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Technical Discussions
John Johnson
John Johnson
08:06 Mar-10-2000
TOFD

I am working on a development project for the inspection of
welds at the Idaho National Engineering and Environmental
Laboratory in Idaho Falls, Idaho, USA. I have been reading about
TOFD and I several questions that I hope that forum members may
be able to answer. Thank you for your time.

1. Why use longitudinal waves rather than shear waves? The only
reason that I have seen in the literature is that one doesn't
need to worry about mode converted waves since they will
arrive at later times. However, I believe that using shear waves
could offer two advantages:

a. Longer time between the two diffracted signals,
allowing for more precise distance measurements

b. I believe that the diffracted signals from shear
wave illumination may be larger than
those from longitudinal wave illumination.
This is certainly true at the same frequency since the
wavelength of the shear waves is smaller.

In addition one could arrange the geometry so that no direct
mode-converted longitudinal waves were received. However, if
one used shear wave illumination, one might receive
mode-converted, diffracted longitudinal waves. These signals
may interfere in the interpretation. Does one observe and use
the mode-converted diffracted shear waves in normal,
longitudinal TOFD?

2. How does one compromise between the seemingly contradictory
requirements of TOFD? On one hand TOFD requires a large
signal-to-noise ratio. This is in direct conflict with the need
for:
a. Wide beams => small diameter transducers => low acoustic
power
=> low incident acoustic energy at the defect
site since the energy is spread over a wide
area.
b. Wide bandwidth => large damping => lower acoustic
transmitted energy and
receiving efficiency.

3. Doesn't the crown of the weld interfere with the lateral wave
propagation? Does the crown of the weld have to be ground smooth
to apply this method?

4. Can TOFD be applied to welds in stainless steels in spite of
grain scattering, anisotropy, and larger attenuation?

5. Are other methods that do not use amplitude measurements
available to measure defect heights?

Thank you again for your time.


    
 
 
J. Mark Davis
Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama, USA, Joined Mar 2000, 85

J. Mark Davis

Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama,
USA,
Joined Mar 2000
85
02:35 Mar-11-2000
Re: TOFD made simple
the geometry so that no direct
: mode-converted longitudinal waves were received. However, if
: one used shear wave illumination, one might receive
: mode-converted, diffracted longitudinal waves. These signals
: may interfere in the interpretation. Does one observe and use
: the mode-converted diffracted shear waves in normal,
: longitudinal TOFD?

Shear waves are used mostly with manual contact time of flight diffraction in the backscatter technique. I find shear waves more effective than l waves. I use l waves when examinaing coarse grain materials.

TOFD in the manner that you described is used more with a pitch-catch using an lwave, i.e., 60 single element l wave.

: 2. How does one compromise between the seemingly contradictory
: requirements of TOFD? On one hand TOFD requires a large
: signal-to-noise ratio. This is in direct conflict with the need
: for:
: a. Wide beams => small diameter transducers => low acoustic
: power
: => low incident acoustic energy at the defect
: site since the energy is spread over a wide
: area.
: b. Wide bandwidth => large damping => lower acoustic
: transmitted energy and
: receiving efficiency.

Generally, you select a frequency which provides a combination of sensitivity, resolution and penetration. For example, if I am examining a very thin material (less than .300 inches) then I would use a higly dampened ( 1 to 1 1/2 cycles) 5 to 10 mhz transducer. Here resloution with a good signal to nioise tip signal is a must to adquately identify the tip difrracted signal. If I am examining a a thick (1 1/2 inch thick stainless steel material a 2.25 medium dampened 1/2 diameter transducer.

Some other considerations are critical flaw size and calibration block design.

: 3. Doesn't the crown of the weld interfere with the lateral wave
: propagation? No>

Does the crown of the weld have to be ground smooth
: to apply this method? No>

: 4. Can TOFD be applied to welds in stainless steels in spite of
: grain scattering, anisotropy, and larger attenuation?

Yes, What is the thcikness of the material and what is the grain size on the SS material.

: 5. Are other methods that do not use amplitude measurements
: available to measure defect heights?

Yes, ID Creeping Waves, Bi_modal and Focused OD Creeping Waves, L-waves and Shear waves are very effective.

: Thank you again for your time.

Please review my flaw sizing handbook listed at NDT.net for an overview on flaw sizing techniques. Also, please check with Mike Anderson ar INEL for a discussion of these techniques.




    
 
 

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