A two-parameter model to predict fatigue life of high-strength steels in a very high cycle fatigue regime

The continuous increment of durability and reliability requirements for many machinery components is significantly enhancing the research activity in the Very‐High‐Cycle Fatigue (VHCF) characterization of metallic materials, in particular of high‐strength steels for critical structural applications. According to the area model, the VHCF strength of high‐strength steels can be estimated from the projected area of the ‘Optically Dark Area’ (ODA), which plays a key role in the VHCF response of high‐strength steels: more than 95% of the total VHCF life is consumed in the ODA formation, with crack growing even though the Stress Intensity Factor (SIF) is below the threshold for crack growth. Following the hydrogen embrittlement theory proposed by Murakami, hydrogen is supposed to assist crack growth within the ODA. The present paper proposes a general SIF formulation for the analytical model of the hydrogen assisted crack growth within the ODA. Starting from the general SIF formulation, a general expression for the material fatigue limit is obtained in the paper. The statistical method for the estimation of the parameters involved in the proposed model is finally illustrated in the paper and numerically applied to an experimental dataset.

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“…It is well‐known that the parameters of

F_{(|, Y_{f}) \sqrt{a_{d, 0}}}

depend on the applied stress amplitude and on the initial defect size. In particular, any monotonic decreasing function of s and

\sqrt{a_{d, 0}}

can be adopted for the mean and any positive function can be used for the standard deviation.…”

Section: Methodsmentioning

confidence: 99%

S‐N curves in the very‐high‐cycle fatigue regime: statistical modeling based on the hydrogen embrittlement consideration

Paolino

Tridello

Chiandussi

et al. 2016

Fatigue Fract Eng Mat Struct

111

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“…(1) The model to predict or estimate the fatigue life and/or fatigue strength is an important topic of VHCF research, which includes the loading stress ratio effect and the specimen size effect on the VHCF behavior. This topic may be referred to [52,64,67,[96][97][98][99][100][101][102][103]. 2The type and the shape of inclusion may affect the crack initiation process, and for some cases, fatigue crack interior initiation may from other inhomogeneities like grain boundaries rather than inclusions.…”

Section: Other Topics Concerned In Vhcf Fieldmentioning

confidence: 99%

The nature and the mechanism of crack initiation and early growth for very-high-cycle fatigue of metallic materials – An overview

Hong

Sun

2017

Theoretical and Applied Fracture Mechanics

Self Cite

145

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Very-high-cycle fatigue (VHCF) is the fatigue damage and failure beyond 10 7 loading cycles. Crack initiation of VHCF for metallic materials is commonly from the interior of specimens and possesses a unique feature of fish-eye (FiE) containing a fine-granular-area (FGA). It is regarded that FGA together with FiE is the characteristic region of crack initiation for VHCF, because it consumes more than 95% of total fatigue life and the value of stress intensity factor range for this region keeps constant for a given material. The aim of this overview is to summarize the research progress on the crack initiation characteristics and the related mechanisms for VHCF of metallic materials. After a brief introduction to the origination of VHCF research in 1980s, this overview contains the following parts: Characteristics of crack initiation region for VHCF, Characteristic region and intrinsic dimensions of crack initiation for VHCF, The formation mechanism and the model of crack initiation region, The new model of Numerous Cyclic Pressing to explain the formation mechanism of FGA, and Verification of proposed NCP model. In the descriptions, the two essential aspects are especially focused: the nature of crack initiation region for VHCF, and the mechanism for the formation of crack initiation region.

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“…The research topic of very‐high‐cycle fatigue (VHCF) has attracted increasing interests for the demanding of fatigue design in engineering applications and for the request of understanding the related fatigue failure mechanism . With the development of engineering technology and the requirement of natural resource conservation, many structural components such as high‐speed train axles and aircraft structures are designed to bear the fatigue life of more than 10 7 cycles, which booms the investigation on VHCF.…”

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

Fatigue Fract Eng Mat Struct

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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

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A two-parameter model to predict fatigue life of high-strength steels in a very high cycle fatigue regime

Cited by 31 publications

References 39 publications

S‐N curves in the very‐high‐cycle fatigue regime: statistical modeling based on the hydrogen embrittlement consideration

S‐N curves in the very‐high‐cycle fatigue regime: statistical modeling based on the hydrogen embrittlement consideration

The nature and the mechanism of crack initiation and early growth for very-high-cycle fatigue of metallic materials – An overview

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

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