Good question.
The answer lies with critical angles. See attached diagram.
When the incident wave in the perspex reaches 27.6° (called First critical Angle), the refracted longitudinal wave in the steel reaches 90° (β=90°, Sin β=1), there total reflection takes place. At this incident angle only shear wave exists in the steel. Thus shear wave probes for testing steel are always greater than 33.2°.
When the incident wave angle is reached 57.2° (called Second critical angle) in the perspex, now the shear waves are converted into surface waves thus no more 'angled' waves are available for scanning within the steel.
Hence, between first and second critical angles, only shear waves exist in the steel and scanning is possible and the transducers/probes s are produced in 35, 45, 60, 70 and 80° only.
Example:
Sin A / Sin B = Velocity in perspex / Velocity in steel
Known: Velocity in perspex 2730 m/s
Velocity in steel 5900 m/s
Sin β=90°=1
Then, Sin A=0.4627, which is 27.5°
PS: The first and second critical angles vary with type of material. .
Respectfully,
Nandesh Kumar A
--- On Wed, 31/10/12, Dixon Davis <ponthokkan.dixon@gmail.com> wrote:
From: Dixon Davis <ponthokkan.dixon@gmail.com>
Subject: Query
To: "Nandesh Kumar" <nandeshkumar@rocketmail.com>
Date: Wednesday, 31 October, 2012, 7:28 AM
Dear all, In manual UT why we are commonly using three types of angle probes( 45,60,70). Is there is any specific reasion...? Regards, Dixon P Davis 8097170585 ---------- Forwarded message ---------- From: Nandesh Kumar <nandeshkumar@rocketmail.com> Date: Wed, Oct 31, 2012 at 11:51 AM Subject: Re: [MW:15819] Can we measure the Ug To: materials-welding@googlegroups.com Ravi,
Following is the extraction of ASTM E94:
Clause 15.2 The radiographic image of an object or feature within
an object may be larger or smaller than the object or feature
itself, because the penumbra of the shadow is rarely visible in
a radiograph.
IMO, you can not measure Ug on a radiograph using a scale. as the Ug will always be very negligible in dimension, usually less than 0.5 mm (if proper techniques are followed)
You can measure the Ug (the penumbra), only if you are making a 'deliberate' setup like large Object to film distance, large source size than the discontinuity size whose Ug is to be measured etc.
And finally, if the Ug is measurable on the film, really it is a bad quality radiograph...!!!
Respectfully,
Nandesh Kumar A
--- On Tue, 30/10/12, Vajjiravelu Ravi <vajjiraveluravi3@gmail.com> wrote:
From: Vajjiravelu Ravi <vajjiraveluravi3@gmail.com>
Subject: Re: [MW:15815] Can we measure the Ug
To: materials-welding@googlegroups.com
Date: Tuesday, 30 October, 2012, 12:24 PMthe question is how it is possible to measure the Ug, anyhow thks for your kind response On Tue, Oct 30, 2012 at 1:33 PM, cbe swamy <cbe_swamym@hotmail.com> wrote: Dear all, T-274.1 Geometric Unsharpness Determination. Geometric unsharpness of the radiograph shall be determined in accordance with: U g = Fd/D where, U g = geometric unsharpness F = source size: the maximum projected dimension of the radiating source (or effective focal spot) in the plane perpendicular to the distance D from the weld or object being radiographed D = distance from source of radiation to weld or object being radiographed d p distance from source side of weld or object being radiographed to the film D and d shall be determined at the approximate center of the area of interest. Please refer Sec -V ASME for more details.
Thank & Best Regards,
M Veera SAMY- Bureau Veritas. Coimbatore -- Vajjiravelu Ravi Welding,Painting,NDT & ISO 9001/2008 take all steps to save our health wealth & environment. -- To post to this group, send email to materials-welding@googlegroups.com To unsubscribe from this group, send email to materials-welding+unsubscribe@googlegroups.com For more options, visit this group's bolg at http://materials-welding.blogspot.com/ http://www.linkedin.com/groups/MaterialsWelding-122787?home=&gid=122787&trk=anet_ug_hm The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents. |
-- Regards, Dixon P Davis QAQC Engineer +919480831188
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