Analysis of temperature and fracture surface of AISI4140 steel in very high cycle fatigue regime

• In the present study, the behaviour of AISI4140 steel was investigated in VHCF regime.
• The fatigue tests were carried out at the f = 20 kHz and R = −1 by means of an ultrasonic fatigue testing equipment.
• The temperature evolution, detected during the whole test by means of an IR camera, shows a trend with the three stages, similar to the one observed in LCF and HCF regimes.
• The thermal response was correlated to the fracture surface analysis, which allowed defining some parameters, related to the fatigue life.

Currently, the technological progress in the engineering fields requires that the design lifetime of many components exceeds 107 loading cycles. The ultrasonic testing machines allow loading the specimen with 20 kHz frequency, allowing reaching gigacycles within a relatively short time, but the high value of frequency produce a high heating of the specimen. A controversy is still opened in literature about the effect of the temperature increase on the fatigue response. This aspect was investigated in the present study. Very high cycle fatigue tests were carried out on AISI4140 steel at frequency f = 20 kHz and stress ratio R = −1 with and without cooling the specimen during the test. The temperature evolution was detected during the whole fatigue test by means of an infrared camera. The fracture mechanism of the investigated steels was evaluated by means of several experimental techniques (Thermography and Optical Microscopy).