ToFD - Collection of Postings
As a NDT services company, we routinely provide AUT using ToFD in India and Saudi Arabia. I wanted to give something back to the outstanding forum on ndt.net and thought that one of the best ways will be to create a compilation of answers on ToFD that are so frequently asked on ndt.net. I plan to update this thread as time permits so that we can all benefit from the collective wisdom of the contributors of this forum. Here are links and brief outlines to important discussions on Time of Flight Diffraction:
Sizing a Defect using ToFD
Contributor Suresh on ndt.net writes:
However, over the last 20 years, TOFD has probably been used more in validations, round robin trials et cetera than any other NDT technique. From this, we have established that TOFD is probably the most accurate sizing technique for embedded defects presently available to the world. A recent test for a major oil company we did on some 30 rootcracks in thin walled material, which were compared to physical measurements after destructive testing showed a mean error on height sizing for TOFD of 0.27 mm with a standard deviation of 0.7 mm. The X-Ray results obviously did not give any height measurement. On the length measurement the accuracy of TOFD was 2.6 mm, with a standard deviation of 4.5 mm. For X-Ray the accuracy of the length measurement was -19 mm (undersized) with a standard deviation of 43.5 mm. By the way, X-Ray only revealed 55% of the cracks, whereas TOFD found 97%.
Demonstration of technique as per 2235-9
Contributor Ed Ginzel writes:
The method of sizing (vertical extent?) for CC2235-9 for examining pressure vessels with a thickness over 13 mm needs to be qualified (demonstrated). It states in i)(3) Flaw Sizing. Flaws shall be sized in accordance with a procedure demonstrated to size similar flaws at similar material depths. Alternatively, a flaw may be sized by a supplemental manual technique so long as it has been qualified by the demonstration above.
Dead Zone Calculation
If I were to bet, this thread is the longest and most frequently asked question with respect to ToFD.
Tom Nelligan of Olympus NDT wrote:
Dead zone -- the interval following the excitation pulse in which potential exchoes are obscured by ringdown -- is a property of BOTH the probe and the instrument. The response of any transducer is highly dependent on how it its pulsed. Changing pulse energy, type (spike, square wave, or tone burst), and damping, as well as receiver gain and filtering, will have a significant effect on the excitation pulse ringdown envelope and hence on dead zone length. As others have suggested, your best approach is to simply measure it under your actual test conditions using appropriate reference blocks.
Ed Ginzel wrote
The "missed data" would be the dead zone. This is not actually a function of the wall thickness but instead, the ring time equivalent depth of the pulse. Frequency, probe spacing and angle used can all be factors. A simple calculator is provided as a download on NDT.net to estimate the Dead Zone heights associated with TOFD setups. Go to http://www.ndt.net/article/v10n06/ginzel/ginzel.htm
For it to be 1/3 wall thickness the specimen you are working with is probably very thin.
Smallest detectable flaw using TOFD
Ed Ginzel once again has a truly informative posting where he mentions:
In clean low carbon steel I have detected pores on the order of 0.2mm diameter using a 5MHz TOFD. That is on the order of 5-6 times smaller than the wavelength! But when the same TOFD technique is applied to austenitic stainless steel with grain size on the order of 50-100 microns, the scatter makes it virtually impossible to detect anything but the largest of flaws. Grain size in chrome stainless steels are typical of this order of magnitude. (see http://www.ultrasonic.de/abstract/wcndt96/data2/165.htm)
He further adds:
Properties of the materials tested (not just the alloy, since an alloy may be made in many forms of different grain sizes) and the purpose of the test will be critically important factors when considering the answer to your question. We can discuss theoretical limits to detections; but in UT (including and especially in TOFD) the response of the indication of concern over the background scatter noise from grain structure will be crucial. If you are expecting to RELIABLY detect flaws smaller than the grain size you will probably not be successful. The lateral wave will cause you problems at the lower end of thicknesses due to the dead zone (but I have seen shear wave TOFD used on 4mm wall in fine grained zirconium tubing). Thick sections (200-300mm) can also be tested by TOFD but accumulation of scatter increases with increasing soundpaths. In all cases the signal to noise ratio you can achieve will be the limiting factor for practical use of TOFD.
LoF vs. Slag when doing ToFD as per B31.3 in Piping
J. Mark Davis wrote:
LOF being a planar defect will tend to have a fast rise and fall time, and a short pulse duration. Slag is a volumetric defect and tends to have a slow rise and a long duration. Both defects can cause a beam redirection to the OD or the ID. Also, please realize that as with radiography, you can have several people look at a radiograph and make a slag or a LOF call, and sometimes not all will be in agreement. The same will hold true with UT and Phased Array. If the indication plots to the weld fusion line this probably is side wall lack of fusion. If the indication plots to the weld centerline, line this is probably Slag. Plus, you still analyze Rise and Fall and Pulse Duration.
Code Case 181
A lot people were searching for the actual code case 181 for ToFD. John O'Brien gave the URL to be as http://cstools.asme.org/csconnect/pdf/CommitteeFiles/15243.pdf on the page
ToFD working Style
Rohit Bafna of TCR Engineering Services wrote:
We perform ToFD Services in India and Saudi Arabia and have a two member team. The first team member does the scanning (we are doing ToFD projects as per API 650 appendix u, Code case 181 for piping as well as code case 2235-9) while the second person has a manual UT machine with him. Prior to the start of any project we give out two documents:
- Procedure for examination
- ToFD operating guide for inspectors which illustrates the step by step machine operations. It also includes the timelines of work during the day. For example we prefer to do the scanning in the morning and reporting in the afternoon.
I encourage others to update this thread whenever they find any interesting article on ToFD. I hope over time, we can help other inspectors and fellow technicians learn andimprove the usage of ToFD in the industry.