Subsurface crack initiation and propagation mechanism in high-strength steel in a very high cycle fatigue regime

The formation mechanism of characteristic region at crack initiation for very-high-cycle fatigue of high-strength steels

With the development of high-strength materials, the existing fatigue strength formulae cannot satisfactorily describe the relation between fatigue strength sigma(w) and tensile strength sigma(b) of metallic materials with a wide range of strength. For a simple but more precise prediction, the tensile and fatigue properties of SAE 4340 steel with the tensile strengths ranging from 1290 MPa to 2130 MPa obtained in virtue of different tempering temperatures were studied in this paper. Based on the experimental results of SAE 4340 steel and numerous other data available (conventional and newly developed materials), through introducing a sensitive factor of defects P, a new universal fatigue strength formula sigma(w)=sigma(b)(C-P.sigma(b)) was established for the first time. Combining the variation tendency of fatigue crack initiation sites and the competition of defects, the fatigue damage mechanisms associated with different tensile strengths and cracking sites are explained well. The decrease in the fatigue strength at high-strength level can be explained by fracture mechanics and attributed to the transition of fatigue cracking sites from surface to the inner inclusions, resulting in the maximum fatigue strength sigma(max)(w) at an appropriate tensile strength level. Therefore, the universal fatigue strength formula cannot only explain why many metallic materials with excessively high strength do not display high fatigue strength, but also provide a new clue for designing the materials or eliminating the processing defects of the materials. (c) 2012 Elsevier B.V. All rights reserved.

General relation between tensile strength and fatigue strength of metallic materials

Propensities of crack interior initiation and early growth for very-high-cycle fatigue of high strength steels

Fatigue propagation threshold of short cracks under constant amplitude loading

The S–N curve obtained from cantilever-type rotary bending fatigue tests using hour-glass-shaped specimens of high carbon-chromium bearing steel clearly distinguished the fracture modes into two groups each having a different crack origin. One was governed by crystal slip on the specimen surface, which occurred in the region of short fatigue life and a high stress amplitude level. The other was governed by a non-metallic inclusion at a subsurface level which occurred in the region of long fatigue life and low stress amplitude. The inclusion developed a fish-eye fracture mode that was distributed over a wide range of stress amplitude not only below the fatigue limit defined as the threshold for fracture due to the surface slip mode but also above the fatigue limit. This remarkable shape of the S–N curve was different from the step-wise one reported in previous literature and is characterized as a duplex S–N curve composed of two different S–N curves corresponding to the respective fracture modes. From detailed observations of the fracture surface and the fatigue crack origin, the mechanisms for the internal fracture mode and the characteristics of the S–N curve are discussed.

S–N curve characteristics and subsurface crack initiation behaviour in ultra‐long life fatigue of a high carbon‐chromium bearing steel

The S-N curve obtained from cantilever-type rotary bending fatigue tests using hourglass shaped specimens of high carbon-chromium bearing steel was clearly classified fracture modes into two groups of different crack origin. One was governed by a crystal slip on the specimen surface, which occurred in the region of short fatigue life and high stress amplitude level. The other was governed by nonmetallic inclusion at subsurface which occurred in the region of long fatigue life and low stress amplitude. The inclusion governed fish-eye fracture mode was distributed in a wide range of stress amplitude not only below the fatigue limit defined as the threshold of fracture due to the slip governed mode but also above the fatigue limit. This remarkable shape of the S-N curve was different from a step-wise one reported in previous literature and was characterized as duplex S-N curves which composed of two different S-N curves corresponding to the respective fracture modes. From the detailed observation of fracture surface and fatigue crack origin, mechanisms of internal fracture mode and characteristics of S-N curve were discussed.

S. Ishihara

Papers

S–N curve characteristics and subsurface crack initiation behaviour in ultra‐long life fatigue of a high carbon‐chromium bearing steel

Duplex S-N curve characteristics and subsurface fatigue crack initiation behaviour in high carbon-chromium bearing steel