A New Method of Predicting Creep Life

Abstract

Creep tests have been made on Nimonic 80A at 750°C under tensile stresses of 92 N/mm² (6 tonf/in²) to 385 N/mm² (25 tonf/in²) and a parameter n, the number of cavities/mm² visible under an optical microscope, has been measured. n increases linearly with strain up to three-quarters of the creep lifetime and appears therefore to provide a good measure of the service life still remaining and of the strain that has occurred. n could be determined from a component withdrawn from service or perhaps even examined in situ. Some measurements of the number of cavities per unit volume have been made and, like the values of n, they are consistent with the idea that there is continuous nucleation of cavities throughout the whole creep test. The results also show that the slope of the line relating n with creep strain increases as the elongation at fracture decreases. Thus, this slope gives an early indication of poor creep ductility.