Ultrasonic Fatigue Properties of High-strength Steel Under Tensile Mean Stress

Abstract

Ultrasonic fatigue tests at 20 kHz were conducted under R=-1 and R=0 for a low-temperature-tempered JIS-SCM 440 low-alloy steel, together with 50 Hz tests using a servo-hydraulic fatigue testing machine. The ultrasonic fatigue testing machine, with which tensile mean stress could be applied, were specially prepared in this research. In these fatigue tests, fish-eye fracture occurred both under R=-1 and R=0. Although the fatigue test results were scattered, difference between the 20 kHz and 50 Hz results were little in case of fish-eye fracture. The origins of fish-eye fracture were mostly an Al2O3 inclusion, whose sizes ranged from 5 to 35μm. ODAs, i.e. optically dark areas, were observed on the fracture surfaces of fish-eye fracture both under R=-1 and under R=0. The ODA sizes under R=0 were, however, smaller than those under R=-1. The scattering of fatigue test results in the fish-eye fracture region were caused by difference of the inclusion sizes and could be reduced in the modified S-N diagrams in which stress amplitudes were normalized with fatigue limits estimated by Murakami's equation. In the modified S-N diagram, the results at 20 kHz showed good agreements with those at 50 Hz under R=0, as well as R=-1. These results supported the validity of the ultrasonic fatigue testing under tensile mean stress in case of fish-eye fracture.