Ultrasonic imaging is widely used in defect detection, localization, and characterization, and has proven effective for identifying defects larger than the wavelength of the incident wave. However, the characterization of subwavelength-scale defects remains a major challenge in ultrasonic non-destructive evaluation. Such defects can be characterized by analysing the scattering information encoded in the transmitter–receiver ultrasonic array data. However, local scattering signals from three-dimensional (3D) features of interest are often contaminated by interference from neighbouring scatterers, leading to reduced accuracy. Although 2D arrays are theoretically capable of resolving 3D local scattering fields, their application is hindered by the vast amount of measurement data and the number of elements required to produce an aperture of sufficient size to allow adequate focussing.
In this study, a volumetric ultrasound imaging method using 1D array is proposed. Ultrasound data are acquired by rotating a 1D array and a 3D image is reconstructed from 2D images corresponding to different angular array positions. During the defect characterisation stage, local regions of interest are identified in 3D imaging volume, and 3D scattering information is extracted using the inverse imaging approach. The method is tested on experimental data obtained from an aluminium and steel samples that contain several flat-bottom holes, and the results are compared with numerical simulations. It is shown that the proposed method enables characterization of sub-wavelength scatterers in 3D space while retaining the high image quality advantages to 1D arrays.

In this study, ultrasonic simulations were conducted to predict the anisotropic refraction behavior of elastic waves in an austenitic stainless steel butt weld. Two anisotropic modeling approaches were applied: the Ogilvy model, which calculates simplified anisotropy using a ray-tracing method, and the MINA model, which predicts grain growth direction and distribution based on welding pass parameters. To quantitatively validate the simulated orientation maps, Electron Backscatter Diffraction (EBSD) measurements were performed on the actual weld specimen. The surface was mechanically polished using 100–2000 grit papers and subsequently polished with 1–9 μm suspensions. A total of 32 EBSD scans were merged to reconstruct a single comprehensive orientation map. Ultrasonic testing was conducted using Phased Array Ultrasonic Testing (PAUT) techniques, considering both Sectorial Scanning and Full Matrix Capture–Total Focusing Method (FMC–TFM) imaging. Data were acquired from far-side inspections targeting notches with depths of 10–30% of the wall thickness. To enable quantitative comparison between experimental and simulation results, a Python-based interface tool was developed. The validated anisotropic simulation model can serve as a foundation for developing reliable NDT systems for far-side inspections of austenitic welds. Furthermore, FMC–Phase Coherence Imaging (FMC–PCI) simulations demonstrated superior resolution in separating tip and corner echoes, confirming the effectiveness of the proposed approach.

This research explores the strengths and limitations of using the Born approximation in ultrasonic phased array setups for characterizing microstructures. Traditionally, the Born approximation has been applied under strict conditions—mainly assuming weak scattering scenarios and focusing on average material properties such as grain size. However, previous studies have rarely examined the validity of imaging results outside these strict conditions or focused on local microstructural details.
In this study, we investigate the performance of the Born approximation specifically for imaging purposes using ultrasonic phased arrays. We use an analytical model based on the Born approximation, which considers only single scattering, to calculate scattering signals and produce simulated images. To evaluate the accuracy and applicability of this approach, we compare these results directly with Finite Element (FE) simulations, which naturally account for both single and multiple scattering phenomena.
By generating realistic polycrystalline microstructures, our analysis specifically considers how grain size and material anisotropy affect the single scattering rate, which directly impacts the agreement between FE and analytical results. Through this analysis, we have established clear parametric conditions defining when the Born approximation is valid for imaging. Additionally, we explore more complex scenarios, including microstructures with multiple scales and large macrozones embedded in finer grains, to test the robustness of the analytical model.
After validating the analytical approach in 2D scenarios, we demonstrate that this method can naturally extend to 3D problems. The analytical model has shown significant speed advantages, making it highly efficient for simulating extensive microstructural configurations in three dimensions. This study provides foundational evidence that validates the analytical model’s effectiveness in both 2D and potential 3D applications, thereby streamlining and accelerating material characterization methods.