Characterization of creep and fatigue damage using laser generated ultrasound

The absorption coefficient of laser generated ultrasound was evaluated for different frequencies to characterize creep and fatigue damages in low alloy ferritic steels. It was found to increase linearly with frequency for head samples and to vary irregularly between high and low values for gauge samples. The absorption coefficient has been correlated with an independent physical parameter viz., decrease in the density which is due to the formation of cavities, and with strain for samples creep tested at the same temperature. Absorption studies were also carried out on samples obtained from alloy 800H specimens subjected to fatigue at different stress and strain rates. The absorption values for samples from gauge portion were normalized with that from head portion. The normalized values of the samples decreased with the number of cycles to fracture (Nf). It has been found that the absorption parameter exhibits less scatter and higher sensitivity than the velocity parameter carries out earlier.