Factors influencing the mechanism of superlong fatigue failure in steels

Abstract

A B S T R A C T When the fatigue life N f of a specimen of 10 mm in thickness is longer than 10 8 cycles, the average fatigue crack growth rate is much less than the lattice spacing (~0.1 Å or 0.01 nm) that is 10 −11 to 10 −12 m/cycle. In the early stage of the fatigue process, the crack growth rate should be much less than the average growth rate, and accordingly we cannot assume that crack growth occurs cycle by cycle.In this paper, possible mechanisms for extremely high cycle fatigue are discussed. Of some possible mechanisms, a special focus was put on a newly found particular fatigue fracture morphology in the vicinity of the fracture origin (non-metallic inclusions) of a heat-treated alloy steel, SCM435, which was tested to NÁ10 8 . The particular morphology observed by SEM and AFM was presumed to be influenced by the hydrogen around inclusions. The predictions of the fatigue limit by the ǰarea parameter model are~10% unconservative for a fatigue life of N f =~10 8 , though it successfully predicts the conventional fatigue limit defined for N=10 7 . Thus, the fatigue failure for NÁ10 8 is presumed to be caused by a mechanism which induces breaking or releasing of the fatigue crack closure phenomenon in small cracks.In the vicinity of a non-metallic inclusion at the fracture origin, a dark area was always observed inside the fish-eye mark for those specimens with a long fatigue life. Specimens with a short fatigue life of N f =~10 5 do not have such a dark area in the fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from the usual fatigue fracture surface in a martensite lath structure.Considering the high sensitivity of high-strength steels to a hydrogen environment and the high hydrogen content around inclusions, it may be concluded that the extremely high cycle fatigue failure of high-strength steels from non-metallic inclusions is caused by environmental effects, e.g. hydrogen embrittlement coupled with fatigue.

Onset Frequency of Fatigue Effects in Pure Aluminum and 7075 (AlZnMg) and 2024 (AlCuMg) Alloys

“…Furuya et al [12] state that frequency generally affects high frequency fatigue tests because: (1) fatigue limits and lives decrease due to the temperature increase caused by plastic deformation [16]; (2) dislocations may not match the applied frequency because dislocation movement is slow compared to sonic velocity [17]; and (3) provided that embrittlement by hydrogen diffusion had an effect [18], fatigue lives would depend on both the number of loading cycles and time. However, Mayer [19] reported also that the HCF behavior of metallic alloys is relatively insensitive to the test frequency, provided that the ultrasonic testing procedure is appropriate (e.g., adequate cooling) and that fatigue-creep interaction and the time-dependent interaction with the environment are negligible.…”

Investigation of high cycle and Very-High-Cycle Fatigue behaviors for a structural steel with smooth and notched specimens

“…According to Murakami's model [5,8,22], fatigue strength at 10 8 cycles, denoted as r w , was predicted by the following equation:…”

“…Additionally, although there have been many studies on the factors those may affect VHCF propensity of high strength steels, such as environmental medium [14][15][16], loading frequency [17,18], microstructure state [19][20][21], etc. [5,8,12], the influence of notch effect has not been fully understood.…”

Effects of inclusion size and location on very-high-cycle fatigue behavior for high strength steels

“…As the most common defect for the fatigue failures of high strength steels, the inclusion of the materials has been paid close attention to and its effect has been included in the models for fatigue life and strength prediction [14][15][16]. These models are empirical formulae based on the experimental results, although the mechanism of the influence of inclusions on fatigue failures is not clear.…”