4.3. Test Effects and Test Techniques

For Ultrasonic Testing, two methods are usually applied: the through transmission technique and the pulse echo technique. For through transmission the hindrance of sound propagation by inhomogeneities is a measure for parts integrity while the pulse echo technique measures this using the reflection caused at the point of inhomogeneity. Austenitic welds and dissimilar metal welds are mainly tested with the pulse echo technique. Therefore only these are described:

4.3.1 Geometry Reflection

Geometry Reflection is present when the sound beam penetrates through the component and is totally or partially reflected. This occurs at pores, voids, also for cracks when the sound beam hits the reflector or part of it perpendicularly.

Fig 4.6: Corner effect and wave modes [104]

4.3.2 Corner effect

The corner effect, the sound reflection from an edge, is the most important test effect for crack detection at the probe's far surface. However, the quality of the corner effect relies on the wave mode used; if it is:
• a vertical polarized shear wave
• a horizontal polarized shear wave
• or a longitudinal wave.

Fig. 4.6 makes those different qualities clear. It shows that the corner effect for longitudinal waves exists only faintly at app. 45°. The vertical polarized shear wave is fully present between 35° to 55°, and the horizontal polarized shear wave (SH- Wave) is present in the full range of incidence angle between 0° to 90°. The reason is that SH- wave exists in isotropy materials, but also in transversal isotropy materials, like austenitic welds; no wave conversion applies, if columnar grains are present in the beam orientation (longitudinal flaw testing).

Fig 4.7: "counterbore" and test effect Test Situation Corner Effect ---- Mode Conversion Fig 4.8: Neighbor Echo 2- Method and Corner Effect. Fig 4.9: Tandem- and equivalent tandem test technique

4.3.3 Mode conversion with Neighbor Echo 2 method

The design features of probes for the Neighbor Echo 2 method are described in item 2.1.4. The Neighbor Echo 2 method makes use of the mode conversion at the part's back surface. Therefore the contour of the back surface must be known. If the part's documentation does not provide this information the contour must be evaluated with a normal beam probe. The effect of the back surface on the echo amplitude is shown in Fig 4.7 which depicts the 'counterbore experiment' and illustrates the need for the careful evaluation of corner effect probe echoes.

Assuming the presence of all necessary conditions for the Neighbor Echo 2 method, this test method offers the highest detection sensibility. This is confirmed in Fig. 4.8 shows this, using a fatigue crack as an example.

4.3.4 Tandem- and Equivalent Tandem Test Technique

For ultrasonic testing applied circularly and perpendicular to the surface, oriented defects results were obtained using the classical tandem test technique, the LLT-Technique, and the Neighbor Echo 1 method.

Each of these three methods provides a specific detection capability, as described in item 5. The principles of these three techniques are outlined as follows:

4.3.4.1 The classical Tandem Technique with vertical polarized Shear Waves

It uses two 45°-Shear Wave Probes, which work on the transmitter/receiver principle. The depth of sensitivity is adjusted via the distance between transmitter and receiver. Fig. 4.9 displays the Tandem principle.

4.3.4.2 The LLT-Technique

A longitudinal wave with small angle of incidence is reflected at the probe's far surface (1. crystal). This reflected wave is received from a 2. crystal and undergoes mode conversion at the point of circular shaped defects. [77] - [79], [196, 197]. The basic principle is displayed in Fig 4.9. The LLT-Technique can be built very compactly into one probe incorporating two test functions, thus a large thickness range is covered. The sound beam in Fig. 4.10 characterizes the working principle of a LLT- compact probe.

Fig 4.10: LLT-compact test technique, test frequency 2 MHz, shown are the real crystals with their true propagation, M 1:2

4.3.4.3 Normal Probe - Mode Conversion Technique (Neighbor Echo 1)

SEL- Probes are used as the basic element of the Neighbor Echo 1 method. Depending on the angle of incidence, different sensitivities and projection distances are provided at the reflector. The schema in Fig. 4.9 confirms this, using a 55°- and a 70°- SEL-Probe as examples.

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