at 09:22 May-13-2013 (Opening).

Dead Zone:
Dead Zone is formed when the crystal is ringing to transmit the sound and thus it cant receive the sound at this moment and is shown as the transmission signal at the start of the time base. Its depth can be seen on a calibrated time base as the amount of time base, occupied by the transmission signal. The dead zone increases when the frequency is decreased, therefore a 5 MHz single probe will have a smaller dead zone than a 2.5 MHz probe. The dead zone is a zone where it is not possible to detect defects.

Near field
Spherical waves interfere with each other and result in a system of maxima and minima in intensity in the region close to the transducer. This region is known as the near field region or Freznel zone. The near field shows a beam having a width approximating the diameter of the crystal. However, it is reduced up to the end of the near field which is called the focus.

 
 Reply 
 
Dhanasekaran
Other, TEAM LEADER NDT
VESTAS WIND TECHNOLOGY INDIA PVT LTD, India, Joined Apr 2012, 15

Dhanasekaran

Other, TEAM LEADER NDT
VESTAS WIND TECHNOLOGY INDIA PVT LTD,
India,
Joined Apr 2012
15
07:35 Jan-05-2016
Re: near field and dead zone
In Reply to mohsen at 09:22 May-13-2013 (Opening).

hi
dead zone is the crystal ringing time to produce the sound waves as the crystal cannot receive/covert the mechanical vibration to electrical pulse while ringing it is called dead zone.
Near zone is the zone where the sound wave pressures at each point is not uniform.as every particles in the crystal combinedly generates sound waves. the waves will undergo constructive and destructive interferences between them.at a point where all the destructive interference wave dies and sound waves become uniform till that point/length it is called near zone.
Regards
R.Dhanasekaran

 
 Reply 
 
Ivo Vanak
NDT Inspector,
Czech Republic, Joined Aug 2016, 5

Ivo Vanak

NDT Inspector,
Czech Republic,
Joined Aug 2016
5
13:30 Aug-28-2016
Re: near field and dead zone
In Reply to Eng. Ahamd at 20:46 Jan-03-2016 .

Hello
My name is Ivo and im having a question regarding the near field length, and its calculation.
We all know the equation N=(D2xf)/4xc
We are using various probes for different tasks, just of the top of my head i´m picking one, 5MHz / 12mm diameter / wedge lenght 20mm - usage for detecting flaws in monolithic composite.
As i was always assuming the probes come with the wedge either for fulfilling some special shape needs or for eliminating the near field effect
So lets assume we are testing a monolithic composite with the speed of sound 3000m/s - speed of sound in plastic is 2700m/s
Since we have a wedge made of plastic i was calculating whether the wedge covers the whole near zone
N=(144x5)/4x2,7 - N=66mm - that indicates that the near zone is also present within the inspected material.
So either my assumption about the wedge purpose was wrong or the wedge is there for whole another reason. So i just dont understand. why Airbus wants probe with precisely 20mm wedge for this purpose.
Thank you very much for explanation!

 
 Reply 
 
Chris
Consultant,
United Kingdom, Joined Dec 2012, 43

Chris

Consultant,
United Kingdom,
Joined Dec 2012
43
01:24 Aug-29-2016
Re: near field and dead zone
In Reply to Ivo Vanak at 13:30 Aug-28-2016 .

I think you are confusing a wedge with a delay line for a zero degree transducer. Wedges are used to generate angled beams, usually shear waves. Delay lines are used to contain both electronic dead zone and any near field effects foe zero degree longitudianl transducers and are normally only used for thin materails or looking for near surace defects. Andrew Hurell is perfectly correct re CW waves and near field. In the case of short pulse (broadband) transducers its informative to think about time difference between waves coming form the centre and edges of the transducer versus the distance into the component. If this is greater than the pulse length there is no interference.

 
 Reply 
 
Ivo Vaňák
NDT Inspector,
Czech Republic, Joined Aug 2016, 5

Ivo Vaňák

NDT Inspector,
Czech Republic,
Joined Aug 2016
5
20:53 Aug-30-2016
Re: near field and dead zone
In Reply to Chris at 01:24 Aug-29-2016 .

I understand that i probably made a mistake calling it a wedge when i should have mentioned delay line.
And i have read the post of andrew hurrel, and i dont quite get it. As i understad what he ments, but we were told that short pulses are normally used when doing PULSE ECHO testing, as the transducer needs to recieve the returning signal in some point. So basically in regular probe with one transducer you only have a short pulse. Why do they taught us so many things about evaluation of the defects and its results and efects wheather in near field or far field.
From what Andrew hurrel says that should not be an issue then, which is obviously false.
So if anyone could please explain this little bit more?
Thank you very much guys

 
 Reply 
 
Chris
Consultant,
United Kingdom, Joined Dec 2012, 43

Chris

Consultant,
United Kingdom,
Joined Dec 2012
43
21:26 Aug-30-2016
Re: near field and dead zone
In Reply to Ivo Vaňák at 20:53 Aug-30-2016 .

Hi, Early ultrasound for NDT used quartz crystals as they were all that was available. Quartz is much less sensitive that the piezoceramic elements used today. The quartz transducers had to be used in a resonant mode (lots of cycles) to get sufficient sensitivity. CW expressions for near field, beam divergence are quite accurate for resonant transducers. While these expression dont fully describe short pulse (broadband ) transducers they give useful guidelines. Its important to avoid near field effects as many defect sizing techniques are based on amplitude of equivalent side drilled holes, this is unreliable in the near field. So it is best to avoid this region. If you really want to know the beam divergence of your transducer you have to measure it. The dead zone also has an electronic component the preamp is protected from the dive pulse but does not fully recover instantly, it may take several microseconds before the preamp reaches its full gain.
It is important to know near fleld and beam divergence equations as you will fail the theory part of UT exams if you dont.

 
 Reply 
 
SAHAI
NPCIL, India, Joined Mar 2018, 1

SAHAI

NPCIL,
India,
Joined Mar 2018
1
08:59 Mar-13-2018
Re: near field and dead zone
In Reply to Andrew Hurrell at 13:52 May-13-2013 .

Sir, With reference to ASME BPVC Section V Article 4 Non Mandatory Appendix B “General Techniques for angle beam calibrations” why is it that the DAC curve drawn with SDH reflectors at 1/4T, ½T & 3/4T for any plate thickness rises in the beginning of curve and then falls (the later part can be attributed to attenuation and beam spread).The initial part was attributed to near zone effects in a UT course attended by me and was asked to leave the ¼ T point for preparing DAC.

Though well illustrated in your article, the complex near field pattern due to head and edge waves do not exist in pulse echo method, I wonder why the curve is drawn like that and it happened practically also for 25mm side square bar with 3mm SDHs (going by memory). Could you please guide me through this?

 
 Reply 
 
Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 889

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
889
14:29 Mar-13-2018
Re: near field and dead zone
In Reply to SAHAI at 08:59 Mar-13-2018 .

Sahai,

Regardless of code, DAC curves are all built on the same principle. First you verify the signal response of each reflector. Once you have establish which one it is, you set its amplitude at 80% FSH. Than without changing the gain, you go back to the reflector that is closest to the front surface and you note the position of each reflector either by drawing the curve on your screen or electronically if your instrument can do so.

The reason why the first reflector sometimes as a lower response than the second reflector is due to the signal being in the near field zone.

 
 Reply 
 

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