Application of BS EN ISO 16827 for characterisation and sizing of discontinuities in welds using advanced ultrasonic testing
TWI Industrial Member Report 1169-2023 [pdf / 16080 KB]
By Kyriakos Flentzeris
Industrial Need
Along with radiographic testing techniques, ultrasonic testing (UT) is a mature approach for identification of unacceptable discontinuities inside the volume of industrial components and structures. The key advantage of UT is that it is non hazardous to human health and is deployable in many contexts cost-effectively, during both fabrication and service. Numerous UT techniques exist in order to suit a large variety of purposes, for example, the application of compression wave UT to austenitic steel components or corrosion mapping techniques for in-service inspection. The use of digital equipment, particularly computing systems, to process the ultrasonic data has allowed for the development of advanced imaging techniques that aid the human operator in more accurate interpretation of the ultrasonic signals from within the component. From the original approach of interpreting a voltage amplitude vs time (the so-called A-scan), to the colour-coded image, the key to effective UT is good technique design, clear operating procedures and training.
This report presents work to assess the existing ISO 16827 (2014) standard for characterising and sizing discontinuities using UT when applied using the advanced ultrasonic techniques making use of images. ISO 16827 is a mature standard containing established concepts but requires the operator to use A-scan signals. Work was done to translate the concepts designed for A-scan signals to the scenario where the signals are presented as images. The sound physical principles of the existing standard were maintained while deriving suitable interpretation procedures for operators viewing images.
The two imaging techniques considered in this work are phased array ultrasonic testing (PAUT) and the full matrix capture / total focusing method (FMC/TFM). Both of these techniques are described in the report, highlighting their similarities and differences. The work was conducted using welded specimens with representative flaws available in TWI for training operators and using specimens fabricated during the project to section implanted flaws to verify their morphology and establish the performance of the investigated techniques.
Key Findings
-
The work has established that the dB drop methods of EN ISO 16827 provide well-recognised conventions that may provide an accurate estimation of the size of a flaw when also using advanced UT technologies. In cases where a flaw size is greater than the beam spot at the area of interest, the 6dB drop sizing technique led to consistent results for sizing the length and height of a flaw. Therefore, the 6dB drop method is recommended for assessing the dimensions of a flaw in both directions of height and length, and it is consistent with existing standards. Investigation of the 3dB drop method indicates similar performance (for height) to the 6dB drop method, but it is not generally accepted in standards.
-
In cases where a flaw signal has a single peak echodynamic, then the 6dB from maximum returns identical results to the 6dB drop tip location (as per ISO 16827 Annex D.5). Therefore, it is recommended and where possible, for length sizing, the 6dB drop tip location method (as per ISO 16827 Annex D.5) to be used in preference to the 6dB drop from maximum technique.
-
UT configurations using beams focused in the region of interest within a weld return similar sizing errors regardless of how the focusing is performed. Accurate sizing and characterising a flaw can be achieved with encoded scan plans. During the design inspection stage, it is recommended that the scan plans implement beams with a focus within the weld volume and the near field range. Imaging techniques such as FMC/TFM did not demonstrate superiority over the different PAUT techniques. However, TFM requires the least amount of setup compared to PAUT from the perspective of the operator and so may be better suited for use in the field.
-
Segmentation algorithms can be used to assist an ultrasonic operator in sizing a flaw, and potentially such algorithms will reduce the required time for data processing. The evidence in this report suggests that the measurement errors, when such tools are appropriately implemented, are close to the limitations of the applied technique. This observation also reinforces the earlier recommendation to use the 6dB drop tip location method (as per ISO 16827 Annex D.5) in preference to the 6dB drop from maximum technique.
-
Flaws lying perpendicular to the incident beam can be sized with greater confidence than flaws at a tilt when using dB drop methods. Therefore it is recommended to size a flaw from the side of the specular signal.
-
The work demonstrated that X-ray computed tomography (XCT) assessment could potentially be used as a supplementary tool in lieu of macro sectioning if appropriate amendments to the procedure are adopted. However, it was noted that XCT might miss cases of minor features of a flaw and hence any results should be used with caution.
-
During the project’s exercises, it was recorded that tip diffraction signals were related to the presence of planar flaws. Thus, it may be concluded that such signals are valuable sources for characterising a flaw and distinguishing it between planar or volumetric.
-
An accurate assessment requires at least two scans on either side of the weld. In cases where only one side is available for inspection, and thus only one scan can be performed, there is a high risk of limited data available for making a reliable assessment. The use of the echodynamics as described in BS EN ISO 16827 may be insufficient to distinguish between different types of flaws (e.g. lack of fusion and cracks), especially when a pattern 1 echodynamic is reported. Probe skewing may be used for the characterisation of a flaw. However, it requires additional time to implement, and its success depends solely on the assessment of the operator, while it is difficult to obtain permanent records. Based on the limited data available, the use of echodynamics in the way described in BS EN ISO 16827 may lead to faulty classification and should be avoided. It is recommended that when tip diffraction signals lead to ambiguous results, further investigation should include a manual assessment by skewing the probe. This may include situations where the measured height size is close to threshold values, such as given by critical engineering assessment.
-
The project did not have the resources to complete an assessment of tip diffraction signals for height sizing. Therefore, tip diffraction signals for height sizing should be used with caution until more detailed work is undertaken.