Study on the Gigacycle Fatigue Behaviors and Fracture Surface of 40Cr Steel

The fatigue behavior of 40Cr steel between 10~5-10~10 cycles was studied with the ultrasonic fatigue testing technique, with a loading frequency of about 20 kHz. The fracture surfaces of specimens were investigated with scanning electron microscopy (SEM). The experimental results show that the S-N curve of 40Cr steel continues to decrease with the increase of the number of cycles between 10~5 and 10~10 cycles and does not generally exhibit a plateau beyond 10~6 cycles. The fatigue strength at 10~7 cycles is found to be 443 MPa and at 10~10 cycles is 235 MPa. The difference between them is very large. The observation of fracture surface shows that the mechanism of fatigue failure is similar in both cases of ultrasonic fatigue and conventional fatigue in long life regime. Cracks initiate from a mass defect of specimen surface or beneath the surface. The fatigue striation areas have been found in the fracture surface of specimens.