Ultrasonic quantification of high temperature cyclic damage in an advanced nickel based superalloy

Present paper discusses about a new methodology to quantify cyclic damage through ultrasonic
measurement. Based on experimental results, correlations have been made between damage accumulated inside the material due to strain excursions and corresponding ultrasonic parameters. It has been
proposed that based on the existing correlation between attenuation coefficient and number of cycles to
failure, fatigue failure characteristics can be partitioned into two regimes (a) failures due to early
nucleation and rapid propagation of cracks and (b) failures due to delayed propagation and crack
coalescence. Plastic strain accumulation and surface crack density have been chosen as two physical
parameters directly influencing attenuation coefficient and it has been observed that with increasing
plastic strain accumulation, ultrasonic attenuation increases. Between two primary echoes of ultrasonic
spectra, some secondary defect echoes have been found. A damage descriptor has been introduced by
taking difference between the bandwidth of defect echo and backwall echo normalized by overall
amplitude frequency distributions of backwall echo. It has been found to bear sensitivity towards surface
crack density. This quantitative estimation differentiates between the classical descriptions of damage
due to dislocation mediated plasticity, micro crack generation and coalescence.