Noncontact acousto-thermal evaluation of evolving fatigue damage in polycrystalline Ti-6Al-4V

Abstract

Non-Contact Acousto-Thermal Signature (NCATS) analysis uses conversion of acoustic energy to heat to characterize evolving damage in materials. In the past, the observed temperature changes were interpreted using phenomenological approaches. This paper presents details of the mechanisms and the theoretical models to predict the temperature change due to conversion of acoustic energy to heat. NCATS experimental measurements performed using 20 kHz high amplitude acoustic waves on as received and fatigued polycrystalline Ti-6Al-4V are compared with theoretical calculations based on the mechanisms of transverse thermal currents, inter-crystalline thermal currents, and dislocation density changes. In the as received samples, the transverse thermal currents contribution has been found to be negligible compared with inter-crystalline thermal currents contribution. The experimentally measured maximum temperature change in the as received sample has been found to be 0.5 °C, and the theoretical prediction based on inter-crystalline thermal currents is 0.08 °C. In the fatigue damaged samples, the maximum temperature change increases with increasing damage that can be attributed to the increasing dislocation density. The theoretical prediction of the maximum temperature attained by a sample that is near failure based on dislocation contribution is 2.0 °C, while the experimental measurements have been found to be 0.95 °C. The differences between the theoretical and the experimental measurements are discussed in the context of the uncertainties in several physical parameters used in the theoretical calculations.