4.7 Visualization of Test Results via Imaging Methods

The direct visualization of a flaws is an old aim of nondestructive testing. With imaging methods such as acoustic holography [18] - [118], [119], [131], [164], the broad band holography [47], [155, 156], ALOK³ [44, 45], [83, 84], [94], [101], [110], SAFT⁴ [65], [165] - [167], [198], FT-SAFT⁵ [125, 126], the Ultrasound-Tomography [113, 208] and other; it is at least possible to show the reflectibility for different wave modes of component area under test. Precondition for the above mentioned methods is that the component is an ultrasonically isotope and homogeneous material [69], a precondition which especially here does not exist and therefore needs a modification of imaging methods for the sound distribution in anisotropy materials. About their principles and technical details readings are available through the attached Literature References.
² Intergranular Stress Corrosion Cracking ³ Amplitude-Laufzeit- Ortskurve ⁴ Synthetic Aperture Focusing Technique ⁵ Synthetic Aperture Focusing Technique with Fourier Transforms

4.7.1 A-scan, B-scan, C-scan and D-scan

In the previous part we focused on the physical and technique problems for testing of austenitic welds. The results were mainly presented as A-scan results.

For validation of test methods, for a first overview of results of components and for the result analysis a A-scan presentation is still appropriated. Especially for testing with longitudinal probes are additional test effects like Neighbor Echo 1 and 2, and the dynamic response of the three wave modes useful and are good displayed via the A-scan presentation. But the test result quality interpreted using A-scan presentations depend highly on the personnel qualification.

Large scale ultrasonic testing, especially in radiation areas, are mainly performed with automated testing and imaging methods. The quality of the image document relies on the data processing (e.g. A-scan digitalization or gate techniques) and the computer.

Test results which are documented by imaging methods are repeatable, describing clearly the tested area and allow an easy separation of geometry indication from real defect indications (e.g. root penetration, counterbore).

Also in laboratory work, during development and optimization of test procedures the image technique (e.g. via B-scan or C-scan) is helpful, since possible additional occurred test effects are better recognized and interpreted. Furthermore the work is better adaptable to the praxis, because with automated testing echoes can't be manually 'peaked'.

Fig 4.39: B-Scan image of a membrane tested in immersion technique. Fig 4.40: C- and D-scan image of a membrane tested in immersion technique. Fig 4.41: Via A-scan to TD-scan image Fig 4.42: C- scan image of fatigue crack; SE-shear wave probe, transmitter: 60°, receiver: 40°, rear positioned.

Definitions: A-scan, B-scan, C-scan, D-scan and TD-scan

These imaging methods - generally well known - are demonstrated as an example on a concentrically ring contoured membrane (15 mm D). Measurement was performed in immersion technique by use of a 20-MHz focused probe.

A-scan: A short impulse is generated into the object and echoes are displayed as function of time (depth).

B- scan: If the A- scan - concept is combined with movement of the probe along the surface, a B- scan is the result. It depicts the acoustical side projection of the object, Fig. 4.39.

C- scan: Via C- scan are echo amplitudes recorded in relation to probe position. It depicts the object top view of amplitude, Fig 4.40.

D- scan: The D- scan is similar to the C-scan, but the echo time of flight (depth) in relation to probe position is recorded. Actually the D-scan is a '3-D visualization' of the object, Fig 4.40.

TD-scan: In a TD-scan ('time displacement') are single shots of a A-scan placed one after each other. The principle generation of a TD-scan image via A-scans is illustrated in Fig. 4.41.

The benefits of imaging methods for test results demonstrates the example as follows: For a compact test technique, which uses in the rear positioned SE - shear wave probes, hinders a fixed positioned coupling echo which occurred while coupling the probe on the component surface. At a reference body, which contained fatigue cracks starting at the probe near surface zone, a gate based c-scan image was performed, Fig. 4.42.

It contains the assumable information as follows:

• The fatigue cracks are detectable with no problems.
• The coupling echo lays like a veil over the test surface.
• And the additional information: The S/R ratio increase when the probe moves over the crack. The reason is the interruption of the coupling echo over the crack. That means cracks can be detected directly and indirectly.

4.7.2 Broadband- Holography

The Broadband- Holography - System [47, 48] was used as analyze equipment for the following described test results. The method improves significant the S/N ratio if geometry and grain echoes are present.

4.7.2.1 Reconstruction of a crack

Fig 4.43: Reconstruction of a crack

A 'cold' crack, which occurred after performed a test welding, was examined with a 45°-Shear Wave-Probe. Fig. 4.43 shows the crack geometry as visibly on the surface, the A-scan images as ALOK presentation and the crack reconstruction.

Fig 4.44: Analysis of a notch reflector through an austenitic weld Scan Results Fig 4.45: Cladding inspection for detection of UPR- reference flaws Scan Results Fig 4.46: Crack analysis in the cladding area Scan Result Fig 4.47: Crack tip detection of a fatigue crack in an austenitic weld

4.7.2.2 Analysis of a notch reflector through an austenitic weld

The analyze was performed by use of a 45°-SEL- Probe and a 60°-SEL-Probe, Fig. 4.44. In both cases it is possible to separate clearly the notch's bottom and tip. Die Image indications at the lower right image corner are either interpretable as Neighbor Echo 1- effect or as shear wave. If it is possible to integrate these image parts into the Holography- Algorithms, so notch tip and bottom can be plan sensitive confirmed.

4.7.2.3 Analysis in the cladding zone.

Fig. 4.45 shows a reference body containing a cladded area over 10 notches. The test was performed by use of a 45°- PVDF- Probe⁶ with a bandwidth of 1.2 MHz and a nominal frequency of 1.6 MHz. Because of the probe's high bandwidth the reconstruction shows a good axial resolution. The low sensitivity of the PVDF- probe is the reason for the noisy A-scan image. Only after noise ratio improvements by averaging as part of the reconstruction method it is possible to achieve a necessary S/N ratio.

Fig. 4.46 demonstrates the analyze of a fatigue crack under cladding which was provided for performing the qualification of the test method. The final destructive test confirmed that measurement results matched good with the true crack depth. A manually analysis (A-scan) was almost impossible due to low S/N ratio and many noise indications (cladding echoes). ⁶ Transducer build with piezoelectric high polymer polyvinylidenfluorid e.g. [154]

4.7.2.4 Analysis of an austenitic weld root

The detection of a crack is illustrated in Fig. 4.21. It was possible by use of the mode conversion, whereas it is predestined that the plane sensitive Neighbor Echo 1 part a depther crack value indicates. The results are dispayed in Fig. 4.47.

The crack tip detection of a fatigue crack in an austenitic weld root is successfully applied by use of a 48° Shear Wave- Probe. However the result should be better by use of a longitudinal probe, that the redundancy of the Neighbor Echo 1 method use.

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