4. Conclusion
Ultrasonic fatigue testing was conducted for high-strength steel using large specimens with a straight section. In
order to confirm their validity, the results were compared with those obtained by conventional servo-hydraulic
fatigue testing. Ultrasonic fatigue testing was also conducted using smaller specimens to investigate the size effects.
Findings of this research are listed below.
1. The high-strength steel used in this research developed fish-eye fractures. Although a few fish-eye fracture
origins were the matrix itself, most of them were an Al2O3 inclusion.
2. The ultrasonic fatigue test results from enlarged specimens with a straight section showed good agreement
with the results of conventional servo-hydraulic tests in case where fish-eye fracture occur.
3. Ultrasonic fatigue testing for specimens of various sizes showed that fatigue strength fell in proportion to
increased specimen sizes. The degradation of the fatigue strength was too large an effect to neglect.
4. The inclusion sizes at the fish-eye fracture origins increased in proportion to increased specimen sizes,
suggesting that this enlargement of inclusion sizes is what causes the degradation of fatigue strength.
5. These results suggest that ultrasonic fatigue testing using large specimens is strongly advisable when
conducting gigacycle fatigue tests of high-strength steel