Abstract
In this paper, we describe the change of nonlinearity by the nonlinear resonant ultrasound spectroscopy (NRUS) during creep damage in an austenitic stainless steel, JIS-SUS304, under 120MPa, 973 K. which is a resonance-based technique exploiting the significant nonlinear behavior of damaged materials. In NRUS, the resonant frequency of an object is studied as a function of the excitation level. As the excitation level increases, the elastic non-linearity is manifest by a shift in the resonance frequency. NRUS exhibits high sensitivity to microstructural change of damaged materials. We use an electromagnetic acoustic transducer (EMAT) to monitor NRUS of bulk shear wave propagating in the thickness direction of the sample. The EMAT operates with the Lorentz-force mechanism and is the key to establish a monitoring for microstructural change during creep with high sensitivity. Furthermore, use of EMAT makes contactless transduction possibility. We also monitor the change of linear ultrasonic characterizations, shear-wave attenuation and velocity. NRUS exhibits much larger sensitivity to the damage accumulation than the velocity. This measured NRUS showed a peak at 50 % of the total life. We interpreted these phenomena in terms of dislocation mobility and restructuring, with support from scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. The NRUS evolution as creep progress is related to the microstructure change, especially, dislocation mobility. This is supported by TEM observation for dislocation structure. This technique has potential to assess the damage advance and to predict the creep life of metals.
