NDT 2010 Conference Topics

Transcription

1 NDT Conference Topics Session 4A (4) Ultrasonics Chairman Dr C Brett 16. Novel conventional and phased-array angle-beam probes for enhanced DGS-sizing Authors - York Oberdoerfer, Gerhard Splitt, Wolf Kleinert The DGS-method was originally developed in the early 6 s for the use with straight-beam probes with circular transducer and was later transferred to the use with angle-beam probes whereas the characteristics of angle-beam probes were found to be in acceptable accordance when compared to straight-beam probes. By the use of nowadays electronics with given precision the authors are able to show that the characteristic of the acoustic pressure originating from the rectangular shape of the crystal of most prevalent angle beam probes can significantly deviate from the characteristic as shown in the general DGS-diagram reflecting rotational symmetric sound fields. In this presentation novel conventional and phased-array angle-beam probes are presented that were designed to overcome the missing rotational symmetry. By means of computer simulations a straight-beam probe with aspired properties is virtually tilted till an angle-beam probe with desired angle of incidence results. The resulting angle-beam probes with unusual transducer geometry now show the same DGSsizing capabilities as straight-beam probes with circular transducer, whereas these capabilities are not only limited to conventional probes but can easily be transferred to angle-beam phased array probes.

2 Novel single element and phased array angle beam probes for enhanced DGS-sizing York Oberdoerfer, Gerhard Splitt, Wolf Kleinert GE Sensing & Inspection Technologies GmbH Huerth, 5354, Germany Abstract The DGS-method was originally developed in the late 5 s for straight beam probes using circular transducers and was later adapted for the use with angle beam probes, based on the assumption that the characteristics of angle beam probes were comparable with those of straight beam probes. Using today s high precision electronics, the authors are able to show that the characteristics of the acoustic pressure originating from the rectangular shape of the crystal of most prevalent angle beam probes can significantly deviate from the characteristics represented in the general DGS-diagram, which relates to rotationally symmetric sound fields. In this presentation innovative single element and phased array angle beam probes are presented that were designed to overcome the missing rotationally symmetry. By means of computer simulations a straight beam probe with specified properties is virtually tilted until an angle beam probe with the desired angle of incidence results. The resulting angle beam probes with novel transducer geometries now show the same DGS-sizing capabilities as straight beam probes with circular transducers. These capabilities are not only limited to single element probes but can easily be transferred to angle beam phased array probes. 1. Introduction To understand the motivation of the authors to develop the novel angle beam probes for enhanced DGS-sizing a short introductory discussion is necessary on the origins of the DGS-method, its designated use and recent measurement results achieved. 1.1 On the origin of the DGS-method The DGS-method was first published by Josef Krautkraemer in 1959 as a proposal for standardization of test results of ultrasound measurements with straight beam probes. (1) The intention was to give operators a standardization method that is independent of the instruments and probes used by developing probe-specific DGS-diagrams, where DGS stands for Distance, Gain and Size. With this diagram the operator is able to compare reflections coming from real flaws with reflections of artificial flat-bottom

3 holes (FBH) with a certain diameter in the same depth. The diameter of the artificial flat-bottom hole, giving the same echo height as the found flaw, is called Equivalent Reflector Size (ERS) and can be used as threshold value by e.g. specifying that any found indication must not exceed an ERS of 3 mm. In 1967 this method was also proposed for the use with prevalent used angle beam probes with Plexiglas (registered trademark of Evonik Röhm GmbH, Darmstadt, Germany) or polystyrene wedges and rectangular transducers that excite shear waves in the specimen by mode conversion at the coupling plane. (2) In this article, the authors transferred the DGS-method by determining the near field length N of the above-mentioned angle beam probes experimentally and taking the DGS-diagram of a straight beam probe with circular crystal having the same near field length. By using contemporary, state-of-the-art equipment the results were found to be in good agreement with the DGS-curves for straight beam probes, even though the sound field of a rectangular transducer mounted on a wedge is not rotationally symmetric (3)(4) as was stipulated in the first work on the DGS-method. (1) The reader should keep in mind, that contemporary equipment of the 195 s and 6 s mean analogue instruments with limited linear amplifiers and cathode ray tubes that enabled, compared with today s standards, imprecise positioning of indications. They bear no comparison with today s digital ultrasound equipment that enable measurement at a level of precision up to.1 db and.1 mm (fig.1). (6) Figure 1. Comparison between an old and modern instrument. On the left side a Krautkraemer USIP4 from 1954 can be seen, on the right side GE Sensing & Inspection Technologies USM Go from 9 is shown. 2

4 1.2 Sound field of angle beam probes having rectangular transducers As described in the previous section, the DGS-method requires a rotationally symmetric sound field (1) but was also proposed for defect sizing using shear wave angle beam probes with rectangular transducers. It was known, that this is only an approximation since the sound field emitted by these angle beam probes does not show the required symmetry. (2) To further understand the characteristics of these sound fields, computer simulations using CIVA (5) have been carried out. In this simulation the sound field of a 4 MHz, 7 angle beam probe with 8 x 9 mm rectangular transducer on a Plexiglas wedge was simulated. Figure 2. Sound field simulation of a 4 MHz, 7 angle beam probe with 8 x 9 mm transducer. The above pictures show the pressure distribution in the plane, perpendicular to the beam axis shortly in front of the near field end, at the near field end and at twice the near field length. The sound pressure distribution ranges from green (low pressure) to blue (high pressure). It can be clearly seen in figure 2, that the sound field is not rotationally symmetric but more elliptical and undergoes a rotation of 9 around the beam axis. Because of the higher beam spread of the shorter transducer dimension, the elliptic characteristic changes from a horizontal orientation in front of the near field end to a vertical one. One should furthermore consider that not only the rectangular shape of the transducer and the different side dimensions fosters this non-symmetry, but also by the refraction at the interface between wedge and test piece. (3) Continuing this thought should then lead to the fact, that the deviation of an angle beam probe with rectangular transducer should be the bigger, the smaller the frequency (leading to a higher beam spread), the bigger the transducer and/or the higher the incidence respectively the wedge angle is (leading to a much bigger cut-off of the portion of higher angles in the beam spread). Results, that confirm this, will be shown in the next section. 1.3 Recent results with angle beam probes Initiated by customer feedback and on-going discussions in the NDT community (7) extensive DGS-measurements with different angle beam probe types were carried out. For this, test blocks were used, that have inclined back walls of 45, 6 and 7, contain nominal 3mm flat-bottom holes (FBH) and allow sound paths up to ~5 mm with a nominal shear wave velocity of 3255 m/s. 3

5 BW / measurement Figure 3. DGS-measurement with GE Sensing & Inspection Technologies SWB6-2(E) angle beam probe at 3 mm FBH in a test block with inclined back wall. Results show serious deviations from predicted 3mm ERS-curve. Observed deviations turned out to be dependable on transducer size, frequency and angle of incidence where the largest deviations of up to 4-6 db from the nominal 3mm ERS curve were measured for 2MHz angle beam probes having rectangular 14 x 14 and x 22 mm transducers and angles of incidence of 6 and 7. It should be noted here, that because of physical principles, this problem applies to all angle beam probes with rectangular and also circular transducers regardless of the manufacturer. 2. The truedgs -technology The following section gives insight into a new technology called truedgs, that provides a solution to all above-mentioned problems that occur when trying to do a DGS-measurement with current angle beam probes. 2.1 Single element probes with truedgs -technology From theory (1) and experience, one knows that DGS-measurements work correctly for straight beam probes with circular transducers where the sound field in the specimen is rotationally symmetric (fig. 4). The idea underlying the truedgs -technology is to start with a computer simulation with a virtual straight beam probe of given transducer diameter (equivalent transducer diameter) with the desired near field length and frequency. The velocity in the specimen is the shear wave speed. Only two constrains are considered, namely Fermat s principle, that results in Snell s law in this case, and the fact, that the difference in distance between the centre of the transducer and a point on the perimeter is exactly half of the wavelength at the end of the near field. (8) 4

6 BW / measurement Figure 4. DGS-measurement with GE Sensing & Inspection Technologies MB2S(E) straight beam probe on a 3 mm steel-rod reflector. Measurements were performed in water (v=148 m/s) for constant coupling conditions. In a first simulation step, a sufficient number of constructional beams are considered, that start at the near field end N, end on the transducer plane in the respective points B i and are angle-wise equally pitched in both planes around the centre beam. Figure 5 represents only for the i-th beam having a tilt angle of i in the view plane. The tilt in the other plane is not shown. Then the length of each beam is calculated in measures of time (TOF i NBi ). Figure 5. Underlying idea of the truedgs -technology. The starting point is a straight beam probe with given frequency and near field length (left). After considering a sufficient number of constructional beams the result is an angle beam probe designed by using truedgs -technology (right). Since the intention is to design an angle beam probe, the aforementioned centre beam is tilted in a second simulation step around the imaginary point of the near field end by the intended angle of incidence and then moved with its end point C into the wedge material to form a delay line. The length of the delay line is chosen in a way that the final wedge gives a sufficient range of allowable wear and the near field end is located in the desired region in the specimen; this may require several iteration steps. When doing so one has to take into consideration that the transition from the lower medium 5

7 (the specimen) to the upper medium (the wedge material) obeys Snell s law using the respective material velocities. The different material velocities also mean that the timeof-flight remains unchanged for all beams but the overall length of each beam, measured in mm, will change. Again it should be noted here, that the shear wave velocity of the specimen has to be taken into consideration. Now this step has to be repeated for each of the constructional beams: The abovementioned angles i (and also the angles in the perpendicular plane) have to be added to the angle of incidence and, again, Snell s law has to be used to describe the transition from one medium to the other. The length of each constructional beam, which was calculated in the first step, is now used to determine the end point of the beam B i (fig. 5) in the medium. The entirety of all points Bi then form the transducer surface that is required to form a sound-beam that is rotational-symmetric and can be compared to the sound beam of the straight beam probe with its circular transducer (fig. 6). Figure 6. Resulting transducer shape of a truedgs -angle beam probe. Since the distance from the near field end to the interface along the different beams and the resulting angle on the interface between both media is unique for each beam, the resulting transducer shape is not circular but can be described more as tongue shaped. The reader should also be aware, that the resulting transducer is not flat anymore but can depending on the simulation parameter be concavely and/or convexly bent in both directions (fig. 7). To proof the model, an angle beam probe that was designed with the truedgs technology has been simulated in CIVA (5) (fig. 8). Again, a 4 MHz probe with an angle of incidence of 7 in steel (3255 m/s) was chosen where the near field length was comparable to the near field length of the probe that is shown in figure 2. Though the truedgs -technology can only be approximated in CIVA, figure 8 proofs that the sound field of a truedgs -probe is much more rotationally symmetric than a current angle beam probe. 6

8 Figure 7. Cross-section of a simulated transducer (left). Though the bending is only small (some tenth of a millimetre), it is not negligible in measures of wavelengths. (right) Longitudinal section of simulated transducer after a coordinate transformation (rotation). One can clearly see a convex as well as a concave bent S-like shape. To realize such an angle beam probe, the bent surface, defined by points B i, also has to be machined into the wedge itself. Nevertheless, with current CAD-milling machines and by the use of modern CAD/CAM-software this is quite feasible. To make the transducer follow the surface contour flexible Piezo-composite material has to be used rather than rigid monolithic Piezo ceramic. By choosing the basic ceramic of the Piezocomposite transducer with respect to the instruments impedances and by adjusting the Piezo-composite s volume fraction with respect to the acoustical impedance of the wedge material, the authors were able to design probes which have besides the rotationally symmetric sound field a relative bandwidth of -5%, that is beneficial to the DGS-method. (9) Figure 8. Sound field simulation of a 4 MHz, 7 angle beam probe designed with the truedgs -technology. The resulting sound field is much more rotationally symmetric than one of current angle beam probe. The sound pressure distribution ranges from green (low pressure) to blue (high pressure). It should be noted here, that, because of the way such an angle beam probe is designed, it can only be used with specimen materials that have within a certain tolerance the same shear wave velocity as the value that was taken during the computer simulation. A 7

9 simple transfer of DGS-curves from one material to another, e.g. from steel to aluminium, by taking the ratio of sound-velocities into account is not possible anymore. Though this seems to be pretty common in the NDT community, the authors want to point out their concerns about this procedure for current angle beam probes after having done the work that is presented in this publication. 2.2 Results of DGS-evaluation with single element, truedgs -probes To verify the truedgs -technology, a complete set of probes were built and evaluated as described in paragraph 1.3 for current angle beam probes. This set contained complements to single element angle beam probes with standard parameters, such as crystal sizes of 8 x 9, 14 x 14 and x 22 mm, angles of incidence of 45, 6 and 7 and 2 and 4 MHz transducer frequency. 5 reference echoes / measurement Figure 9. DGS-measurement with 4 MHz, 6 degree probe with truedgs - technology. The accuracy is now comparable to those of straight beam probes. To allow sound-attenuation to be neglected and to avoid its necessary compensation, a back wall reference echo, being in the same depth as the flaw itself, was recorded for each flaw echo. Consequently, the DGS-evaluation could, distance-wise, be done on a vertical, straight line in the diagram (fig. 9). The result of this evaluation is an angle beam probe whose DGS-accuracy is comparable to that of straight beam probes as shown in figure 4. The underlying DGS-curve was derived from the general DGS-diagram for a transducer diameter equal to the equivalent transducer diameter from the simulation. 2.3 Phased array angle beam probes with truedgs -technology The starting point to design a phased array probe is the same as for a single element probe where the wedge angle is chosen to be in the middle of the specified steering range. The resulting transducer shape is then divided into the desired number of equally spaced elements. As a next step, this calculation is repeated for all other angles that are to be realized by means of electronical steering later on. This is not being done by simple calculation of 8

10 delay laws based on geometrical considerations but by designing a truedgs -probe for each phasing angle (fig. ). Of course, since the mechanical size of the transducer cannot be changed from angle to angle, the resulting probes have to show a congruence with the probe from the starting step in a way that the rays, defining the outer perimeter, are tangential to the outer perimeter of this starting probe. To achieve this, several iteration steps may be required and may lead to a simulated probe, that has a slightly different transducer shape respectively size and wedge dimensions. Figure. Schematic showing the simulation of phased array probes. Starting from a probe with the later wedge angle and a pre-defined phasing angle pitch, a consecutive number of phased array probes are simulated. The delay laws for each element and each angle is calculated from the location-dependent distance. Schematic for negative phasing angle (left). Schematic for positive ones (right). For negative phasing angles the equivalent transducer diameter can vary by 5-%. Since this has direct influence on the resulting near field length, delay line length and transducer-to-flaw ratio (fig. 11), this has to be considered in the DGS-evaluation resulting in highly angle dependent DGS-curves. equivalent transducer diameter / mm 15, 14,5 14, wedge angle delay line length / mm wedge angle phasing angle / phasing angle / Figure 11. The equivalent transducer diameter (left) and delay line length (right), that has to be taken for the DGS-evaluation, depends on the specified phasing angle. 9

11 2.4 Results of DGS-evaluation with truedgs phased array probes Again, several probe types were built using truedgs -technology allowing direct comparison with similar GE Sensing & Inspection Technologies standard single element probes, namely MWB, SWB and WB. Frequencies were chosen to be either 2 or 4 MHz whereas the number of elements was 16 in all cases. To prove the enhanced DGS-sizing capabilities, a set of 5 test blocks was used to cover a steering range between 45 and 7 in steel (3255 m/s). All test blocks contain 3mm flat-bottom-holes and have inclination angles of 45, 53, 6, 65 and 7. To compensate possible sound attenuation, reference echoes were recorded from the same depth as the reflections from the FBH themselves. 45 steering 53 steering 5 6 reference echoes / measurement reference echoes / measurement steering 65 steering reference echoes / measurement 6 6 reference echoes / measurement 6 7 steering reference echoes / measurement 6 Figure 12. DGS-measurement with 2 MHz, 16 element phased array angle beam probe for different steering angles. For all tested angles, namely 45 (upper left), 53 (upper right), 6 (middle, left), 65 (middle, right) and 7 (lower left), the results show a consistent accuracy, again comparable with straight beam probes.

12 A GE Sensing & Inspection Technologies Phasor XS flaw detector was used where the different delay laws were calculated externally and entered manually into the set-up file instead of using the internal delay law calculator. On the receiving channels an additional 12 db attenuation was realized to be able to record back wall echoes over the complete, specified evaluation range. As seen in figure 12, the results show a DGS-sizing capability of phased array probes, designed according to truedgs -technology, that is comparable to that of single element probe as described in prior sections and of course to that of straight beam probes. Nevertheless this accuracy is not limited angle-wise but can be realized for a predefined, very fine angle pitch, whereas the maximum steering range is limited by the actual element pitch, count and frequency. 3. Outlook Since the theoretical model underlying the truedgs -technology is purely physical, it can be applied to all situations that can be described mathematically. Specifically, the application of DGS-evaluation to any other material than steel can now be done without any constraints since the model accounts for different sound velocities and gives, as a result, the adapted transducer shape. Also the adaption to surface geometries other than flat ones (D obj < *l ps, D obj < * w ps ) is now possible as long as these geometries can somehow be parameterized. Thus, it is no longer necessary to exclude DGS-evaluation for probes that have to be matched to surface geometries as been described in the EN 583-2:1-4. Figure 13 (picture taken from EN 583-2:1-4). For any object with diameter smaller than the ten-fold length and/or width of the wedge the wedge have to be matched to the surface. With this matching a use for DGS-evaluation of these probes is not allowed currently. () 11

13 4. Conclusion The authors presented a new, theoretical model, that transfers the DGS-sizing accuracy from straight beam probes to angle beam probes. Unlike single element angle beam probes with rectangular or circular transducers, the model accounts for the fact, that the DGS-method requires a rotationally symmetric sound field. TrueDGS-single element angle beam probes have been built in accordance to the model and measurements have demonstrated a close fit to standard DGS-curves. True DGS-probes overcome the drawback of missing symmetry in the sound field of standard angle beam probes with rectangular or circular transducers can deviate as to current knowledge up to 6 db from the general DGS-diagram, giving rise to oversizing of flaws. Furthermore, the applicability of this model to phased array angle beam probes was demonstrated. These probes allow DGS-evaluation for all angles, that can be realized acoustically by the given element size, count and frequency. An adaption to different materials and/or surface geometries should be possible as long as sound velocities are known or surface geometries can be described mathematically. 5. Acknowledgements The authors want to thank Willi Warkowksi and Marek Parusel for their help to measure the vast number of DGS-curves. References 1. J Krautkraemer, 'Fehlergroessenermittlung mit Ultraschall', Archiv fuer Eisenhuettenwesen, Vol, No 11, pp , November R Frielinghaus, U Schlengermann, 'Ueber die Bestimmung der Nahfeldlaenge von Winkelpruefkoepfen fuer die Ultraschall-Werkstoffpruefung', Materialpruefung, Vol 9, No 12, pp , Dezember H Wuestenberg, 'Untersuchungen zum Schallfeld von Winkelpruefkoepfen fuer die Materialpruefung mit Ultraschall', BAM Berichte, Vol. 27, August H Wuestenberg, E Schulz, W Moehrle, J Kutzner, 'Zur Auswahl der Membranformen bei Winkelpruefkoepfen fuer die Ultraschallpruefung', Materialpruefung, Vol. 18, No 7, pp 223-2, July CIVA software is being sold by Extende (Orsay, France) 6. For more details visit GE Sensing & Inspection Technologies homepage: 7. See discussion on 'Near field in Angle beam testing' on 8. W Kleinert, Y Oberdoerfer, G Splitt, 'Influence of asymmetric sound fields on DGS-evaluation', th ECNDT Conference, Moscow, June 9. J Krautrkaemer, H Krautkraemer, 'Ultrasonic Testing of Materials', Springer, Berlin, 199. EN 583-2:1, 'Non-destructive testing - Ultrasonic examination - Part 2: Sensitivity and range setting', April 1 12