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- since 1996 -
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Technical Discussions
Gowrisanthosh
NDT Inspector,
Royscan engineering pte ltd, Singapore, Joined May 2008, 52

Gowrisanthosh

NDT Inspector,
Royscan engineering pte ltd,
Singapore,
Joined May 2008
52
01:47 Mar-14-2012
Ultrasonic testing for Low thickness m/l

Most of the codes mention the thickness range for UT. this is almost more than 8 mm(for example AWS- 8~200). But I heard that even low thickness below 8mm can do UT. IF it is like that what are the reasons behind that range mentioned in specific codes. And why? And what are the limitations?

    
 
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bradley moxon-holt
bradley moxon-holt
10:01 Mar-14-2012
Re: Ultrasonic testing for Low thickness m/l
In Reply to Gowrisanthosh at 01:47 Mar-14-2012 (Opening).

With conventional UT, the reason for the thickness range is because of the limitations with probes. UT probes have 3 regions that are known as the dead zone, near zone and far zone.
The dead zone is the time that it takes the crystal in the probe to stop ringing and be able to receive a signal back - as such any responses in this time could not be interpreted as either a flaw or the crystal still ringing from its original pulse.
The near zone is after this period when pressure hugely varies and effects the signal received back - again making it highly difficult to interpret the response correctly - as the amplitude may not be relative to the size of the flaw as you can inspect deeper into the material when you are operating in the far zone.
The length of each zone varies with the material you are testing (ie velocity of sound in that material), the probe frequency, crystal size among other factors.
A procedure will give a range and type of probe that can be used, which when the factors mentioned above are taken into consideration will give an operating thickness.

There are way to perform UT on thinner walled items using both advanced and conventional methods.
With conventional methods, using twin crystal angle probes removes the effect of a dead zone, so near surface discontinuities can be inspected, you still however have the near zone effect so flaws may not be truly represented, and as the dual crystal probes are focused, the thickness range that can be inspected is limited.
Secondly items can be inspected by performing inspections at 1.5 / double skip distances, (in theory doubling the thickness you are inspecting) however, this again has limitations as over a further distance, any deviations in angles are amplified, irregularities in the surface profile of the material can alter your angle more and the spread of the beam will be greater so again the signal of the response received may not be a true representation of a flaw, making both sizing and categorization of the flaw more difficult.
This is why surface testing such as MPI/DPI should be used in conjunction with UT (there are plenty of other topics already on the forum detailing the advantages of multiple techniques).

    
 
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Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
13:45 Mar-14-2012
Re: Ultrasonic testing for Low thickness m/l
In Reply to Gowrisanthosh at 01:47 Mar-14-2012 (Opening).

In my opinion, codes tend to focus exclusively on common weld inspection cases and don't cover other ultrasonic applications that are quite viable with the right equipment. For straight beam inspection, if you use single element broadband delay line transducers you can resolve metal thickness as low as 0.25 mm (at 20 MHz). These are commonly used in thin metal thickness gaging applications and are available from a number of vendors, inclusing us. For angle beam testing, small 10 MHz wedge assemblies are available that can be used to test butt welds and seam welds in metal as thin as 3 mm or so. High frequency immersion testing is another option when the inspection is being done in a manufacturing environment.

    
 
 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
14:51 Mar-14-2012
Re: Ultrasonic testing for Low thickness m/l
In Reply to Tom Nelligan at 13:45 Mar-14-2012 .

Dear Tom (I hope it is OK to address you like that - if not my apologies) - you are perfectly right. Since I have written many times about the UT or RT of thin tubes and welds, I did not respond this time for fear of being accused of repetition (once some member made some rather caustic and ungentlemanly remarks about my frequent posts on this topic).

So, with advance apologies to those who are bored of reading this again, I reiterate that real life situations are not always covered by the main codes and there are many other specifications and standards. We at NFC routinely inspect very thin seamless tubes and seam welds, butt welds etc. in Zirconium Alloys and Stainless Steels. You just need the right equipment for manual contact testing and or automated immersion testing.

With best regards,

Swamy
Retired QC Manager, Nuclear Fuel Complex

    
 
 Reply 
 
MarioTalarico
NDT Inspector,
Italy, Joined May 2010, 411

MarioTalarico

NDT Inspector,
Italy,
Joined May 2010
411
16:59 Mar-14-2012
Re: Ultrasonic testing for Low thickness m/l
In Reply to Gowrisanthosh at 01:47 Mar-14-2012 (Opening).

When a code require limits we must respect, and that's it. To understand why should be present when the limits are set by people who have years of experience.
In many cases the customer asks for checks not covered/outside of code or standard, often before and after PWHT, to secure a higher quality than the minimum allowed under the regulations.

Respect to welding, as already mentioned from colleagues, it is reasonable go below the limits of thickness. Contact probes with single crystal, however, must provide a complete excursion of geometries root (assuming that the width of the outer cordon permitting) with acoustic paths outside the conditions of Fresnel (near zone).

For common applications are standard probes dn 6 mm 5 MHz. Often with high angles, depending on the thickness, allow a scanning 0.5 skip for the root and 1 skip scan for the body of welding in rebound, to below the external bead.

Scan the root .... If this condition can not be realized the root must be covered with 1.5 skip scan, with angles as a function of the thickness and width esternal cap. This scan is performed more or less easily starting from position 1 skip pointing esternal cap, then moving back from welding to scan the root pass. The space action is really very narrow, with a long time..., because signals travel in reverse respect to a standard control. It 'a good idea to put everything on paper first, even ( especially some would say! ) if you think are skilled operator. A reference on the piece for the guide-probe helps a lot for training in the first welding of a given configuration. In some cases you may need a mock-up and acquire the feasibility by simulating the actual geometry. Avoid amletic walking on production piece .
Greetings
mario

    
 
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