Specimen Size Effect on Gigacycle Fatigue Properties of SUP7 Spring Steels

Abstract

Ultrasonic fatigue tests up to gigacycle regimes were conducted for two heats of SUP7 spring steels using φ7×20 mm and φ3 mm specimens whose risk volumes were 912 and 33 mm³, respectively. As the result, the φ7×20 mm specimens revealed much lower fatigue strength than the φ3 mm specimens, showing fish-eye fracture regardless of the specimen types. The fish-eye fracture origins were an Al₂O₃ inclusion in most cases, while in case of the φ3 mm specimens of heat C, a TiN inclusion and matrix itself also caused fish-eye fracture. The Al₂O₃ inclusion sizes of the φ7×20 mm specimens were larger than those of the φ3 mm specimens, which was the reason why the φ7×20 mm specimens showed lower fatigue strength. The minimum size of the Al₂O₃ inclusion observed at the fish-eye fracture origin was about 10 μm. This implied that when the specimen contained no Al₂O₃ inclusion exceeding 10 μm in the risk volume, a TiN inclusion or matrix itself could cause fish-eye fracture as in case of the φ3 mm specimens of heat C. These results meant that the effects of the specimen size were large in case of fish-eye fracture, recommending us to use large specimens, such as the φ7×20 mm specimens, in conducting gigacycle fatigue tests on high-strength steel.