Effect of Test Frequency on Fatigue Strength of Low Carbon Steel

The ultrasonic fatigue tests (test frequency : 20 kHz) and the conventional tension compression fatigue tests (10 Hz) have been conducted using the annealed and 10% pre-strained specimens of 0.13% carbon steel. Two small artificial holes (diameter and depth : 100μm) were introduced onto the specimen surface to invistigate the effect of test frequency on the crack initiation and growth behavior. In order to suppress the temperature rise of the ultrasonic fatigue specimens, the intermittent ultrasonic loading method and the air cooling were used. The dynamic stress concentration factor and the stress intensity factor under the ultrasonic fatigue tests were calculated. It has been revealed that the dynamic stress concentration factors for a small circular hole and a small spherical cavity and the dynamic stress intensity factor for a penny-shaped crack are almost the same as those at low strain rates from the conventional tension compression fatigue tests. On the other hand, the fatigue properties were dependent on the test frequency. The ultrasonic fatigue tests showed longer fatigue life and lower fatigue crack growth rate for the annealed and 10% pre-strained specimens. The evident slip bands were not observed in the neighborhood of the crack in the annealed specimen under the ultrasonic fatigue tests, while the slip bands were observed in the wide area around the crack under the conventional fatigue tests. Slip bands were observed slightly in the 10% pre-strained specimen under the ultrasonic fatigue tests. In order to determine the stress-strain curve for 0.13% carbon steel under the high strain rates, the Split Hopkinson Bar tests were carried out. The yield stress and deformation stress were increased with the strain rate. Thus, the effect of test frequency on fatigue strength can be explained from the viewpoint of the effect of test frequency on fatigue crack growth for 0.13% carbon steel.