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The complex structure of inhomogeneous welds poses a long-standing challenge in ultrasonic non-destructive testing. Elongated grains with spatially varying dominant orientations can distort and split the ultrasonic beam, hindering inspection data interpretation. One way to tackle this problem is to include material information in imaging and signal analysis; however, such information is often only gathered using destructive methods. This paper reports the development of a physics-based weld inversion strategy determining grain orientations using a ray tomography principle. The considered approach does not rely on a macroscopic weld description but may incorporate it to facilitate inversion. Hence, it is more general than other available approaches. The methodology is demonstrated in both numerical and experimental examples. The experimental work focuses on mock-up samples from the nuclear industry and a sample manufactured during this research. The 'ground truth' for the latter comes from an EBSD evaluation—the most accurate (yet destructive) examination technique available. Across the considered specimens, our methodology yielded orientation maps with average errors well below 20∘ , leading to time-of-flight errors below 0.05 μs. Applying the result from inversion to ultrasonic imaging offered between 5 and 14 dB signal-to-noise ratio improvement for defect signatures. [ABSTRACT FROM AUTHOR]

This paper uses three-dimensional grain-scale finite-element (FE) simulations to appraise the classical scattering theory of plane longitudinal wave propagation in untextured polycrystals with statistically equiaxed grains belonging to the seven crystal symmetries. As revealed from the results of 10 390 materials, the classical theory has a linear relationship with the elastic scattering factor at the quasi-static velocity limit, whereas the reference FE and self-consistent (SC) results generally exhibit a quadratic relationship. As supported by the results of 90 materials, such order difference also extends to the attenuation and phase velocity, leading to larger differences between the classical theory and the FE results for more strongly scattering materials. Alternatively, two approximate models are proposed to achieve more accurate calculations by including an additional quadratic term. One model uses quadratic coefficients from quasi-static SC velocity fits and is thus symmetry-specific, while the other uses theoretically determined coefficients and is valid for any individual material. These simple models generally deliver more accurate attenuation and phase velocity (particularly the second model) than the classical theory, especially for strongly scattering materials. However, the models are invalid for the attenuation of materials with negative quadratic coefficients. This article is part of the theme issue 'Wave generation and transmission in multi-scale complex media and structured metamaterials (part 1)'. [ABSTRACT FROM AUTHOR]