Fractography analysis of smooth and notched high cycle fatigue specimens in TC21 alloy at 315°C

The HCF fatigue tests of smooth and notched specimens in TC21 alloy were performed at 315°C, and the fractography was observed by SEM, the effect of notch on fatigue life was investigated. The results indicate that, the fatigue strengths ratio of the smooth and notched specimens decreases with the decrease of fatigue life. The main crack length of smooth specimens increases with the decrease of the maximum cyclic stress, but the ultimate fracture stress of the final fracture zone increases with the decrease of the maximum cyclicstress, which means that the ultimate fracture of fatigue specimens is controlled by the destabilization of fatigue cracks. For the notched specimens, there are multiple crack initiation sites on the fractography. When the nominal section stress multiplied by stress concentration factor equals the nominal stress of the smooth specimens, the main crack length of notched specimens is longer than that of the smooth specimens, meaning that the weight percent of fatigue cracks propagation life in overall life of notched specimens is larger than that of smooth specimens.