Characteristics and Formative Factors of Fracture Surfaces on Subsurface Initial Crack Growth in Very High Cycle Fatigue

Abstract

In the very high cycle regime over 107 cycles, a fatigue crack often initiates from subsurface of materials. In subsurface fractures, a unique fracture surface with a fine concavo-convex pattern called Optically Dark Area (ODA) is known to exist around the fracture origins. This type of fracture surface has been observed not only in high strength steel, but also in Ti alloy. To reveal the formative factors of the fine concavo-convex pattern, this study focuses on a unique environment around subsurface cracks. Subsurface cracks are not surrounded by atmosphere; therefore, the environment around them can be similar to vacuum. Based on this idea, fatigue crack growth tests using high strength steel (SNCM439) and Titanium alloy (Ti-6Al-4V) were conducted in high vacuum under various conditions, and fracture surfaces were investigated by SEM analyses. As a result, the fine concavo-convexo morph similar to ODA in subsurface fractures was observed on the fracture surfaces obtained in the crack growth tests. In addition, it was clarified that the unique fracture surface was not formed during crack growth process but formed by the long term repeating contacts of already initiated fracture surfaces in high vacuum. The fine concavo-convexo morph can be formed regardless of materials when the following two conditions are satisfied ; one is a vacuum environment and the other is a long term repeating contacts of fracture surfaces over about 107 cycles.