Very high cycle fatigue crack initiation mechanism according to a 3D model of persistent slip bands formation in α‐ferrite

Abstract

A B S T R A C T Many studies on the behaviour of very high cycle fatigue (VHCF) showed that there appear some distinguishable features compared with the low and high cycle fatigue. The understanding of mechanism of fatigue crack initiation in VHCF is very important.In the present work, 1 mm thin sheet specimens on commercial pure iron were employed to perform tests under tension-compression cyclic loading at 20 kHz ultrasonic frequency. In situ observation was carried out with thermal and optical imaging. Local deformation marks on the specimen and the fracture surface near the crack initiation were investigated by scanning electron microscope. A micron abreast pipes 3D model has been constructed to illuminate the damage accumulation of irreversible deformation. It was found that the micron abreast pipes develop during previous cyclic loadings and remain unaffected at the fracture surface. A mechanism was established to explain the relationship between micron abreast pipes and fatigue crack initiation.Keywords irreversible deformation; fatigue crack initiation; threshold of fatigue crack; very high cycle fatigue; persistent slip bands. N O M E N C L A T U R E3D = three-dimensional b = Burger's vector Δd = elevation of irreversible deformation density E = modulus elasticity N = number of cycles ΔK th = crack propagation threshold in fatigue R = stress ratio

A general scenario of fish‐eye crack initiation on the life of high‐strength steels in the very high‐cycle fatigue regime

“…Slightly decreasing the stress amplitude will need a much larger area of initiation because of the square relationship and certainty of ∆K th . Irreversible slip bands at the initiation site form very slowly in the VHCF regime and are strongly dependent on microstructure characteristics, leading to a progressively weaker area with discrete nanoscale damage. Accordingly, the slope of the S‐N curve is lower.…”

Surface crack initiation mechanism for body centered cubic materials in the gigacycle fatigue domain

“…Observations at the specimen surface were made during the tests (for 3 tests) by light microscope. These observations showed that irreversible slip bands (PSBs) were formed at the beginning of the fatigue loading [1,21]. These slip bands multiplied or grew within the same grain during subsequent cycle loading.…”

“…A specimen showing pronounced PSBs on three perpendicular surfaces, Fig. 3a, was investigated by SEM to gain a 3D understanding of mechanism of PSB formation [21]. The axial and cross surfaces were mechanically polished and etched with 4% Nital.…”

“…Three materials were studied: two FCC materials (pure copper and a-brass) and one BCC polycrystalline single phase material (Armco iron with a small amount of carbon). Some results concerning pure copper and Armco iron have already been published [1,19,21,29,30].…”

Very high cycle fatigue for single phase ductile materials: Comparison between α-iron, copper and α-brass polycrystals

“…The aim of this paper is to investigate the qualitative correlation between the dissipated energy and the slip markings for the three materials of interest during fatigue tests conducted at 20 kHz using an ultrasonic device and to compare the material responses. Some results obtained on copper and a-iron have been published [10][11][12][13][14][15]. These results are summarized and completed in order to compare the fracture, thermal and microstructural responses in VHCF.…”