Pitting damage imaging by non-linear ultrasonic technique: comparison between resonance and non-resonance modes

A non-linear ultrasonic method was applied for evaluating the cavitation damage, so-called pitting damage, in the mercury target of the Japan Spallation Neutron Source (JSNS). First, pitting damage was imposed on disk specimens in mercury using the magnetic impact testing machine (MIMTM) with various numbers of impacts (0, 104, 105, 106, 107 and 108). The pitting damage was imaged with resonance and non-resonance modes, respectively, by employing a non-linear acoustic imaging system. Waveform attenuation and generation of higher harmonics, which have high relevance to the degree of pitting damage, were useful for the imaging. Combined with the attenuation phenomenon, the resonance mode imaged the micropits with higher harmonics more effectively with higher harmonics, compared with the non-resonance mode.