4.8 Comparison of EMAT- S_H- and SEL Technique for Testing of Dissimilar Metal Reference Bodies

4.8.1 History

It has long been common knowledge that theoretically, the EMAT-S_H- Testing Technique is provides good predictability when applied to testing of dissimilar metal welds [104].

Because the horizontal polarized Shear- (S_H-) Wave combines in transversal isotropic materials, like austenitic welds, good sound penetration with a perfect corner effect can be obtained if the columnar grains are present in the beam orientation (s. also Chapter 1 and 2).

In practice the EMAT-S_H- Technique is not used much, because the relatively large probes were difficult to manipulate and provided low sensitivity.

That has changed since the end of 1991, when detection by use of redesigned EMAT-S_H- Probes was proved [105], [106], [100], [112].

4.8.2 Test Results at a Dissimilar Metal Reference Bodies

Fig 4.48: Fatigue crack analysis of an austenitic weld 0°- Propagation through a dissimilar metal reference body: Buffer Inconel 182; Width of weld and buffer at the crown: 35 mm; Wall thickness: 40 mm; two options of dissimilar metal reference bodies: with 8 mm depth fatigue crack with 2, 4 and 8 mm depth notch at the interface between weld rood and buffer

Fig 4.49: Detection of notches with EMAT- generated SH- Waves in a dissimilar metal reference body as described in Fig. 4.48; Depth of the notches: 2, 4 and 8 mm Fig 4.50: Detection of notches with a 55°- SEL- 2 MHz- Probe (top) and a 63°- SEL- 2 MHz- Probe (bottom) in a dissimilar metal reference body as described in Fig. 4.48; Depth of the notches: 2, 4 and 8 mm Fig 4.51: Detection of a fatigue crack with a EMAT-SH- Wave Probe in a dissimilar metal reference body as described in Fig Abb. 4.48; fatigue crack depth: 8 mm Fig 4.52: Detection of a fatigue crack with a 55°- SEL- 2 MHz Probe (top) and a 63°- SEL- 2 MHz- Probe (below) in a dissimilar metal reference body as described in Fig Abb. 4.48; fatigue crack depth: 8 mm

Because of all the above, that method has been examined since 1992 - the probe was improved through further developments of EMAT-S_H- Probes and Transmitter- Receiver- Module [106, 100] and the equipment was improved with a redesigned ALOK- System [50]. The investigation was performed on a 40 mm thick dissimilar metal reference body with cladded notches (of 2, 4 and 8 mm depth) at the interface of weld rood and buffer; as well as a fatigue crack (of 8 mm depth) also at the interface of weld rood and buffer.

In order to quantify the test results of EMAT-S_H- Technique, a measurement with SEL- Probes, specially designed and manufactured for that purpose was also done.

An overview of the test problem is illustrated in Fig. 4.48. It shows the amplitude distribution of a backwall echo in different materials (cladded ferrite material, buffer Inconel 182, austenitic weld rood, austenitic parent material) evaluated with 0°- incidence angle of a piezo- probe:

• Through the weld not even the backwall is visible

• The different absorption in the cladded area is clearly depicted left part in Fig. 4.48).

In Fig 4.49 the result of notch detection is illustrated as top view, front view and side view. The measurement was performed by use of gate-free ALOK- test equipment. All three notches are indicated, the notches with less depth of 2 and 4 mm and a S/N ratio of 6 dB. The test was performed from the austenitic side, that means through the weld.

In the upper part of Fig. 4.50 the result of the 55°- SEL 2 MHz-Probe is displayed, that and all other measurements as well were performed with SEL- Probes and a gate based C-scan presentation. The left image part illustrates the propagation from ferrite to austenitic material side and shows a better S/N ratio than the right image part which shows the propagation from the austenitic side. The increase of the noise level through the weld is clearly visible here, but the S/N ratio is not lower compared to the EMAT-S_H technique.

In the lower part of Fig. 4.50 the result of the 63°-SEL 2 MHz-Probe is displayed. The result is qualitatively similar to that of the 55°-Probe.

Fig 4.51 illustrates the detection of the 8 mm depth fatigue crack by use of the EMAT-S_H-Technique from the austenitic side. The S/N ratio is greater than 20 dB.

Fig 4.52 illustrates the result achieved with the SEL-Probe which was used before. The S/N ratio is significantly lower than that of the EMAT-S_H- Technique, high amplitude noise signals are indicated, and the test results are affected by slightly different angle of incidents.

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