Chapter 15 - The Mechanism of Fatigue Failure of Steels in the Ultralong Life Regime of N>107 Cycles

Abstract

This chapter investigates the mechanism of fatigue failure of steels in the ultralong life regime of N >107 cycles. The fatigue fracture surfaces of specimens containing different levels of hydrogen showed very different fracture surface morphologies, and the influence of hydrogen was crucial for the elimination of the fatigue limit in a cycle region that was longer than in conventional fatigue testing. Compressive residual stresses of approximately 500 MPa were present at a specimen surface. All fractures occurred from internal inclusions. The locations of these inclusions were at depths from the surface greater than 1700 μm. This was because the core of a specimen was softer than the surface, and also because the residual stress at the surface was compressive. To evaluate the influence of inclusions, the fatigue limit σw of each specimen was estimated by the √area parameter model. Although the residual stress at the fracture origin inclusion was unknown, for a tentative calculation, it was assumed that the stress ratio R was −1 because the residual stress in the core region of a specimen was thought to be very small.