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Re: TOFD made simple
Posted by: J. Mark Davis
(PID_553), E-mail: mkdavis@mindspring.com, on March 11, 2000 at 22:35
:In Reply to: TOFD posted by : John Johnson on March 10, 2000 at 18:06:
: 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?
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.