Mechanism of fatigue failure in ultralong life regime

Abstract

The fatigue fracture surfaces of specimens of heat treated hard steels which failed in the regime of N = 105 to 5 × 108 cycles, were investigated by optical microscopy and SEM. Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark and in the previous paper it was named the Optically Dark Area, ODA. The roughness inside ODA is larger than outside ODA. The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of ultra long life fatigue failure. Direct evidences of existence of hydrogen at the inclusion at fracture origin are presented. It is presumed that the ODA is made by the cyclic stress coupled with the hydrogen which is trapped by the inclusion at the fracture origin. To verify the influence of hydrogen, specimens containing different levels of hydrogen were prepared by different heat treatments. The results obtained by fatigue tests of these specimens suggest that the hydrogen trapped by inclusions is a crucial factor which causes the ultra long fatigue failure of high strength steels. Aspects of the double S–N curve are also discussed in terms of experimental methods, specimen size and statistical distribution of inclusions sizes.

Effects of loading condition on very-high-cycle fatigue behaviour and dominant variable analysis

“…The behaviour of VHCF fatigue crack initiation and propagation may become comparably significant for airworthiness. The influence of loading condition, namely the control volume and loading frequency, on VHCF has been studied, in which the concept of control volume was first proposed by Murakami et al [3] to interpret the decrease of fatigue strength under axial loading. Control volume is defined as the volume of the tested part of the specimen, in which potential fracture origins exist.…”

Fractography and Crack Initiation of Very-High-Cycle Fatigue for a High Carbon Low Alloy Steel

“…The fish-eye fracture is an internally originating type of fatigue failure, which eliminates the traditional fatigue limit of high strength steels. Hence, inclusions decrease the fatigue strength of high strength steels and cause VHCF [4,5] . This paper attempts to further investigate the behavior and mechanism of VHCF for metallic materials.…”

Evolution Mechanisms of MgO·Al₂O₃ Inclusions by Cerium in Spring Steel Used in Fasteners of High-speed Railway

“…The fatigue failure is the main factors affecting the service life of spring steel used in fastener of high speed railway. 1,2) There are many factors affecting the fatigue life including inclusions, heat treatment, surface quality, corrosion resistance, and microstructure and grain size etc.…”