Application of high-speed IR imaging during mechanical fatigue tests

High-speed infrared (IR) imaging provides researchers a noncontact and nondestructive tool of studying fatigue behavior of materials. In this study ULTIMET superalloy manufactured by Haynes International, Inc. was used. High-cycle fatigue tests (20 Hz) were conducted at the University of Tennessee using two MTS (Material Test System) machines. An IR camera recorded temperature evolution during the fatigue test. Four distinct temperature regions were observed: (1) the initial temperature rise was found to be dependent on the damage condition of the specimen. A pre-damaged specimen showed a slower rate of initial temperature rise than the as-received specimen; (2) The equilibrium temperature of a specimen depends on the applied stress, test frequency, and thermophysical properties of the material. The equilibrium temperatures varied from 23 °C to 50 °C; (3) Further temperature increase was detected 5,000 cycles before final failure, indicating the onset of a fatal crack; and (4) Final temperature drop was found due to the separation of the test sample. High temperature regions were observed at the crack tip as a result of stress concentration. High-speeding IR imaging also revealed the thermoelastic effect during cyclic loading.