THERMAL FATIGUE.

Creep and thermal cycling tests with cold-drawn and standard heat treated Nimonic 80A wires were performed at several tensile stress levels. The interrelationships of deformation and fracture behavior under constant and cyclic temperature conditions were investigated quantitatively. The changes of microstructure and of several physical and mechanical material properties, such as electrical resistivity, internal friction and hardness after precreep and thermal precycling have been studied intensively. The acceleration effects of thermal cycling on creep rate and fracture were explained by thermal microstresses arising at the precipitates on account of different coefficients of thermal expansion of matrix and precipitate. A modified creep law is proposed. The effects of cyclic stress superimposed to a constant mean tensile stress on the creep behavior of cold-drawn Nimonic 80A were investigated with tests carried out on a tension testing machine combined with a high temperature furnace. The creep laws under constant and alternating stress conditions were quantitatively compared by a suitable parameter-method. Author