|NDT.net - September 1999, Vol. 4 No. 9|
Celle, Mai 10-12, 1999
Berichtsband published by DGZfP
|TABLE OF CONTENTS|
|Fig 1: Probability of Detection for NDT Methods (by AEA QT News Article )|
TOFD was originally intended for sizing of flaws which have been detected with other ultrasonic methods [3,4]. This technique was used in Germany, when it was applicable and useful. Only later was it published that the TOFD technique had also been used for flaw detection . No doubt this technique has some capability for flaw detection, however its limits must be clearly specified. The limits must recognize the type of defect and essentially its position within the component, not only its physical limits. This paper focuses on crack-like surface braking defects. This kind of defect is seen more as products age, since with load and environment the risk of failure on the surface increases. For that reason standards require NDT methods, e.g., in the KTA guidelines for pressurized vessels.
|Material and Weld Configuration||Thickness of the Base Material (d in Millimeter)|
|d < 8||8 £ d < 15||15 £ d < 40 1) 2)||40 £ d < 60 1) 2)||60 £ d <- 100 2)||d > 100 2)|
|Ferrit Butt Weld||RT||RT or (UTB)||RT or UTB||UTB or (RT)||UTB and Tandem or UTD||UTD|
|Ferrit T - Weld||RT||RT or (UTB)||RT or UTB||UTB or (RT)||UTB||UTD|
|Austenitic Butt Weld||RT||RT||RT or (UTD)||UTD or RT||UTD or (RT)||UTD|
|Austenitic T - weld||RT||RT||RT or (UTD)||UTD or RT||UTD or (RT)||UTD|
| Remark 1: If two methods are mentioned, the method in brackets is less preferred.|
Remark 2: For ultrasonic testing the test class is mentioned, whereas UTB is equal to the class B of EN 1714.
1) For Steel 3,4 and 6 are both UT and RT or UTD required for wall thickness 15 £ d < 40 and 40 £ d < 60
2) UTD required a special inspection procedure. For mechanized ferrite steel ultrasonic testing is recommended. This can be a mechanized Impulse - Echo -Technique or the time of flight diffraction method (TOFD). For austenitic steel of d > 40 mm a mechanized Impulse - Echo -Technique is recommended.
3) The TOFD Standard is currently under preparation.
If now, as in , a standard for unfired pressure vessels is introduced for the wall thickness range from 15 mm the TOFD technique is equal to the "conventional" ultrasonic- and radiography method (table 1). The time is coming to investigate TOFD with respect to its capability for defect detection, especially since national standards must be withdrawn, according to the resolution of May 7, 1985 appendix II, harmonization of standards (85/C 136/01). For pressurized vessels in Germany public safety plays an important role. Therefore the interest in TOFD technique is not only from an academic point of view. Besides this standard draft for pressure vessels, it also raises a high interest in discussion since publications and notes on the subject of TOFD suggest replacing radiography with this technique  - a technique based on a very different interaction. This comes at a time of increased discussion regarding how best to use certain NDT methods in combination with each other, as opposed to replacing one with another. Another reason for BAM's activity was the fact that very aggressive marketing by commercial interests has taken place with regard to this technique.
|Fig. 2 Example of a TOFD inspection|
Based on the illustration of "Basic Principles" the four shown A-scan signals and B-scans are typical TOFD indications. The lateral wave generates the impulse with the shortest time of flight. The time of flight of this impulse can be used as a reference for the coupling condition, and - as described in the standard - it is used to determine the distance of the two probes. At a later event and therefore with a higher time of flight as the lateral wave, crack edge indications are visible. These crack edge diffracted echoes are of double significance since they indicate first, a defect in the material exists, and second, if the signals are clearly visible as in the example shown, the length of the indication can be determined by the time difference .
The example shows that with the TOFD Technique the presence of a defect as well as its size can be determined. The question is now, what are the limits of this technique, compared to radiography?
A comprehensive work about acceptance of the TOFD technique was done in the Netherlands which was published as a KIND study (The Dutch Society on Quality Surveillance and NDT) [9-12]. The aim of this very exhaustive work was the recording of TOFD results , to present an overview of the inspection work with TOFD , to build a correlation of the TOFD results and those results found by fracture mechanic tests , and to present recommendations for the practical evaluation of TOFD results. One result of this work is that in some cases radiography can be replaced by the TOFD technique. However it must be mentioned that radiography inspection Class A was used. Unfortunately the general discussion - known or unknown - did not mention clearly that by using radiography as well as the TOFD technique a surface crack inspection is necessary. That means: the volume inspection is replaced but not the surface crack inspection. It is questionable how the result of the work would be viewed if instead of inspection Class A the Class B had be used, or mechanized radiography, since the angle of incidence is essential for the detection of two dimensional defects by use of radiography. The comparison is allowed, since the TOFD technique is generally carried out mechanized.
|Fig. 3: Coplanar laminography in a cylinder coordinate system|
|Fig. 4: Test specimen|
The recording camera is located at the other side of the tube, 180° from the position of the x-ray source, connected with a common PC to record the data.Each turn delivers a scan of 100 mm width. Re-positioning the x-ray source of one scan width in axial direction of the tube and scanning again delivers another scan of the tube at this position. Since the camera stays in the same position this kind of installation obtains a projection from the weld zone, shot under different angles. It is clear that this multi-angle-technique can detect better crack-like defects. All scans can be evaluated separately or sandwiched, as shown in Fig. 3. The result of this reconstruction is a 3D visualization or the visualization of single layers.
With such an installation it is not only possible to detect crack-like defects, but also to determine the depth propagation in the direction of the tube wall. This method and a slightly modified radiography method - the planar tomography - will be examined more closely later, in the investigation used for comparison of radiography and the TOFD technique.
An austenitic tube of 13 mm wall thickness and 140 mm diameter was used as a specimen (Fig. 4). Near the weld a crack was generated, starting at the surface and propagated in the direction of the tube wall 1. The surface braking crack was made visible by liquid penetration and is clearly visible in the photo.
1 Note: Because of the crack's depth propagation it was not possible to use it for evaluation of the limits of this method. For this purpose measurements at notches in a specimen have been used.
Fig 5: Manipulator for the Planartomography
Fig 6: Cracks indicated in the range of ± 8,8°
Fig7: Cracks at the clamping device
Fig8: 3D Visualization of the crack
Fig 9: Prinziple of the Planartomography
Fig 10: Crack detection with Planartomography
Fig 11: Crack indication results with TOFD Technique
Fig 12: Crack detection with TOFD Technique (weld testing)
Fig 13: Crackdetection with TOFD Technique (weld testing)
Fig 14: Crack detection with TOFD Technique (weld testing)
The result of the reconstruction is shown in Fig. 8. The crack depth is evaluated with the 3D image to 12 mm; the length to 52 mm. That corresponds nicely with the result obtained with the liquid penetration method. Such exact geometric matching, in this case how it corresponds to the caterpillar contour, is possible by comparing the photo to the reconstruction. The caterpillar contour is clearly visible in both images. The result shows also that by the use of novel radiography, cracks can not only be indicated, which was always described as a disadvantage of radiography, but the depth propagation can be determined as well. With an x-ray opening angle of 40°, that can be applied with this kind of installation, it is possible to indicate cracks with ± 20° oblique position with good contrast. Of course, the inspection time needed for 5 scans is not negligible and therefore costs must be considered. For that reason it is recommended that a "normal" Radiography image at the tubing be performed and evaluated. Once the results are in hand it can be decided if further inspections are necessary. In that case planar tomography would be recommended.
With this method, as shown in Fig. 9, the x-ray source is moved in a longitudinal direction along the tube and a camera records the data. Later on the data can be drawn along this line scan, which was performed with this crack in Fig. 10. The resulting image can be used to evaluate the crack depth. The data processing and reconstruction along such a line scan is much faster and thus more inexpensive. If the depth geometry of the crack is of interest, it is possible to perform such line scans at two or three positions. The limits of this method were determined in the laboratory to a wall thickness thinning of < 2%. In practice this value must be slightly corrected. With confidence it is possible to indicate wall thickness changes of 6%, that means in the presented case a crack depth of 0.8 mm is detectable.To compare the results of this novel radiography technique with the TOFD technique the crack was measured with a TOFD image. The result is shown in Fig. 11. The image indicates the lateral wave at 40 mm, the back wall echo coursed by the longitudinal wave is visible at 47 mm and the back wall echo generated by a combination of transversal- and longitudinal wave at 52 mm. In the center of the image the interruption of the lateral wave and the typical diffraction image are clear visible as shown in Fig. 2.
Fig 11 also shows an A-scan image based on the area of the diffraction signal, which is not visible here as a single indication since it interferes with the back wall echo. The sound path of the diffraction signal calculated with the A-scan is 46 mm. With this value the crack depth can be calculated to 11 mm. A value which corresponds well with the radiography result. The installation is depicted in Fig. 11, in that the probe scan is performed circumferantially. Based on published results it is clear that such a crack can be indicated clearly with TOFD technique, since the shadowing of the back wall echo shows that it must be a big crack. This example clearly shows that setup probes with high bandwidth are necessary for TOFD. These probes are available on the ultrasonic market and were used in this work, that means the results have been measured with these probes. A separation of the diffracted signal and the back wall echo was not possible according to this example. The impulse ring is about 3 wave lengths long. The question is whether this limit must also be considered for small cracks.
For that purpose a specimen with notches of different depth was machined. The TOFD technique results are shown in Fig. 12. At the left hand the result of a 5 mm notch is shown. The diffracted signal is clearly visible. The evaluation result was 5.3 mm. At the right hand the result of a 3 mm notch is shown. At no place in this image is an indication of a defect visible. We can conclude that these are the limits of the method. Based on these results and investigations of surface braking cracks and notches, at the back wall side we can conclude that surface braking cracks of two wave length cannot be indicated. For this reason in [9-12] the TOFD technique is always discussed as a surface crack inspection. Another issue which is always discussed for TOFD technique is that a wall thickness up to 70 mm with a probe distance which is optimized for the middle wall can be tested .
Fig 13 shows the result of a welding with nonfusion and surface braking cracks. The distance between the two probe indexes was 86 mm. The result shows clearly that only the nonfusion defects could be indicated.In the A-scan picture shown it is near the back wall echo - no indication visible. When the probe distance is increased to 138 mm, optimized for a higher wall thickness, the indications of the surface braking cracks should become visible. Fig 14 shows the result. Near the back wall echo there is a visible indication that comes from the cracks. Both examples clearly show that the probe distance according to  must be modified in practice. However, the better way is to inspect the near surface area as described in [15, 16] using the impulse echo corner effect (mirror effect). The weld inspection could be tested with a combination of TOFD and Impulse Echo techniques as depicted in Fig. 15 on the left. By use of novel ultrasonic equipment, for example phased array technique, it is possible to inspect such a welding completely with two probes, as shown in Fig. 4. Hence with the advantage of focusing, for instance on the root zone (optimized by variation of the effective crystal size), a separation between back wall and defect indication is improved.
|Fig 15: Probe set-up for weld inspection|
As recent publications indicate, very early euphoric statements about modified test equipment are withdrawn in the face of sober consideration of considering reliability and safety. This means a complete inspection always uses a method for inspection of the near surface zone, in addition to the TOFD technique. Additionally, the results of the radiography method have been corrected in recent publications. If a single beam technique is used, e.g., along the flanges - in Fig 16 called Bevel Radiography (Technique No. 11) - the probability of detection in relation to crack propagation can be significantly increased. If a multi-angle-technique is used in planar tomography, it can not only improve the detection of planar defects, but the determination of crack depth as well. The investigations at the BAM have shown, that in the future with respect to reliability and safety, especially for pressurized vessels, more consideration must be given to how NDT methods with their various interactions with the component can support each other as opposed to which method should replace which other.
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