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14:05 Sep-28-2009
Sensitivity comparison between Shear & Longitudinal wavee

Could any one clarify whether there is any sensitivity difference between Shear & Longitudinal wave ,if so Why?Why is Longitudinal wave used in ToFD.Please reply.

04:41 Sep-29-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Rupesh

One needs to be cautious using the term "sensitivity". The ndt.net encyclopaedia defines sensitivity as "The characteristics of an ultrasonic instrument and probe combination which determine the ability to detect small signals limited by the signal-to-noise ratio. In order to perform ultrasonic systems for flaw detection to the requirements of various products it is necessary to adjust the sensitivity level."

ASTM E-1316 defines senstivity as, "a measure of the smallest ultrasonic signal which
will produce a discernible indication on the display of an ultrasonic system."

The ASNT handbook on ultrasonic testing 3rd edition defines sensitivity as "Ability of signal to change with small changes of measured quantity"

For the 1st part of your question. Using compression and shear waves of equal wavelength (i.e. different frequency crystals) and equal soundpath distance and a side-drilled hole as the reflector, I can only imagine that, without mode conversion (i.e. compression - compression or shear - shear cases), the reflected sound energy difference will be minimal unless the mode of propagation of the incident wave results in more sound energy being diffracted or otherwise diverted away from direct reflection.

For the 2nd part of your question, use of longitudinal waves for TOFD is dictated by the greater sound velocity thus meaning that all reflected shear waves return to the receiving probes after the reflected longitudinal wave from the backwall giving a more easily interpretable area of interest.

09:42 Sep-29-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Dear Nigel

Thank you for the reply.

For the first part of the question if the type of defect was a transverse notch will it make a difference in the sensitivity?,as the diffraction & mode conversion phenonmenon are more dominant.

11:12 Sep-29-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Rupesh

May I suggest a literature search using such names as JA Ogilvie, GA Georgiou, JM Coffey, RK Chapman amongst many others who have written papers on the complex subject of soundfield - defect interaction and scattering phenomenon.

Good luck and best regards

13:08 Sep-29-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

The “sensitivity” definitions cited by Nigel are too general to give a definite answer to the Rupesh question. But if we define the sensitivity of an ultrasonic system as a relative echo amplitude from the reflector of a given shape, size and orientation as compared to the BW echo received from the same distance we can say something about relative sensitivities of L and T waves.

The general answer following from the elasticity theory is Yes, the amplitude of reflected/back scattered wave depends on the mode (L or T) of incident wave. The strict solutions for cylinders and spheres are available in advanced books on elastic wave propagation theory. I could also risk a statement that generally (for most reflectors) the share wave is more “sensitive” than longitudinal one assuming both wave have the same wavelengths.

The explanation is simple and follows directly from the Snell’s Law. Just consider that L wave undergoes partial transformation (energy loss) for every incident angle on the reflector surface except 0 deg. The T wave undergoes transformation only for incidence angles less than critical angle, 33 deg for steel-air interface. So statistically energy losses on reflections due to transformation are grater for L waves than for T waves.

Having this in mind you can predict that relative “sensitivity” of T waves will be better than L waves when detecting SDH and notches but comparable to L waves when detecting FBH.

Best Regard

16:25 Sep-29-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Rupesh

Are your two questions connected? It reads very much as if you wish to present a case for use of shear waves for TOFD inspections.

19:28 Sep-29-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Nigel

Your guess is right. As with slawomir"s explanation if the shear component is more sensitive than the longitudinal wave(with respect to the energy transformation loss ) then why cant we use the shear part of the wave in ToFD for resolution & sizing. It is possible to view the shear part even after the L-component wave.

Best regards

12:51 Sep-30-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Rupesh

Your question - Why can’t we use the shear wave for TOFD inspection is quite reasonable and worth of detailed discussion. The Nigel’s argument that the L-wave is faster does not prohibit the use of T-wave. You can use, for example, the standard T60 shear wave probe and have no problem with interfering L-wave, because it doesn’t exist in this case.

The main problem with T-wave in TOFD is that the so called “lateral wave” is closely followed by interfering Rayleigh surface wave traveling with slightly lower speed (about 7% for steel) than the share wave. Due to this effect you have unwanted waveforms trailing the actual lateral wave and pretending or obscuring relevant indications from the volume of tested material. Of course there are some other problems connected with directivity patterns of diffracted waves but in my opinion they are not critical.

Best Regards

13:23 Sep-30-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Slawomir
Thank you for the detailed reply.Even with the presence of rayleigh wave a depth equivalent to approx. 1 Lamda may get misinterpreted,which even is missed with respect to the Lateral wave.The chances of detecting & evaluating the surface breaking defects may get in uncertainity.But does it effect the volume embedded indications.Please comment.

Regards
Rupesh

15:05 Sep-30-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Rupesh/Slawomir

A Google search on the phrase "TOFD using shear wave probes" brings up some interesting papers on the topic. G. Baskaran, Krishnan Balasubramaniam and C.V. Krishnamurthy report on success with this method for thin-walled materials (6 - 7 mm).

I would advise very careful validation of any such inspection especially due to the lower diffraction coefficient and beamspread of shear wave from those of longitudinal wave.

do you have a specific application in mind Rupesh?

15:44 Sep-30-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Nigel, I belive you are using the same Google :-)

No results found for "TOFD using shear wave probes".
Results for TOFD using shear wave probes (without quotes):

19:30 Sep-30-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Thanks for the corrrection Rolf. No quotation marks Rupesh!

19:35 Sep-30-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Nigel

I dont have any special application in mind,but the usage of L-wave literally generated a feeling of having some better advantages in usage as in comparison with S-wave .As I was unware of that I put it in discussion.
Anyway I found the aricle posted in the web by the name"TOFD using shear wave probes".I have put a request to the author (Prof.Krishnan Balasubramaniam )whom I have met before to get his precious comments if possible.Thank you for the guidance Nigel.

Regards

02:35 Oct-01-2009
Re: Sensitivity comparison between Shear & Longitudinal wavee

Thanks to you too, Rupesh for starting the discussion. From that paper it seeems that S-TOFD has applications. Many of us accept the givens (i.e. TOFD = L-wave) without questioning why its a given and does it always have to be so.

Cheers!