5. Measurement results of the TOFD-Technique

The sensitivity adjustment was performed at a side drilled hole of 3 mm diameter. The side drilled hole is placed 15 mm deep into a 53 mm thick reference block. As shown in Fig 5 the amplitude of the hole's diffracted signals relies on the probe distance. More is shown in the A- and B-scan displays Fig 6. Besides the diffracted signals of the longitudinal wave, indications of higher time of flight are also visible. Those are generated by mode conversion and if defect position is unknown it can lead to misinterpretation. können.

TOFD measurements have been performed at the welds of the specimens P1, P2, P3, P5 and P8. The specimen's dimensions, the weld shapes and the defect types are listed in Tabele 1. The scan was performed with probe beams perpendicular to the weld seam axis and the probes moved parallel to the weld seam axis. Scan results were reproducible by use of varied probe spacing. The measurement results with its B-scan images are presented in the following figures and are commented here as:

Specimen P2:
The B-scan image which was performed with a 65°-probe is displayed in Fig 7. The probe spacing was 54 mm optimized on the wall thickness middle zone The Durchschweißfehler defect in 9 mm deep is clearly visible. The defect runs through the total 300 mm long weld seam. The A-scan image (Fig 8) shows the different phases of both diffracted signals. Based on the time of flight, a defect size of 2.5 to 4 mm can be determined

Specimen P1:
The B-scan image which was again performed with a 65°-probe (see Fig 9) shows no diffracted signals between the lateral wave and back wall indications. This could be caused by the 1 mm deep Durchschweißfehler defect, even when the pixel resolution was increased from 0.03 mm to 0.015 mm with comparative gain.

Specimen P3:
Specimen P3 provides a wall thickness of 60 mm, thus suitable 60° probes for this range have been applied on the unground and ground surfaces (see B-scan Fig 10) Again the probe spacing was optimized suitable to the wall thickness middle zone. Measurements applied from both surface sides show defect indications. However, in contrast to the B-scan image measured through the ground side the image from the unground side is clearly noisier. The indications are calculated to exist at 23 mm and 37 mm deep, which results in a 14 mm defect size. B-scan images which have recorded through the ground and unground surfaces are compared. We see that the parent material beside the seam shows a defect-free image while the record through the unground surface shows pseudo defects (see Fig 10a and Fig 10b).

Specimen P5:
Again the spacing of the 60° probes was adjusted on the wall thickness middle zone. B-scan image from the weld indicates a diffraction signal at 19 mm deep (see Fig 11). The defect's elongation is clearly visible.

Specimen P8:
In contrast to specimen P3 and P5, P8 provides a non fusion defect on both sides of the seam at its middle zone and in the root. The middle zone defect could be detected with 86 and 138 mm probe spacing (B-images see in Fig 12 and Fig 13) while the root defect is only weakly visible with 138 mm applied probe spacing in the first third of the specimen's width. A B-scan image, which was measured with a MWB45 N2 probe in Impulse echo technique, shows a Flankenbindefehler defect in the root at 70% of the total weld length and its echo height approaches the registration level.

Testkörper T63-1:
The results of Fig 9 demonstrates that the TOFD technique cannot detect defects at the probe's far surface when the probe spacing is adjusted to the wall thickness middle zone and not to the range of the far surface zone. The 5 mm deep notch (notch width 0.1 mm) at the probe's far surface of the 63 mm thick specimen is only visible when the probe spacing is optimized on the far surface zone. (see Fig Fig 15 and Fig 16) The 1 mm deep notch (notch width 0.1 mm) at the probe's far surface is impossible to detect even when the optimized probe spacing of 174 mm is used (see Fig 17 and Fig 18), while this notch is easily detectable with a MWB45 N2 probe using the corner effect method.