Based on a theory of elastic wave propagation in arbitrarily oriented transversely isotropic media, which has been presented recently, the radiation characteristics of ultrasonic transducers in these media are determined. Using the directivity patterns for normal and transverse point sources on the free surface of such (semi-infinite) materials—the derivation is based on the reciprocity theorem—the radiated wave fields are obtained by the method of point-source-synthesis, i.e., by superposing the wave fields of numerous point sources located within the transducer aperture. Since ultrasonic inspection of anisotropic materials, especially weld material in nuclear power plants, suffers from the well-known effects of beam splitting, beam distortion, and beam skewing, valuable information in view of an optimized inspection is provided. Focusing on transversely isotropic weld material specimens, numerical evaluation is performed for several grain orientations with respect to the transducer-normal. The approach presented is particularly useful in view of an appropriate extension to inhomogeneous welds and the consideration of time-dependent RF-impulse functions.

Transducer-modeling in general transversely isotropic media via point-source-synthesis: Theory

The use of ultrasonic arrays has increased dramatically within recent years due to their ability to perform multiple types of inspection and to produce images of the structure through post-processing of received signals. Phased arrays offer many advantages over conventional transducers in the inspection of materials that are inhomogeneous with spatially varying anisotropic properties. In this paper, the arrays are focused on austenitic steel welds as a representative inhomogeneous material. The method of ray-tracing through a previously developed model of an inhomogeneous weld is shown, with particular emphasis on the difficulties presented by material inhomogeneity. The delay laws for the structure are computed and are used to perform synthetic focusing at the post-processing stage of signal data acquired by the array. It is demonstrated for a simulated austenitic weld that by taking material inhomogeneity and anisotropy into account, superior reflector location (and hence, superior sizing) results when compared to cases where these are ignored. The image is thus said to have been corrected. Typical images are produced from both analytical data in the frequency domain and data from finite element simulations in the time domain in a variety of wave modes, including cases with mode conversion and reflections.

/pdf/correction-of-ultrasonic-array-images-to-improve-reflector-444hl1l4sq.pdf

Correction of ultrasonic array images to improve reflector sizing and location in inhomogeneous materials using a ray-tracing model

The quality of an ultrasonic array image depends on accurate information about its acoustic properties. Inaccurate acoustic properties can cause image degradation such as blurring, mislocation of reflectors, and the introduction of artifacts. In this paper, for the specific case of an inhomogeneous and anisotropic austenitic steel weld, Monte Carlo Markov Chain (MCMC) inversion is used to estimate unknown acoustic properties from array data. The approach uses active beacons that transmit ultrasound through the anisotropic weld; the ultrasound is then captured by a receiving array. A forward model of the ultrasonic array data is then optimized with respect to the experimental data using an MCMC inversion. The result of this process is the extraction of a material property map that describes the anisotropy distribution within the weld region. These extracted material properties are then used within an imaging algorithm - the total focusing method in this paper - to produce autofocused images. This MCMC inversion approach is first applied to simulated data to test the convergence, robustness, and accuracy of the method and its implementation. The extracted weld map is used to show improved imaging of defects within the weld, relative to an image formed assuming a constant velocity. Finally, the MCMC inversion approach is used on experimental data from a 110-mm-thick steel plate containing an austenitic weld. Here the extracted weld map is used to show that defect location errors of greater than 5 mm are reduced to around 2 mm when the extracted weld map is used.

Monte carlo inversion of ultrasonic array data to map anisotropic weld properties

The degradation of ultrasonic array images due to propagation through an anisotropic material presents a significant inspection problem to the engineering industry. If the distribution of anisotropy is known, ray-tracing algorithms can be used to predict the path of sound through the material and hence correctly image anisotropic components. Conversely, ray-tracing can be used as part of an inversion procedure to infer the anisotropic properties from measured time-of-flight data. However, inversion methods often require thousands of ray-traces to map a single weld and as such, a rapid ray-tracing algorithm is essential for use. This paper explores the use of two path-finding algorithms as applied to a ray-tracing scenario: Dijkstra's algorithm and the A ⁎ algorithm. Although prevalent within computer science applications due to their low computation time, both algorithms have seen little use within the Non-Destructive Evaluation (NDE) field. This paper aims to both describe the algorithms and to demonstrate their relative merits for application in NDE. Dijkstra's algorithm was applied to an anisotropic weld inspection and the optimal parameters explored, drawing comparison to an equivalent inspection using a beam-bending algorithm to ray-trace. A comparison of accuracy and computation time between Dijkstra's algorithm and the A ⁎ algorithm shows them to maintain similar accuracy, but the A ⁎ algorithm to exhibit significant reductions in computation time.

Novel ray-tracing algorithms in NDE: Application of Dijkstra and A* algorithms to the inspection of an anisotropic weld

Ultrasonic inspection of austenitic welds is challenging due to their highly anisotropic and heterogeneous microstructure. The weld anisotropy causes a steering of the ultrasonic beam leading to a number of adverse effects upon ultrasonic array imagery, including defect mislocation and aberration of the defect response. A semi-analytical model to simulate degraded ultrasonic images due to propagation through an anisotropic austenitic weld is developed. Ray-tracing is performed using the A⁎ path-finding algorithm and integrated into a semi-analytical beam-simulation and imaging routine to observe the impact of weld anisotropy on ultrasonic imaging. Representative anisotropy weld-maps are supplied by the MINA model of the welding process. A number of parametric studies are considered, including the magnitude and behaviour of defect mislocation and amplitude as the position of a fusion-face defect and the anisotropy distribution of a weld is varied, respectively. Furthermore, the use of the model to efficiently simulate and evaluate ultrasonic image degradation due to anisotropic austenitic welds during an inspection development process is discussed.

https://research-information.bris.ac.uk/files/67856003/Drinkwater_As_submitted_Manuscript_5_1_.pdf

Ultrasonic Array Imaging Through an Anisotropic Austenitic Steel Weld Using an Efficient Ray-tracing Algorithm

The strength of austenitic welds will be considerably reduced if cracks are present. To guard against this possibility good workmanship is essential; quality is monitored by inspecting the welds after manufacture. Ultrasound is the favored method of inspection but there are difficulties associated with this in the coarse-grained columnar weld structures often associated with austenitic materials. We present two different approaches to modeling such inspections: ray tracing and full numerical solution of the governing wave equation. We show that the two approaches, although very different in physical nature and numerical method, give similar and consistent results.

JA Ogilvy

Papers

Modeling ultrasonic inspection of austenitic welds