Simulation in NDT. Online Workshop in in September Software Tools for the Design of Phased Array UT Inspection Techniques

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1 Simulation in NDT Online Workshop in in September 2010 Software Tools for the Design of Phased Array UT Inspection Techniques Daniel RICHARD, David REILLY, Johan BERLANGER and Guy MAES Zetec, 875, boul. Charest Ouest, Québec (QC), Canada G1N 2C9. Phone: +1 (418) , FAX: +1 (418) ; Abstract Software modeling tools are required for the design and validation of both conventional UT and phased array UT inspections. However, they are perhaps more essential for inspections involving phased arrays as these are generally applied on complex geometries and can involve multiple beams generated from a single probe: for example, the increasing application of 2D arrays that can generate beams with a range of refraction and skew angles. Two modeling tools that are commonly used are ray tracing and beam simulation. 3D ray tracing can be utilized, for a given component and defect combination, to determine where a probe should be placed and what refraction and skew angles are required to insonify the defect. Beam simulation can be utilized to design probes to meet a specification. Alternatively, it could be used to assess whether or not an available probe will be suitable for a particular application. The paper will give a description of features of the 3D Ray Tracing and Beam Simulation tools available in Zetec s UltraVision 3 software. Keywords: Phased Array; Ultrasonics; Modelling and Simulation 1. Introduction Software modeling tools are required for the design and validation of both conventional UT and phased array UT inspections. However, they are perhaps more essential for inspections involving phased arrays as these are generally applied on complex geometries [1] and can involve multiple beams generated from a single probe: for example, the increasing application of 2D arrays that can generate beams with a range of refraction and skew angles [2]. Two modeling tools that are commonly used are ray tracing and beam simulation. 3D ray tracing can be utilized, for a given component and defect combination, to determine where a probe should be placed and what refraction and skew angles are required to insonify the defect. Information can be extracted regarding the incidence angle on the defect and the angle with respect to the probe beam axis of the ray incident on the defect: enabling an initial assessment of detectability. Ray tracing also provides information regarding travel times for echoes scattered from the defect and for geometric echoes, in the component. 3D rays can be used to provide a graphical representation of focal laws and their formation. Multiple rays can provide an indication of coverage achieved either for a single crystal or a phased array probe. Beam simulation can be utilized to design probes to meet a specification. Alternatively, it could be used to assess whether or not an available probe will be suitable for a particular application. Beam simulation can also provide information regarding: can the specified focal law be generated; the

2 presence of sidelobes and grating lobes (when plotted superimposed on the component, the possibility of a geometric echo that may interfere with an echo from a defect can be assessed); the relative strength of both transverse and compressional components when using a wedge; the effect on the beam structure of dead elements and non-uniform array excitation or performance. Beam simulation is also an integral part of defect scattering models [3]. The initial approach adopted by Zetec in the creation of these tools was to develop them as customized add-ons to AutoCAD. The beam simulation was then implemented in Zetec s Advanced Phased Array Calculator. Now both the 3D ray tracing and beam simulation tools are available in Zetec s UltraVision 3 software, which provides a platform for inspection design, control of Zetec s DYNARAY phased array controller, data capture and data analysis. An essential element in the use of any ray tracing or simulation tool is an accurate representation of the component. Within UltraVision 3 this is provided by Spatial s 3D modeling engine (ACIS software development kit), which enables the creation of specimens from a range of templates within UltraVision 3 as well as importing files from common CAD packages, for instance AutoCAD. 2. Specimens and postulated defects Specimens can be created within UltraVision 3 by modifying the default parameters of a number of templates, e.g. plate-butt weld, pipe-butt weld, T-joint, or by importing an ACIS format file (*.sat) created in a CAD package. Once created the material parameters can be set by selection from a database (that is modifiable by the user). Figure 1 illustrates a tyical layout containing four views (top, side, end and 3D view which the user can rotate to any orientation) and the Specimen Settings dialog which allows the parameters of the specimen, in this case a plate-butt weld to be altered. The Specimen Settings dialog also enables the user to include postulated defects within the specimen using the Defect Specification tab. Defect templates have been implemented for rectangular, circular and elliptical defects. Once created the defect dimensions, location and orientation can be altered. Surface breaking defects can be created. Any number of defects can be created. In addition, custom defects can be imported (*.sat file). In Figure 1, two defects have been included in the plate-butt weld specimen: one on the weld face and the other at the weld root.

3 Figure 1 Plate-butt weld specimen example containing two postulated defects 3. 3D ray tracing In the 3D ray tracing tool both contact and immersion rays can be generated. The ray path can include reflection/transmission at an interface, including mode conversion, as illustrated in Figure 2 for an initial compression ray segment (shown in blue), reflected at the back-wall (blue) and mode converted at the back wall (red). Also shown in Figure 2: in the view are blue spheres which are grip points that enable the user to change the ray location, refracted angle and skew angle; the 3D Ray Tracing dialog which contains: the 3D ray tracing command buttons; a display of all the rays in the current document; a properties grid displaying the parameters for the current selection. As well as enabling the user to alter ray parameters the properties grid reports ray information, e.g. ray segment length, travel time for the segment, incidence angle, etc. Within the 3D ray tracing tool commands are available to: 1. Create multiple rays based on selected rays to provide a representation of beam spread and probe skewing; 2. Optimise the refracted and skew angles to mimimise the distance between the ray start point and end point of any segment of the ray; 3. Create (polar or linear) arrays of rays to represent scanning of a probe to generate coverage maps; 4. Create rays from the centre of the selected defect, thus allowing the optimal probe location, refraction and skew angles to be determined; 5. To attach a ray to a scanner representation (another feature available in UltraVision 3, not covered in this paper) and create coverage maps;

4 Figure 2 Example of 3D ray tracing For the example in the plate-butt weld specimen shown in Figure 1, a possible detection technique for the defect on the weld face would involve a reflection off the back wall, off the defect, off the back wall and returning to the start point. The path is illustrated in using a refracted angle of 59º in Figure 3(a). When the refracted angle is changed to 60º the path to the defect and the return path are the same, as shown in Figure 3(b). By copying the ray twice and moving the new rays, the region on the surface within which the defect will be detected can be determined, as shown in Figure 3(c). 4. Advanced phased array calculator Prior to calculating a beam simulation, a phased array assembly (probe and wedge) has to be created. This can be achieved using features within Zetec s Advanced Phased Array Calculator, included in UltraVision 3. 1-D Linear and 2-D Matrix arrays, individually or in a Transmit- Receive configuration, can be created either in direct contact with the specimen or placed onto an appropriate wedge (solid or liquid). The wedge/component interface can be planar, cylindrical or fit-to-surface (e.g. for Transmit-Receive arrays on a conical surface). There is also an option for fitting a flexible probe (1-D or 2-D) to a surface. Following the creation of the phased array assembly, focal laws can be defined and the excitation delays calculated. Subsequently, the focal laws and their formation are displayed using ray tracing, as shown in Figure 4.

5 (a) (b) (c) Figure 3 Example of 3D ray tracing applied to Plate-butt weld specimen containing defects.

6 Figure 4 Example of phased array assembly creation and focal law definition/calculation. 5. Beam simulation The beam simulation that has been implemented within UltraVision 3 (and the 3D Package for AutoCAD) is a time domain solution that sums the pulsed signals from sample points on the phased array elements, taking into account the travel time from the sample point to the observation point (including refraction at wedge/component interface), the effective range of the travel path (to account for beam spreading/converging introduced by refraction at the interface) and the transmission coefficient at the wedge/component interface. This approach allows the beam to be simulated for flat, cylindrical and complex wedge/specimen interfaces. 5.1 Initial checking of the beam simulation approach Extensive validation of the beam simulation has still to be carried out. Initial checking of the beam simulation with other simulation packages has demonstrated good agreement. In addition, an initial comparison of experimental and simulated results has also demonstrated good agreement. The experimental configuration consisted of an OmniScan PA unit with the phased array assembly placed on a test block with 1.0mm side drilled holes (SDH). The maximum amplitude pulse-echo signal from the SDH was plotted and compared with the simulated response for a 16 element 2.25MHz array in conjunction with two wedges: one for nominal 45º longitudinal wave and the other for nominal 60º shear wave. Two situations were considered: the first, generating a single focal law and moving the probe relative to the SDH; the second, a stationary probe generating a range of focal laws. The comparison between experimental and simulated data for the compression wave wedge is shown in Figure 5. As can be seen there is relatively good agreement between the data sets. The larger response is for all 16 elements used in the focal law and the other response for a subset of the 16 elements used in the focal law. In the latter case, both the experimental and simulated data

7 exhibited an expected reduction in amplitude and increase in beam width. The comparison for the shear wave probe is shown in Figure 6. Again, good agreement between the data sets is evident. Normalised Amplitude (a) 1.00E E E E E E E E E E E Distance (mm) Figure 5 Comparison of experimental and simulated data for compressional wave wedge: (a) Using all 16 elements (larger response) and a subset to generate a single focal law and moving the probe relative to sdh; (b) Using all 16 elements to generate multiple focal laws from a stationary probe relative to sdh. Normalised Ampltiude 1.00E E E E E E E E E E E+00 (b) Focal Law Angle (Degrees) E E E E E-01 Normalised Amplitude 8.00E E E E-01 Normalised Amplitude 7.00E E E E E E E E+00 (a) Distance (mm) E+00 (b) Focal Law Angle Figure 6 Comparison of experimental and simulated data for shear wave wedge: (a) Using all 16 elements to generate a single focal law and moving the probe relative to sdh; (b) Using all 16 elements to generate multiple focal laws from a stationary probe relative to sdh. 6. Applications The 3D Ray Tracing and Beam Simulation modules developed for AutoCAD have been used in a number of application developments within Zetec, e.g. pipe inspection, turbine blade root inspection, pinned root inspection and rotor teeth probe inspection. For example usage of the new tools in UltraVision 3, some of the steps in the technique development are repeated in the following sections. 6.1 Pipe inspection[2]

8 Zetec and EPRI collaborated to qualify an encoded manually driven phased array procedure for homogeneous pipe welds, using a portable phased array system, for the detection of IGSCC. The inspection is conducted from the OD, with two-line scans using both transmit/receive and pulseecho techniques, as illustrated in Figure 7(a). The phased array probe assemblies consist of either one or two 15 element 2D arrays, at nominal frequencies of 1.5MHz, placed on exchangeable rexolite wedges. For the detection of axial flaw, single array assemblies were used, as shown in Figure 7(b). The search unit allows for pulse-echo operation with shear waves. The wedge assemblies were optimized to generate beams from 35º to 65º in 2.5º increments, at five skew angles between 22.5º and 52.5º; a total of 65 beams fired at each location. For the detection of circumferential flaws a dual array assembly was used, as shown in Figure 7(c). This search unit allows for transmit/receive (T/R) side-by-side operation with either shear or longitudinal waves. The wedge assemblies were designed to generate beams with refracted angles between 40ºand 70º in 1º increments, at three discrete skew angles, -15º, 0º and +15º; a total of 93 beams at each location. (a) (b) (c) Figure 7 Encoded manually driven phased array inspection. (a) Scan paths of the PA probes, (b) Single PA search unit for the detection of axial flaws, (c) Dual PA search unit for the detection of circumferential flaws. Example simulations for both search units are shown in Figure 8. The conclusion of the theoretical study was that the search units could generate the required beams.

9 Figure 8 Example beam simulations for the single and dual array search units. 6.2 Turbine blade - pinned root inspection The inspection of turbine blade pinned roots is to detect cracks originating in the pin hole at locations ±30º from the hole centre line, a postulated defect has been inserted in the pinned root example shown in Figure 9. Investigation with conventional UT probes has demonstrated that defects can be detected using a skip off the internal surface. However in order to detect defects away from the hole centre line, skewing of the beam is required which is not easily achieved with a single probe. The use of a 1D linear array placed on a wedge as shown in Figure 10 was investigated. This configuration of array and wedge can generate beams with approximately the same refraction angle, but with variable skew angle as illustrated in Figure 10(a). Using 3D ray tracing it was determimed, Figure 9, that a pulse-echo path exists to detect the defects from 0º to 30º on the front of the hole using a corner trap mechanism. However, it is only possible to detect defects up to 15º on the back of the hole. In order to detect the defects the array has to be capable of producing beams with ±30º skew. Example beam simulations for skews of 0º and ~30º are shown in Figure 10. As can be seen the array and wedge configuration are capable of producing the required beams. 7. Summary Modelling is an essential tool in the design of phased array probes and the specification of focal laws. 3D ray tracing can be used to determine what focal laws are required and to assess the required coverage is being achieved. Beam simulation can be used to design phased array/wedge assemblies and to assess if the specified focal laws can be generated. Future work for the modelling and simulation tools within UltraVision 3 includes an extensive validation of the beam modelling approach and an extension of the modelling capability to include defect interaction.

10 Figure 9 Defect detection mechanism for pinned root inspection. (a) (b) (c) Figure 10 Pinned root inspection (a) focal laws, example beam simulations for (b) 0º and (c) ~30º skews. 8. References 1 R. Lyon, Confirming plant integrity by specialized NDE inspections, Insight, Vol 47, No 9, pp , September G. Maes and J. Berlanger (Zetec), J. Landrum & M. Dennis (EPRI NDE Center), Appendix VIII Qualification of Manual Phased Array UT for Piping, 4 th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, London (UK), December J.M. Coffey and R. Chapman, Application of elastic scattering theory for smooth flat cracks to the quantitative prediction of ultrasonic defect detection and sizing, Nucl. Energy, 22, No. 5, pp ,October 1983.