TOFD images can be corrupted by incorrect setups or other problems such as electrical noise. The following images were all made on the same plate to show some Figure N-481(f)
Flaw Location and TOFD Display Showing the Image of the Concave Root Flaw
1
2 3
Figure N-481(g)
Flaw Location, TOFD Display Showing the Image of the Midwall Lack of Fusion Flaw, and the A-Scan
1 2
4 3
of the typical problems that can occur. Starting first with an acceptable scan, and then subsequent scans made to show various corruptions of this image.
(a) Acceptable Scan [Figure N-483(a)]. The gain and gate setting are reasonable, and the electrical noise is minimal.
(b) Incorrect Low Gain Setting [Figure N-483(b)]. The lateral wave and some of the diffracted signals are start- ing to disappear. At yet lower gain levels, some of the dif- fracted signals would become undetectable.
(c) Incorrect High Gain Setting [Figure N-483(c)]. The noise level increases to obscure the diffracted signals; this can lead to reduced probability of detection, and poor siz- ing. High noise levels can also arise from large grains. In this case, the solution is to reduce the ultrasonic frequency.
(d)Correct gate settings are critical, because TOFD A-scans are not that easy to interpret since there are mul- tiple visible signals. As a minimum, the gates should en- compass the lateral wave and longitudinal wave backwall signal; the gate can extend to the shear wave backwall, if required. Typically, the best signal to use as a guide is the first (longitudinal wave) backwall, since it is strong and always present (assuming the transducer separation is reasonably correct). The following figures show examples of incorrect gate positioning, which will inherently lead to poor flaw detection.
The first example,Figure N-483(d)(1), shows the gate set too early, the lateral wave is visible, and the backwall is not. Any inside (ID) near-backwall flaws will be missed.
Figure N-481(h)
Flaw Location and TOFD Display Showing the Image of the Porosity
1 2
3
Figure N-481(i)
Flaw Location and TOFD Display Showing the Image of the Transverse Crack
1 2
3
The second example,Figure N-483(d)(2), shows the gate set too late. The lateral wave is not visible. The first signal is the backwall, and the second signal is the shear wave backwall. With this setup, all the outside (OD) near- surface flaws will be missed.
The third example,Figure N-483(d)(3), is with the gate set too long. Though this is not technically incorrect, the image will show the diffracted backwall shear-shear wave signal. These S-S waves may show additional and confir- matory information. The diffracted shear waves show the porosity more clearly than the diffracted longitudinal waves and there is a strong mode-converted signal that occurs just before the shear wave gate, which could cause interpretation problems. Normally, the gate is set fairly short to enclose only the lateral wave and the longitudinal wave backwall to clarify interpretation.
(e)Incorrect (too far apart) transducer separation [Figure N-483(e)] results in the backwall signal becoming distorted, the lateral wave becomes weaker, and some of the diffracted signal amplitudes drop.
(f) Incorrect (too close together) transducer separa- tion [Figure N-483(f)] results in the lateral waves becom- ing stronger, and the backwall weaker. Again, the TOFD image of the flaws is poor.
(g)If the transducers are not centered on the weld [Figure N-483(g)], the diffracted signal amplitudes will decline to the point where flaw detection is seriously impaired.
(h)Noise levels [Figure N-483(h)] can seriously impair TOFD interpretation. Noise can come from a number of sources such as electrical, ultrasonic, grains, and coupling.
Typically, ultrasonic and grain noise appears universally across the TOFD image. Electrical noise appears as an in- terference pattern, depending on the noise source. Once the occurrence of the electrical noise increases beyond a certain point, interpretation becomes essentially impossible.
Figure N-481(j)
Schematics of Image Generation, Flaw Location, and TOFD Display Showing the Image of the Interpass Lack of Fusion
Receiver Transmitter
Back wall Lateral
Reflected
Reflection
L B
1 2
3
Figure N-482(a)
Schematic of Flaw Locations and TOFD Image Showing the Lateral Wave, Backwall, and Three of the Four Flaws
2 – Incomplete sidewall fusion
3 – Slag 4 – Incomplete
fusion at root
2 Top
3
4 1 GENERAL NOTES:
(a) Root crack (right): ~ 1.6 in. (40 mm) to 2.5 in. (64 mm) from one end.
(b) Incomplete sidewall fusion (mid-left): ~ 4 in. (100 mm) to 5 in. (125 mm).
(c) Slag: ~ 6.4 in. (163 mm) to 7.2 in. (183 mm).
(d) Incomplete fusion at root (left): ~ 9.3 in. (237 mm) to 10.5 in. (267 mm).
Figure N-482(b)
Schematic of Flaw Locations and TOFD Display Showing the Lateral Wave, Backwall, and Four Flaws
1 – Incomplete fusion at root
2 – Toe crack 3 – Porosity 4 – Incomplete sidewall fusion
2 3
4
1 GENERAL NOTES:
(a) Incomplete fusion at root (left): ~ 0.6 in. (15 mm) to 1.8 in. (45 mm) from one end.
(b) Toe crack (top left): ~ 3 in. (80 mm) to 4 in. (100 mm).
(c) Porosity: ~ 5.5 in. (140 mm) to 6.25 in. (160 mm).
(d) Incomplete sidewall fusion (upper right): ~ 8 in. (200 mm) to 9.25 in. (235 mm).