Very high cycle fatigue of normalized carbon steels

Fatigue properties in the very high cycle regime of normalized carbon steel with carbon content 0.61% C (Ck60) and 0.15% C (Ck15) have been investigated with the ultrasonic fatigue testing technique at load ratio R = -1 and cycling frequency 20 kHz. Both steels show a distinct change of slope in the S-N curves at approximately 10 ⁷ cycles. No specimen failed above 2.2 × 10 ⁸ cycles although 25 specimens were stressed to over 10 ⁹ cycles. Calorimetric measurements were used to evaluate the cyclic plastic strain amplitude at constant stress amplitude. Ck60 shows initial cyclic softening followed by cyclic hardening at stresses above and below the mean fatigue limit at 10 ⁹ cycles. The lower the stress amplitude, the lower was the maximum plastic strain amplitude and the higher was the number of cycles when cyclic softening started. Cyclic hardening in Ck60 is seen to continue until 10 ⁹ cycles. Ck15 undergoes cyclic softening mainly at stresses above the mean fatigue limit and specimens can fail at very low values (10 ^-6) of the cyclic plastic strain. No sub-surface crack initiation was detected.