Magneto-mechanical damping and ΔE-effect in ferritic steel at different levels of ultrasound strain amplitudes

The magneto-mechanical damping and Young's modulus (ΔE-effect) of reactor pressure vessel steels were investigated in a wide amplitude range for both amplitude-independent (AID) and amplitude-dependent (ADD) damping ranges. Two JRQ (Japanese reference quality) steel samples, one in the as-received and the other in the thermally aged condition were used in the experiments. The measurements were made using the composite oscillator technique at resonance frequencies of about 100 kHz. Longitudinal vibrations of the rod samples were used. Magnetic field up to 26,000 A/m was applied along the length of the rod. Amplitude and field dependencies of the damping and resonant frequencies (Young's modulus) have been measured. Computer controlled acoustic measurements were made simultaneously at two amplitude levels. The data about the influence of high amplitude ultrasonic excitation (HAUE) of the samples (with the amplitudes in the ADD range) on hysteretic field behaviour of AID, ADD and resonant frequencies (ΔE-effects) have been obtained. The experiments showed that the ΔE-effect at low amplitudes (AID range) was qualitatively changed by HAUE, whereas the AID remained almost the same during the experiments with and without HAUE. It was found that annealing of the JRQ steel for 3 years at 300 °C slightly increased magneto-mechanical damping and ΔE-effects for both AID and ADD ranges, but it qualitatively changed neither amplitude nor magnetic field dependencies. The analysis of the experimental data shows that the influence of annealing can be explained by relaxation of internal stresses in the sample material. This is confirmed by the higher level of flow stresses for the `as received' sample as compared with the stresses for the `thermally aged' one. The newly found effect of HAUE is supposed to be caused by reversible changes (due to high vibrational amplitude) of the internal stresses as well.