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Microstructural variability from manufacturing and post-fabrication activities have been attributed to a significant portion of failure in grade 91 steels. For example, heat treatment from welding can change the microstructure locally, which can change the hardness, phase content and grain size. This article explores the use of ultrasonic coda waves to detect microstructural changes in martensitic grade 91 steel. Tube samples with eight different microstructural conditions were simulated using different heat treatments and were tested using coda waves. Six different differential features were extracted from the coda wave signals. Only three features: Loss of correlation, differential curve length and coda wave interferometry showed accuracy > 50%. Further, clustered analysis to test the ability of the features to classify good, mid-level and bad microstructures showed that only loss of correlation had good accuracy. Overall, the study suggests that coda waves can be used to detect subtle changes in microstructure. [ABSTRACT FROM AUTHOR]

This article aims to study the interaction of Rayleigh or surface wave with a varying angled wedge using numerical simulations. This work uses numerical tools to understand this complex problem and fills some existing gaps such as the influence of frequency and geometry of the wedge (curved vs. sharp transition). Quantitative analysis was carried out by calculating the transmission and reflection coefficients, and qualitative analysis used displacement vector plots to study the scattering and mode conversion phenomenon. The results suggest a strong dependence of transmission and reflection coefficients on the frequency and geometry of transition, which has not been reported before in the literature.

This article reports on the characterization of the "as-fabricated" state of Inconel 718 samples fabricated using laser directed energy deposition (DED). Laser-DED is known to produce complex metastable microstructures that can significantly influence the baseline ultrasonic response compared to conventional processing methods. The present work uses three parameters to characterize the samples: (a) ultrasonic velocity, (b) an attenuation coefficient, and (c) a backscatter coefficient. The baseline ultrasonic response from the DED sample was compared against the ultrasonic properties of conventional IN718 samples reported in the literature. The results suggest that strong grain boundary scattering from large macrograins can lead to attenuation and backscatter values that are significantly higher than conventional samples. Additionally, the results including velocities, attenuation and backscatter coefficients were found to be dependent on the fabrication direction, with the build direction being different from the transverse directions. Finally, destructive analysis was used to develop conjectures to explain the experimentally observed ultrasonic response. [ABSTRACT FROM AUTHOR]

This article reports on the destructive and nondestructive evaluation of dry spots or patches in thick unidirectional glass/epoxy laminates. Dry spots in composites can act as weak regions potentially resulting in catastrophic failure if undetected. Composite samples with dry spots were fabricated and nondestructive evaluation was first carried out using non-contact air coupled ultrasonics. Two types of dryness were investigated: surface and mid-plane dryness. Both types were tested using two ultrasonic testing methods. The ultrasonic velocity was used to determine stiffness and dryness fraction maps. The results showed that the dryness could be detected and characterized using the ultrasonic nondestructive evaluation. To validate the results, a destructive analysis was carried out by cutting the sample into small pieces and burning them to obtain the weight of matrix and fibers. This was further used in micromechanics models to obtain void and dryness fractions. Both model and ultrasonic results show that dry spots result in spatial gradient of mechanical properties around the dry spot. Nondestructive characterization of dryness helps in deciding if the part must be scrapped or repaired based on the severity in terms of loss of stiffness. [ABSTRACT FROM AUTHOR]

The present article investigates the possibility of using nonlinear resonance ultrasound spectroscopy to determine the acoustic nonlinearity parameter (β) and third order elastic constant by developing an inverse problem. A theoretical framework was developed for nonlinear forced vibration of a cantilever beam using material nonlinearity (stress-strain nonlinearity). The resulting nonlinear equation was solved using method of multiple time scales to obtain the nonlinear frequency shifts. The present works focuses only on classical nonlinearity and, therefore, a diverse group of intact, classic nonlinear materials were chosen. The samples were tested using nonlinear resonance ultrasound spectroscopy, and the developed theory was used to invert the experimental frequency shifts to obtain the nonlinearity parameters. The third order elastic constants and β were calculated using their analytical relationship with the nonlinearity parameter. The experimentally determined C111 and β values for all various materials agree well with literature values. In addition to determining β, determination of the sign, or phase of β was also explored theoretically and experimentally. [ABSTRACT FROM AUTHOR]