In an attempt to improve the ultrasonic testing capability of a conventional C-scan system, a flaw detection method using an ultrahigh gain is developed in this paper. A time-dependent threshold for image segmentation is applied to identify automatically material anomalies present in the sample. A singly scattered response model is used with extreme value statistics to calculate the confidence bounds of grain noise. The result is a time-dependent threshold associated with the grain noise that can be used for segmentation. Ultrasonic imaging experiments show that the presented method has advantages over a traditional fixed threshold approach with respect to false positives and missed flaws. The results also show that a low gain is adverse to the detection of microflaws with subwavelength dimensions. The forward model is expected to serve as an effective tool for the probability of detection of flaws and the inspection of coarse-grained materials in the future.

The spatial correlation properties of ultrasonic backscattering signals in random media have important implications. For example, they can be used for microstructural characterization and flaw detection in engineering materials. However, the traditional spatial correlation coefficient (SCC) is only a leading order quantity that does not capture the true spatial correlations of random media. This is caused by neglecting confounding variables such as non-zero means or other non-zero odd-order moments. Here, the SCC is generalized from zeroth- to general-order through partial cross-correlation analysis. A series of indicators are defined to quantify the SCC curve at zero time lag, and the maximum time shift curve, which are both functions of lateral separation between two sensor positions. A stainless-steel specimen and a focused ultrasonic transducer are used to verify the method. Scattering measurements show that the higher-order SCC can consistently capture spatial correlations whereas the zeroth-order SCC is inadequate. The zeroth-order SCC is shown to predict a step size that can be more than six times too large. Thus, the present method can provide better understanding of statistical correlations and conditions to measure uncorrelated backscattering signals.

Accurate measurements of the ultrasonic shear wave attenuation coefficient are essential for the quantitative nondestructive evaluation of the microstructure of a material. This work provides an experimental technique for measuring this coefficient using a contact method. The first and second pulse-echo shear wave signals are measured and are used to calculate the frequency-dependent attenuation coefficient; in this process, the diffraction and the diffraction-affected partial reflection coefficient are corrected to improve the accuracy of measurement. The diffraction coefficient is calculated from measurements of the shear wave velocity, and the partial reflection coefficient is determined experimentally by using another transducer on the opposite side and taking into account wave diffraction effects. The shear wave attenuation coefficient is determined for samples of different thicknesses, and these results show good agreement over a broadband frequency, thus validating the proposed method. The advantage of discriminating the porosity of additive manufacturing materials using shear wave attenuation is demonstrated, and the practical benefits of accurately measuring the shear wave attenuation coefficient using the proposed method are also discussed.