Some Influencing Variables on Internal Fatigue Crack Initiation in Structural Materials

Abstract

Major variables which cause internal fatigue crack initiation are nonmetallic inclusions in high strength steels, surface roughness and notch in structural components and corrosive environments. This paper focused on the effect of stress concentration factor and corrosive environment for internal fatigue crack initiation of structural materials mainly based upon author's experimental results.In the results of conventional and ultrasonic fatigue testing on notched specimens with stress concentration factor of 1.5, 2.0 and 2.5 for 0.65 mass % carbon matrix high speed steel with HRC 60.7, subsurface fatigue crack initiation was observed on all notched specimens at failed number of cycles over than 106. On the contrary fatigue crack initiated from notched specimen surface for powdered high speed steel with HRC 67 with stress concentration factor of 2.0. The fatigue crack initiation mode in notched specimens at high cycle region may be governed by surface hardness of high speed steels.The different effect of corrosive environment on fatigue crack initiation is demonstrated in the plate bending fatigue testing results of 1% carbon steel and13Cr-1Mo steel in refrigeration compressor gaseous environment. For 1% carbon steel fatigue crack initiated from subsurface at lower fatigue life of 5x106 cycles,while fatigue crack initiated from surface corrosion pit at higher fatigue life of 5x106 cycles. For 13Cr-1Mo steel subsurface fatigue crack initiation was predominantly observed in air and in refrigeration compressor gaseous environment.