Mechanisms and modeling of subsurface fatigue cracking in metals

It is well‐known that the high cycle fatigue (HCF) strength of steel components is influenced by a lot of factors depending on both material, loading (including environment), specimen or component geometry (design), and manufacturing process. Based on a literature review of a lot of experimental data, a synthesis is proposed in this paper to discuss the effect of the structural and operational factors on the very high cycle fatigue (VHCF) characteristics of steels. HCF and VHCF regimes are distinguished in terms of failure mechanisms and S‐N curve shapes for high and low strength steels. Then, the effect of the microstructural and mechanical features on the VHCF resistance is debated as different parameters (microstructure, inclusion size type and depth, hydrogen, environment, maximum tensile strength, and residual stresses). Next, the influence of the loading conditions is addressed by taking into account both the frequency effect, the highly stressed volume, the loading type, and loading ratio. Finally, the influence of the testing techniques used in VHCF experiments is discussed.

Section: Effect Of Microstructural and Mechanical Features On The Vhcmentioning

confidence: 99%

A review about the effects of structural and operational factors on the gigacycle fatigue of steels

Jeddi

Palin‐Luc

2018

“…17 Because nonmetallic inclusions are the crack origins for most VHCF cases with interior crack initiation, the effects of the inclusions on the behavior of VHCF have been intensively investigated, and the influential parameters are their size, elastic properties, and adhesion to the matrix. 23 Each of these proposed mechanisms encountered difficulties for more general cases. 17 Several models have been proposed to explain the formation mechanism of FGA, including "hydrogen assisted crack growth", 19,20 "decohesion of spherical carbide", 8 "formation and debonding of fine granular layer", 4,21 "local grain refinement at crack tip and debonding", 22 and "vortical plastic flows to produce nanostructure layer and debonding".…”

Section: Introductionmentioning

confidence: 99%

Crack growth rates and microstructure feature of initiation region for very‐high‐cycle fatigue of a high‐strength steel

Sun

Hong

2018

The S‐N data up to very‐high‐cycle fatigue (VHCF) regime for a high‐strength steel were obtained by fatigue tests under constant amplitude and variable amplitude (VA) via rotating bending and electromagnetic resonance cycling. Crack initiation for VHCF was from the interior of specimens, and the initiation region was carefully examined by scanning electron microscopy and transmission electron microscopy. Crack growth traces in the initiation region of fine‐granular‐area (FGA) were the first time captured for the specimens under VA cycling by rotating bending. The obtained crack growth rates in FGA were upwards to connect well with those in fish‐eye region available in the literature and were associated well with the calculated equivalent crack growth rates in FGA. The observations of profile samples revealed that FGA is a nanograin layer for the specimens under VA cycling, which is a new evidence to support the previously proposed “numerous cyclic pressing” model.

“…29,30 Thus, it received a number of investigations with different explanations for its formation mechanism, including 'hydrogen assisted crack growth', 31,32 'decohesion of spherical carbides', 33 'formation and debonding of fine granular layer', 27,34 'local grain refinement at crack tip and debonding', 35 and 'vortical plastic flows to produce nanostructure layer and debonding'. 36 These proposed mechanisms encountered difficulties in the explanation of different experimental cases including a recent result 37 for a martensitic 12% Cr steel, for which the morphology of FGA was observed in the specimens at R = À1 but never found in the specimens fatigued at R = 0.1, 0.5 and 0.7. Recently, it was revealed that this characteristic region of FGA is a nanograin (NG) layer of several hundred nanometer thick on both crack surfaces for the cases of negative stress ratios, while the NG layer diminishes for the cases of positive stress ratios.…”

Section: Introductionmentioning

confidence: 99%

Nanograin layer formation at crack initiation region for very‐high‐cycle fatigue of a Ti–6Al–4V alloy

Liu

Sun

et al. 2016

A B S T R A C T The microstructural features and the fatigue propensities of interior crack initiation region for very-high-cycle fatigue (VHCF) of a Ti-6Al-4V alloy were investigated in this paper. Fatigue tests under different stress ratios of R = À1, À0.5, À0.1, 0.1 and 0.5 were conducted by ultrasonic axial cycling. The observations by SEM showed that the crack initiation of VHCF presents a fish-eye (FiE) morphology containing a rough area (RA), and the FiE and RA are regarded as the characteristic regions for crack initiation of VHCF. Further examinations by TEM revealed that a layer of nanograins exists in the RA for the case of R = À1, while nanograins do not appear in the FiE outside RA for the case of R = À1, and in the RA for the case of R = 0.5, which is explained by the Numerous Cyclic Pressing model. In addition, the estimations of the fatigue propensities for interior crack initiation stage of VHCF indicated that the fatigue life consumed by RA takes a dominant part of the total fatigue life and the related crack propagation rate is rather slow.Keywords crack initiation; fatigue life; nanograins; Ti-6Al-4V alloy; very-high-cycle fatigue. N O M E N C L A T U R E2a FiE = equivalent diameter of FiE 2a RA = equivalent diameter of RA ffiffiffiffiffiffiffiffi ffi area p = equivalent size of FiE or RA A, m = material parameters BFI = bright field image BSE = back scattered electron (imaging) EG = equiaxed α grain FIB = focused ion beam FiE = fish-eye FGA = fine granular area HAADF = high angle annular dark field (imaging) ΔK FiE = range of SIF in FiE region ΔK RA = range of SIF in RA ΔK th = threshold of SIF LM = lamellar structure NCP = numerous cyclic pressing NG = nanograin N f = total fatigue life N i = fatigue life of crack initiation RA = rough area SAD = selected (electron) area diffraction SIF = stress intensity factor VHCF = very-high-cycle fatigue