ABSTRACT
The sound fields generated by ultrasonic transducers can be modeled using the Quasi-Monte Carlo (QMC) method with a high level of accuracy and efficiency from Zhang [J. Acoust. Soc. Am. 149(1), 7-15 (2021)]. In this work, this method is extended to simulate transmitted wave fields through complicated interfaces. When a wave propagates in two-layer media, the vibrating waves over the interface radiated by the transducer can be treated as the source for generating waves in the second medium, thus, a nested-form Rayleigh integral expression can be used as a model equation for the transmitted wave calculation. When the QMC method is used to solve the nested integral, pseudo-random samples for constructing the transducer and the interface are sampled separately and the transmitted wave fields are obtained using the final sample mean. Numerical examples and results are presented when the wave transmits normally or obliquely through planar or curved interfaces. The results indicate that the high level of accuracy and efficiency remains when the QMC method is used to model the transmitted wave fields. One important advantage is that wave fields can be well simulated using the QMC method when the wave transmits through a complicated interface as long as the interface can be constructed using pseudo-random samples.
ABSTRACT
Previously, a transverse-to-transverse single scattering model (T-T SSR) was developed for a pulse echo configuration, which may have limitations for strongly scattering materials. In this work, a transverse-to-transverse double scattering model (T-T DSR) is presented to model the transverse ultrasonic backscatter more accurately. First, the Wigner distribution of the transducer beam pattern is extended to a transverse wave. Next, the multiple scattering framework is followed to derive the transverse and longitudinal components of the second-order scattering. Then, a quasi-Monte Carlo (QMC) method is used with Graphics Processing Unit (GPU) acceleration to calculate numerical results of the final expression which contains a five-dimensional integral. The correlation length, the focal length of the transducer, and incident angle are used to investigate differences between the T-T DSR model and the T-T SSR model. Finally, a backscatter experiment is performed on two stainless steel specimens with different grain sizes to determine the respective correlation lengths. The results show that the T-T DSR model has better performance over the T-T SSR model for evaluating the grain size of these relatively strongly-scattering specimens.
ABSTRACT
Diffuse ultrasonic backscatter measurements have been shown to enhance the detection capability of sub-wavelength flaws when combined with extreme value statistics. However, for a normal-incidence immersion measurement, a "dead zone" created by the ring-down of the front-wall echo will hide near-surface flaws. In this article, a pulse-echo transverse wave backscatter measurement is used to detect near-surface flaws under high gain. The approach is validated using a magnesium specimen with side-drilled holes. The confidence bounds of the grain noise from this specimen are given by a transverse-to-transverse scattering model, which takes the grain size distribution and the hexagonal crystal symmetry into account. The upper bound is then treated as a time-dependent threshold for the C-scan. Experiments show that the developed method has good performance for detecting sub-wavelength, near-surface flaws, and can suppress both missed detections and false positives.
