Ultrasonic diffraction technique was also employed as a test method. In the USA this method was first applied for crack detection and sizing (depth, length) in austenitic pipe welds. The tested welds were investigated destructively and the ultrasonic results were compared with those obtained via metalography. The inspection method and its parameters were validated.
UT with the 70°SEL technique was qualified for testing of austenitic piping with 5mm to 15mm wall thicknesses. This uses the mode conversion effect on transversal waves at the far surface , . By use of longitudinal waves, which penetrate well in austenitic materials, and applying with a large (70°) angle of incidence , a signal from the reflector surface (main echo) can be achieved . Rood defects of less depth are distinguished from defects with greater depth by the absence of neighboring echoes (triangle reflection in fig 1a.). That is performed as a detection technique. The height of the main echo cannot be used to determine the crack depth. Fig 1b. shows the so-called notch diagram of the 70° SEL small probe (KWU) (main and neighboring echoes). The different test notches and the edges of the reference block display an echo amplitude similar to the main echo. A clear echo resolution exists for determination of the registration level (echo amplitude of the 1.5 mm notch minus 6 dB). Those echo signals which exceed the registration level and the typical neighboring echoes are classified as defects. The echo amplitude of the neighboring echoes (triangular reflection) increases with crack depth. The evaluation using the displayed echo signal pattern is only qualitative by means of a yes/no decision, since the echo amplitude changes significantly with the weld root geometry.
Fig. 2c shows an example of a crack open to the far surface (artificial notch), which is detectable by the diffraction echoes and the absence of the back wall echo. The crack depth can be depicted as a linear B-scan image. The echo amplitude is displayed as gray scale, usually zero amplitude light gray (negative maximum amplitude black, positive maximum amplitude white). An accuracy of approx. +/- 1 mm can be achieved for defects 10 mm under the surface . The lateral wave hinders the evaluation in a depth of approx. 5mm.
The diffracted signals have a low amplitude ( approx. 20 dB less than a 3mm Ø -side drilled hole). A problem is that the test has to be performed with high gain, increasing chances that small inhomogeneities or slake will generate significant signals. Our task was to qualify TOFD as a detection technique on gladding components . The results showed that a gladding nonuniform surface interfered with the backwall echo and lateral wave; these could be evaluated as surface cracks or loss of coupling. That necessitated verification by use of the impulse echo technique. It is very difficult to distinguish significant signals in the area of noise (rough surface, conclusions). When TOFD is employed as a detection technique the scan of the total surface must be performed with clear and sufficient sensitivity to produce a reliable test result.
When applied on austenitic welds an insufficient signal to noise ratio is another problem. TOFD employed as an additional analytic method for verification of Impulse Echo Technique or Radiography results seems to be more applicable.
To identify a diffracted echo a corner echo from the crack is needed. The corner echo is generated by use of the corner effect between far surface and crack surface. There are two techniques:
Twin probes with 60° or 45° generating shear waves are applied. To make the crack detection more reliable for finding cracks in deeper areas as well, the diffracted echo is also evaluated after travelled as V-transmission; that means after it has been reflected on the far surface. A test with 60° longitudinal waves is subsequently performed. When welds of piping that can be only reached from one side of the weld are tested and the echo must be transmitted through the weld, only longitudinal probes are applied.
The crack depth can be easily calculated, thus measuring the time of flight when the peak amplitude of the diffracted signal is detected. There are also methods that measure when the signal registers half value or just disappears within the noise. That should prevent the error on measuring diffracted signals from a lower part of the crack instead from the tip. All signals higher than noise and echo dynamic and time of flight are evaluated as possible cracks. If a diffracted signal does not exist, but an echo signal from the far surface appears, the result is classified as a crack less than 1 mm.
In the 80's, this technique was investigated in the USA by the EPRI Institute for detection and sizing of cracks . An American standard describes the necessity of using ultrasonic testing for crack depth measurement. The performance of the method used and the personnel qualifications must be demonstrated .
Our task was the inspection audit of austenitic piping which was tested by the service company General Electric. The test was performed mechanically by the Smart 2000 system; all A-scans can be stored. The evaluation is performed with post processing of all data after the test is finished. Even though the company had a lot of experience in our specific area (other component geometry, weld geometry) further study was necessary in order to achieve a reliable result. The results of the ALT Method on notches and nature cracks are displayed in fig 4. Our comments for these test results:
Since it is not certain that all crack edges generate diffracted waves, the optimized ultrasonic pulse echo technique, (described above) should be used. That method uses reflected signals from the crack surface or corner. In case of a suspected defect other tests, e.g., radiography or ultrasonic analysis, need to be employed to obtain a test result which is based on all test methods.
For more information see: TOFD in UTonline 09/97
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