Grain size is an important parameter in evaluating the properties of microstructures in metals. In this paper, the attenuation coefficient of Rayleigh waves is introduced to characterize grain size in heat treated 316L stainless steel. Rayleigh wave attenuation is measured using an angle beam wedge transducer as the transmitter and an air-coupled transducer as the receiver. The results show that the grain size in 316L stainless steel increases due to heat treatment time, the hardness decreases accordingly, and the attenuation coefficient of Rayleigh waves increases. This indicates that the Rayleigh wave attenuation is sufficient in distinguishing the changes in the properties of the heat-treated stainless steel. It is found that compared with the measurement method using an angle beam wedge receiver, the measured results are efficient, more stable and less influenced by the surface state when an air-coupled receiver is used. In addition, comparison results also show that the Rayleigh wave attenuation is more sensitive to changes in material properties than the longitudinal wave attenuation, as the wavelength of the Rayleigh wave is shorter than that of the longitudinal wave at the same frequency.

This paper presents a new method to determine the third-order elastic constants (l, m, and n) using thermally induced ultrasonic bulk (longitudinal and shear) and Rayleigh wave velocity changes. The equations of relative wave velocity changes of bulk and Rayleigh waves caused by homogeneous temperature variation in an isotropic medium are presented. Two materials (Aluminum 6061 and SS304 stainless steel) were tested using the thermal modulation test and the uniaxial compression test. The measured relative velocity changes in the thermal modulation test were used to correct the temperature effect on the results of the compression test. The results show that the third-order elastic constants determined from the thermal modulation test agree with the experimental values from the uniaxial compression acoustoelastic test.