Abstract
Uniaxial tension fatigue tests of high strength steel, SNCM439, were carried out in air and high vacuum environments. In order to clarify how different environments affect the fatigue fracture surfaces and interior fatigue crack propagations, fracture surfaces were compared through fractographic analysis under air, high vacuum, and inner environment of materials. The following results were obtained; (1) Compared with surface fractures in air, the fatigue life of surface fractures in high vacuum tended to be longer. (2) High vacuum environment strongly influenced the fatigue fracture surface in initial stage of crack propagation. (3) When the transition of crack propagation from Optically Dark Area (ODA) to Stage II a occurred, there was the possibility that some mechanical factors were involved in the transition. The formation of ODA was closely related to the mean stress. (4) Inner environment of materials gave more similar effects on crack propagation with high vacuum rather than air. (5) When fracture origins were non-metallic inclusions, interior fatigue cracks of materials in Fish eye were propagated in the following order; ODA → Stage II a → Stage II b → Stage II c.
