ABSTRACT EXPERIMENTAL VALIDATION OF CIVA ULTRASONIC SIMULATIONS S. Lonné (*), L. de Roumilly (*), L. Le Ber (*), S. Mahaut (*) and G. Cattiaux (**). *Commissariat à l Énergie Atomique, CEA Saclay, SYSSC, Bât. 611, 91191 Gif-sur-Yvette Cedex, France sebastien.lonne@cea.fr ** Institut de Radioprotection et de Sûreté Nucléaire, B.P.17 92262 Fontenay-aux-Roses Cedex, France Ultrasonic simulation tools have been gathered in the Civa software developed at CEA in order to conceive, optimize and predict the performances of various inspection techniques. Those models are based on semi analytical kernels and numerical integration, so that realistic 3D and time transient configurations may be dealt with. Specific emphasis is made to optimize time computation, so that these models can be used for parametric studies in spite of potentially complex configurations. Civa simulation tools include beam propagation and defect scattering models, thus the whole inspection performances may be predicted for a wide range of applications. In addition to existing skills in terms of probes and specimens, recent developments have been achieved so that realistic flaws can be taken into account. Application cases studied in this paper consider realistic ultrasonic inspections in the framework of collaboration with IRSN. These cases include acquisitions made on several mock-ups containing calibration reflectors (Side Drilled Holes, Flat Bottomed Holes) or artificial complex shaped defects (planar defects with CAD defined contours or multi facetted flaws). Experimental results are then discussed and compared to the predictions obtained with Civa in such configurations. The comparison between Civa simulations and experimental results for both calibration flaws and complex shaped defects shows a good agreement in the range of approximations of the simulation models. INTRODUCTION Ultrasonic modeling tools have been gathered in the CIVA software developed at CEA for multiple techniques of Non Destructive Testing (simulation of inspections, data acquisition and processing) [1-2]. Those models allow to predict both the beam transmitted by an arbitrary transducer into the specimen under inspection, as well as the interaction of the beam with defects, thus one can simulate the whole ultrasonic examination process of a specimen (prediction of A, B or C-scans) for an a priori knowledge of the piece (which may either be parametric or CAD defined), of the defects within (calibration reflectors or crack-like planar defects, backscattered noise, solid inclusions) and of transducer radiation, sensitivity and scanning. Echoes arising both from defects or specimen boundaries are computed. In this paper, theses models are shortly described, with a specific emphasis over the defect scattering model, and examples are presented in relation to two dedicated applications carried out in collaboration with IRSN. The first one is related to the inspection of thick steel components (representative of the vessel of a nuclear power plant) with immersed focused probes, showing the influence of tilt and location of defects close to the backwall. The second one concerns complex shaped defects contained in a planar specimen and aims at evaluating simulation performances over the detection of such defects. UT MODELING TOOLS IN THE CIVA SOFTWARE
UT modeling tools developed and integrated into the CIVA software allows to simulate the inspection carried out with an arbitrary probe over an arbitrary specimen, containing arbitrary flaws. Transducer Arbitrary Arbitrary transducer: transducer: discretisized discretized as as Field radiated at M? Pencil source source points points M M FIGURE 1: Field prediction sketch. In order to predict the interaction between the beam and the flaw, the first step is to predict the radiated beam which will arise at the defect. This simulation takes account of arbitrary transducers (standard or phased arrays, planar or focused, single or twin-crystals) in immersion or in contact, with an arbitrary waveform. It also takes into account the mode conversion at the interface and with or without after a rebound on the backwall of the tested specimen. The specimen can be either a homogeneous or a multi-layered medium, isotropic or anisotropic and of arbitrary geometry. In order to calculate the wave field radiated at point M (see Fig. 1), the transducer is discretized as a series of source points over the transducer s surface. For each point source, elementary contributions are obtained through a pencil method [3-4] (high frequency approximation). These contributions are calculated following these steps: firstly, a pencilmatrix formulation in order to predict wave-front radii of curvature along the wave path as well as the amplitude and the time of flight, and secondly, a plane wave transmission/reflection coefficient at all interfaces is associated to each pencil. The impulse responses are then synthesized from the contributions and convoluted with the input signal: s(m,t) = v(t) h(m, t) The overall principle for the field-to-flaw interaction [5] will therefore be constituted of three computation stages: incoming transient ultrasonic field arising on the defect, field-toflaw interaction according to the Kirchhoff approximation, and -prediction of the sensitivity at reception using the Auld s reciprocity theorem. The field radiated by the probe will be simplified as the form of a temporal distribution (the waveform delayed by the time of flight between the probe and the computation point over the defect) multiplied by the amplitude value transmitted at this point: s(m,t) AE(M) v(t tm ). The interaction of this incident beam and the defect will be computed using the Kirchhoff approximations, which assumes that the wave does not propagate into the flaw, which is valid for spherical voids, cracks, but not for solid inclusions, for which other scattering models are therefore used in CIVA (modified Born approximation [6]). The prediction of the sensitivity at reception, which finally allows predicting the signal observed by the reception probe, is obtained thanks to an argument based on Auld's reciprocity theorem [7]. This feature allows to simulate the whole inspection using only one computation of the radiated beam (or two beam computations if two different probes should be used at transmission and reception, like Tandem configurations [8]), and the interaction of this beam with a surface meshing of the flaw. The summation of all elementary surface contributions scattered echoes of the flaw will therefore give rise to the simulated signal at Transmission/Reception.
Different types of defects can be dealt with in the Civa software. In the last Civa 7.1 release, we were able to take into account canonical defects such as side-drilled holes, flat/hemispherical-bottomed holes, spherical and rectangular defects. In the Civa 8 version, it is now possible to define and compute ultrasonic responses from more realistic defects: - CAD contour planar defects are contained in a rectangular area, in which a complex profile is defined by defining its contour (see Fig. 2). We have simulated an inspection with longitudinal waves in normal incidence over a planar specimen containing a star-shaped contour planar defect. Assuming a 2D displacement of the probe (scanning, increment), we obtain a Cscan image shown on Fig. 2, on which we can observe the contour of the inspected defect. - Multifaceted defects are defined with two lines, one defining the profile of the flaw and the other one defining its extension in the perpendicular direction. This extension is not necessarily straight (see Fig. 3). We have simulated an inspection over such defects, and results are presented below (see Fig. 4). FIGURE 2: Inspection of a CAD contour planar defect. FIGURE 3: Definition of multifaceted defects FIGURE 4: Inspection of different multifaceted defects.
APPLICATION AND VALIDATION OF THE SIMULATION TO PERFORMANCES PREDICTION Two applications cases have been studied, with support from IRSN in the framework of R&D projects. The first one is related to the inspection of thick steel components (representative of the vessel of a nuclear power plant) with immersed focused probes, showing the influence of tilt and location of defects close to the backwall. The second one concerns complex shaped defects, either contained in a planar surface with an irregular profile or multifaceted. All simulations were performed using the Civa 8 version and comparisons with experimental results have been done for both applications. Inspection of planar and cylindrical specimen with surface breaking vertical and tilted defects with immersed focused probes This application is dedicated to the assessment of inspection performances carried out with immersed focused probes over planar or cylindrical thick specimen, representative of the reactor vessel wall of nuclear power plants. Experiments have been carried out over planar mock-ups containing artificial defects (electro-eroded notches), embedded or emerging at backwall. Varying defects depths and probes (each probe being optimized for an inspection range) were modeled and investigated for these applications. The results discussed hereafter concerns planar defects emerging at backwall, vertical or tilted by 10. The images displayed on Fig. 5 hereafter show experimental and simulated results for three vertical and 0 tilted planar defects at backwall (for a planar ferritic steel specimen of 250mm thickness), of 35mm extension, and respective heights of 6, 12 and 25mm (from left to right). Experiments and modeling results are presented on rectified Bscan images related to the specimen coordinates. The transducer used for this inspection is a large aperture probe, 1 MHz central frequency, which radiates 45 shear waves focused about 160mm depth. FIGURE 5: Simulated and experimental results for the inspection of a planar mock-up with different height vertical and tilted notches.
For those different defects, one can observe the main corner echo arising from a combined reflection at the backwall and at the defect, as well as very low diffraction echoes at the defects tip. Values reported on the Fig. 5 correspond to the amplitude loss of corner echo indications with respect to a calibration reflector (a 2mm side drilled-hole, 100mm extension, located at 170mm depth). For vertical defects, the corner echo amplitude increases as function of the defect height, because the beam is larger than the defects height (the beam spot is about 15 mm at 250 mm depth). This is observed both for experimental and simulation results. Also, as expected, the corner echo amplitude decreases for tilted defects, as the geometrical path of the corner echo is not optimal for a 45 shear waves examination as soon as the defects are not vertical anymore. It is also observed that the corner echo amplitude over tilted defects is not increased as a function of defect height: the 25mm height defect has lower amplitude (about 6 db fall) than the 6mm defect. The same behavior is observed on the simulated result. On the whole set of defects (vertical and tilted), the agreement between experiments and simulations, in terms of prediction of defect echoes level compared to the calibration reflector, is about 1dB, which demonstrates that the inspection simulation tool allow to predict actual performances of inspection for different defects height as well as tilt orientations. Similar acquisitions and simulations have been carried out over a cylindrical configuration. The selected specimen was an available pipe of 445mm outer diameter and 75mm thickness, for which artificial defects (2mm side drilled holes and three emerging vertical notches) have been made. The objective of this study was therefore to validate the ability of the models over a cylindrical configuration. Fig. 6 shows experimental and simulated reconstructed Bscan views of the three notches at backwall, of respective heights (from left to right) 5, 10 and 15mm, and extension 20mm. The reported amplitudes correspond to the amplitude falls compared to a reference side drilled hole of 2mm diameter, located at 60mm depth. FIGURE 6: Simulated and experimental results for the inspection of a cylindrical mock-up with different height vertical and tilted notches
The inspection was carried out in immersion, with a large aperture focused probe, radiating 40 longitudinal waves, focused at 60mm depth. The agreement between simulation and experiments for these three defects is below 1dB. It can also be observed on the reported superimposed echodynamic curves of the experimental and simulated results that the overall shapes of the simulated echoes accurately fits the experimental echoes. Inspection of a planar specimen containing complex flaws This second application aims at evaluating performances of simulation tools developed in Civa for ultrasonic inspections over more realistic flaws, such as CAD contour planar defects and multifaceted defects. In the framework of collaboration with IRSN, we have several ultrasonic measurements obtained on planar mock-ups containing complex electro-eroded flaws, and thanks to the new developments achieved in Civa 8 release, we succeeded in simulating ultrasonic acquisition over these types of defects. Comparison is going to be presented in this section, after a brief description of the studied configurations. The first configuration that is considered in this part corresponds to an inspection with an immersed focused probe radiating shear waves at 45. The specimen is made of ferritic steel and a defect with an elliptical contour has been machined (see Fig. 7 for the description of this defect). For this configuration, the probe is moving along two perpendicular directions so that a Cscan image can be obtained. Then, from this Cscan image, we can display a Dscan image which is an increment/time representation of the signals: at each increment position, the maximum amplitude among all the scanning positions is extracted as a function of time. Considering the amplitude loss at -6dB compared to the maximum amplitude of the diffraction echo arising from the top of the defect, we can evaluate the length of this diffraction echo along the increment direction for both simulation and experiment. The agreement is very good since we measure an echo of 13mm length for experiment, whereas simulation predicts a 12mm length echo (see Fig. 7). FIGURE 7: Ultrasonic inspection of an elliptical shaped defect.
The second configuration we present in this part concerns another ferritic steel mock-up containing two different geometries of faceted flaws, as shown on Fig 8. The probe considered here is a non-focused contact probe radiating 45 degree shear waves. In this application, we try to simulate the different echoes that occur during the inspection of such complex flaws, taking into account direct, corner and indirect echoes. The comparison between experimental and simulated results is presented on Fig. 8 for the two defects, and the amplitudes reported on those results are referenced to a 2 mm side-drilled hole. Concerning defect number 1, a very good agreement is seen not only for the corner echo but for the diffraction arising from different edges of the faceted flaw. On the other hand for defect number 2, we also find a fair agreement for the corner echo created between the backwall of the specimen and the bottom of the defect, but we can observe on the simulated image that the corner echo due to the reflection of the waves between two facets of the flaw is not predicted. This is a limit of our model, which doesn t take into account this type of complex phenomenon. FIGURE 8: Experimental and simulated results of the ultrasonic inspection over two faceted flaws contained in a planar ferritic steel specimen. Conclusion Ultrasonic simulation tools gathered in the Civa software have been briefly presented. Those models, based on semi-analytical formulations kernels and numerical integration to deal with generic applications, include beam modeling and field-to-flaw interactions. A whole inspection (prediction of scattered echoes received by the probe, for each scanning position)
can therefore be simulated using the incident field and the defect scattered echoes, using an argument based on the Auld's reciprocity theorem. Two applications have been presented, the first one for immersed probes over a planar mock-up with surface breaking notches at backwall, vertical or tilted, as well as a cylindrical mock-up. The second one concerns an ultrasonic inspection over a planar specimen containing complex shaped defects with an immersed focused probe and a contact non-focused probe. For both cases, a fair agreement is observed between simulations and experiments, especially in terms of amplitude echoes level with comparison to the reference calibration reflector. These simulation tools therefore demonstrate their ability for quantitative prediction of inspection performances. REFERENCES 1) Calmon P., Lhémery A., Lecoeur-Taibi I. and Raillon R., Integrated models of ultrasonic examination for NDT expertise, in Review of progress in QNDE, edited by D. O. Thompson and D. E. Chimenti, Vol. 16B, Plenum Press, New-York, 1997, pp. 1861-1868. 2) Mahaut S., Chatillon S., Kerbrat E., Porré J., Calmon P. and Roy O., " New features for phased array techniques inspections : simulation and experiments", 16th World Conf on Non Destructive Testing, Montréal, 2004. 3) Deschamps G.A., Proceedings of the I.E.E.E., 60, 1022 (1972). 4) Gengembre N., "Pencil method for ultrasonic beam computation", Proc. of the 5th World Congress on Ultrasonics, Paris, 2003. 5) Raillon R. and Lecœur-Taïbi I., Transient elastodynamic model for beam defect interaction. Application to nondestructive testing, Ultrasonics, 2000 38 527-530. 6) Darmon M., Calmon P. and Bele C., Modelling of the ultrasonic response of inclusions in steels, in Review of progress in QNDE, Vol. 22A, AIP publishing, 2003, pp. 101-108. 7) Auld B.A., Wave motion, 1979 1 3-10. 8) Raillon R., Lozev M., Spencer R., Kerbrat E. and Mahaut S., "Application of Tandem techniques with contact monoelement or phased array probes : simulation and experiments., to be published in Review of progress in QNDE, op.cit., Vol. 24, 2005.