Experimental Investigation with vertical and horizontal polarized Shear- and Longitudinal Waves at "Narrow Slot Welds"

4.4.1 Comparison of vertical and horizontal polarized shear waves and longitudinal waves.

The advantage of T_H- (or S_H-) waves is that like the longitudinal wave it can propagate relatively easily through polarized grain.

The T_H wave works better at the probe's far surface than longitudinal waves. The T_H wave must be generated with EMAT probes, whereas the EMAT technique did not allow for a standard weld test until recently.

Fig 4.11: TH-, Tv and L-Waves reference body A-scan 1 A-scan 2 A-scan 3 A-scan 4 Fig 4.12: Tv and L,- Wave- reference body A-scan 1 A-scan 2 A-scan 3 A-scan 4 Fig 4.14: "Ghost Echoes" from the weld center line of the Narrow Slot Welds; Probe: MB 4 SN

Experimentally it is possible to generate obliquely oriented T_H- waves with 0°- shear wave probes, which are placed on a reference body of a specific size Fig. 4.11. The polarization level is adjusted by simply turning the 0°- probe, so both the T_H- and the T_v- Wave characteristic can be examined with one probe. It's easy to compare the T_H- and T_v- Wave with the longitudinal wave; it uses a 0°- longitudinal probe of the same wave length.

Fig 4.11 shows the comparison at an 45°- Narrow Slot Welds specimen; a 2 mm depth notch is tested through the parent material and through the weld, and delivers an expected result:

• The T_v-Wave can detect the notch through the weld seam about as well as the T_H-Wave!
• The longitudinal wave shows a high indication of the weld root, that confirms its good through penetration, but notch detection is less efficient.

The verification of the T_v- Wave results by use of 45° shear wave- probes produced the expected result. On the reference body (Fig. 4.12) it was possible to detect with a good S/N ratio the notch through the parent material as well as through the weld.

This first surprising results can be explained by beam propagation modeling [148], see also chapter 2, which according to Fig 4.13 predicts a good sound propagation of the T_v-Wave.

4.4.2 Interface- Reflectionability of the Narrow Slot Welds

The Tandem test on the Narrow Slot Welds showed that the tandem defect at the weld flange is indicated with a shorter time of flight than the interface reflection [170].

The test with 45°- Shear Wave Probes in the root zone is hindered by indications. These are illustrated in fig. 4.12 and their time of flight result is not related to the corner effect, which is built at the probe's far surface and the weld flange.

The two examples prove the hypothesis that the source of the interface reflections could lie in the weld center. The following experiment confirms that:

In a 'Narrow Slot Weld Reference Body' , two circular reflectors in the front and back of the weld flange were applied to surfaces left and right of the weld symmetrically to the weld center. A test with 0°-Longitudional Probes shows the "interface indication" in the middle of the circular reflectors, that means it lies in the middle of the weld, Fig. 4.14.

If the hypothesis is right - (certainly it must be sujected to further experimentation) - that the "interface indications" occur mainly from the weld center - it would affect the test practice thusly: that by use of precise manipulators and high resolution imaging equipment, reflector indications of the weld flange could be easily be separated from those of the interface.

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