4.1. State of the Art

Testing of austenitic and dissimilar metal welds is generally limited, since truly homogeneous ultrasound wave propagation does not exist. The testing and manufacturing methods must find economical solutions for achieving testability for those components. With today's advantages in forgings and welding processes a more homogeneous grain size could be achieved and the ultrasonic propagation will possibly be more quantifiable in the future. In practice, test examples show promising results for crack testing in the near surface and volume zones, this can be achieved by using Transmitter Receiver Longitudinal Probe Techniques (German term: SEL).

There are disadvantages for crack shaped reflectors in the probe far-surface area since the corner effect for longitudinal waves is weak. This disadvantage may be improved in the future by use of horizontal polarized shear waves (abbr. T_H or S_H) which can be generated with Electromagnetic Acoustic Transducers (EMAT or EMUS for the German term). Very early mechanized measurements in reference standards for composite welds show great potential for EMAT-S_H -Techniques. Test validation should be always checked by using the same type of reference standards.

4.2. Probes and Testing Techniques

Fig 4.1 : Equivalent Wave Propagation

4.2.1 Longitudinal Probes

The best test results for crack testing in the near surface and volume area are achieved by using longitudinal double crystal probes (SEL), as long as they are carefully selected for the specific inspection task. Parameters are frequency, incidence angle, sensitivity, pulse length, parasite shear waves, acoustical isolation between transmitter and receiver elements and the acousto-electro impedance matching. All these together with expert probe manufacturing can deliver the needed signal to noise ratio. The following examples will describe some typical SEL probes.

4.2.1.1 55°- SEL-Probe

The probe design predicts beam geometry and thus the sensitivity and the wave mode propagation. The probe delivers a good longitudinal intensity for the detection of volume shaped defects. Geometry based reflection perpendicular to the surface-oriented defects will be minimized. The probe also generates a shear wave which will reflect after mode conversion at the far surface as a longitudinal wave. This is displayed together with the directly generated longitudinal wave as a so-called neighboring echo. In figure 4.1 the equivalent wave propagation is displayed. The probe is highly applicable to volume inspection, crack tip detection, and for crack classification of more depth cracks.

4.2.1.2 70°-SEL-Probe

This probe's working principle is similar to that of the 55° probe. But with its high incidence angle and in conjunction with further probe design, high sensitivity under the surface is already achieved . The neighboring echo shows much less amplitude and is more projected farther positioned as shown with the 55° Probe. In addition to the first neighboring echo a second neighboring echo, which can be used for flaw detection, is converted at the far surface Fig. 4.1.

4.2.1.3 Creeping Wave Probe

The principle of a Creeping Wave Probe [71] is that the wedge angle is equal to the total reflection angle of the longitudinal wave. Ideally, according to Snell's law no longitudinal wave will propagate.

However, Snell's law also tells us that a composite of waves can exist, Fig. 4.2 [120, 168]:

• the direct shear wave (T)
• the direct longitudinal wave (L) (its angle of incidence relies on the ratio of crystal diameter d to wave length )
and
• the creeping wave which propagates with the longitudinal velocity speed (its intensity relies also on the d/ ratio).
• there is yet another shear wave, the head wave (K); its index point is shifted forward (in relation to the d/ ratio).

A key characteristic of a creeping wave probe is its big d/ ratio (> 15). It can reach the longitudinal angle of incidence up to app. 85° with a high intensity of the creeping wave.

Fig 4.2:' Snapshots' of the longitudinal creeping wave and another from a creeping wave probe generated wave [168]. 1 = Wave of the right probe edge 2 = Wave of the left probe edge L = Longitudinal wave K = Head wave T = Shear wave R = Rayleigh wave Fig 4.4: Sound beam and A-scan display of the ADEPT-Probe. Fig 4.4: Focus diagrams

The creeping wave probe is mainly applied for testing the near surface and the near surface area (Fig. 4.1).

2.1.4 Mode Conversion Probes

The conversion [71, 161] is used especially for testing austenitic pipes. It is used at the probe's far surface, from shear waves to longitudinal waves or creeping waves. This test method has advantages vis a vis the corner reflection effect:

• It is less hindered by welding root propagation.
• It is very sensitive - good for detection of small cracks.
• It is possible to determine the crack depth by evaluation of the special beam propagation.

Fig 4.1 depicts both beam propagations for the so-called Neighboring Echoes 1 and 2. This beam propagation also generates a Creeping Wave Probe. However the neighboring echoes 1 and 2 of the creeping wave probe are limited for practical use for thin piping, since the high longitudinal and creeping wave intensity causes more wave mode conversions.

The neighboring echo 2 of a mode conversion probe works like the creeping wave probe for the total reflection angle of the longitudinal probe. The difference from the creeping wave probe is a smaller d/ ratio.

This affects a low creeping and longitudinal wave intensity, and the direct shear wave incorporates with the headwave at the probe index. Probes based on this design have the advantage of being built as a small user-friendly single crystal type.

Optimum Neighbor Echo 2 gains sensitivity together with the surface sensitive Neighbor Echo 2 effect. These are especially adaptable for testing the probe's far surface area.

4.2.1.5 ADEPT-Probe

The ADEPT-Probe ¹ differs in design from the previously described SEL-Probe; its crystals are not parallel but positioned to the rear. The sensitivity zone of the 3 MHz- probe beam provides parallel borders; so the probe shows the characteristics of a angle beam probe without divergence, Fig. 4.3.

Wide adaptability is a feature of this probe. It shows a sensitivity for the surface area similar to the creeping wave probe and with its longitudinal wave covers the zone between 0 mm to ca. 25 mm. A dominant neighbor echo 1 part also allows the planar sensitive detection of perpendicular oriented reflectors (Fig. 4.1). The small housing of these probes allows the combination of two opposite sound exiting probes in one - less probe fixtures for mechanized testing!
¹Advanced Dual Element Probe Technology

4.2.1.6 Conclusion

The previously described probes are not the only one probe solutions for testing of austenitic welding. The description shows how different longitudinal probes, especially SEL probes, can be designed. In addition to the acoustic propagation image, the longitudinal probes' sensitivity diagrams in Fig 4.4. illustrate (Fig. 4.1) the probes' various properties. Also typical are also the C-scan images in Fig 4.5.; these were performed using a reference containing different depth notches. The dynamic characteristics of the 70°-SEL-Probe are totally different, due to its (in Fig. 4.5 mentioned) different design. Longitudinal probes must be carefully adapted or designed for specific test problems.

Fig 4.5: Notch detection with a 70°-SEL-Probe

4. 2.2 Shear Wave Probe

4.2.2.1 Piezoelectric Probes

If welds are accessible from both sides and if a test is all that is needed aside from the weld seam, the use of usual piezoelectric probes, which generates vertical polarized shear waves, is sufficient.

If low frequency is used for testing, in some cases complete penetration of the weld seam is possible, (mainly in the case of "shear wave friendly" welds, for instance for Narrow Slot Welds). (s. item 4.4). In respect to probe design the part 2.1 shall apply. Also shear wave probes must be carefully adapted to the inspection task.

4.2.2.2 EMAT - Probes

Electromagnetic Acoustic Transducers (s. also Book Chapter 5) generate horizontal polarized shear waves (TH- or SH-waves). Four essential characteristics:

1. It works without couplant.
2. If the columnar grains are contained in the plane of incidence , no mode conversion is occuring. This is the case during longitudinal flaw testing. (See also chapter 2 of the book.)
3. The horizontally polarized shear wave is penetrating columnar grained material as effective as the longitudinal waves does.
4. If the columnar grains are contained in the plane of incidence, the corner reflector is unrestricted in the whole range of incidence angles between 0° and 90°

Fig. 5.6 shows the principle of an EMAT Probe [104], which is applied as a phased array transmitter /receiver technique. More useful reading concerning probes and testing techniques is available in [] and [].

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