Evolutions of Nonlinear Acoustics and Microstructure Induced by Plastic Strain in a Low Carbon Steel

Abstract

Electromagnetic acoustic resonance (EMAR) is a contactless resonant method with an electromagnetic acoustic transducer (EMAT). This method enables not only to measure exact ultrasonic attenuation of measured sample but also to eliminate nonlinear acoustic effect between the sample and transducer. In this study, the EMAR was applied to investigate the relationships between nonlinear acoustic characterizations; resonant frequency shift, three-wave mixing and birefringence acoustoelasticity and microstructural changes induced by tensile plastic strain in a low-carbon steel, JIS-S25C. Furthermore, we developed a single bulk-shear-wave EMAT which was composed of three-layer elongated coils and a pair of permanent magnets to measure in three-wave mixing. The EMAT transmits and receives shear wave propagating in thickness direction of a plate specimen. Three nonlinear acoustic parameters and ultrasonic attenuation increased with increase in tensile plastic strain. This phenomenon is interpreted as resulting from microstructure changes, especially, dislocation density and crystal misorientation. This is supported by X-ray observations for dislocation density and EBSD (electron backscattering diffraction) for the misorientation.