Effects of specimen size on fatigue life of metallic materials in high‐cycle and very‐high‐cycle fatigue regimes

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: Influence Of the Loading Frequency And Risk Volume (Or Higmentioning

confidence: 99%

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

Jeddi

Palin‐Luc

2018

“…The observation was focused on the region of crack initiation and early growth. 39,40 The fatigue test data of VA cycling are characterized by the cumulative damage D f , which is based on Miner cumulative damage theory 41 and is written as, 35 Before the cutting, a Pt protection layer was coated on the target location to protect the surface of initiation region from damage.…”

Section: Fracture Surface Observationmentioning

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.

“…It has been known that a fatigue crack always initiates from the interior of specimen for VHCF, 9,10 which is different from the surface crack initiation mechanism for low-cycle fatigue and high-cycle fatigue. 11,12 In addition, the unique influences of stress ratio, [13][14][15] strength level, 16 loading frequency, 16,17 specimen size 18,19 and environmental condition 20,21 on the behavior of VHCF have been reported. However, the theories of physics and mechanics for the explanation of VHCF experiments still need further development.…”

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

Self Cite

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