Gigacycle fatigue initiation mechanism in Armco iron

Abstract

a b s t r a c tMicrostructure irreversibility plays a major role in the gigacycle fatigue crack initiation. Surface Persistent Slip Bands (PSB) formation on Copper and its alloy was well studied by Mughrabi et al. as typical fatigue crack nucleation in the very high cycle fatigue regime. In the present paper, Armco iron sheet specimens (1 mm thickness) were tested under ultrasonic frequency fatigue loading in tensioncompression (R = À1). The test on the thin sheets has required a new design of specimen and new attachment of specimen. After gigacycle fatigue testing, the surface appearance was observed by optical and Scanning Electron Microscope (SEM). Below about 88 MPa stress, there is no PSBs even after fatigue cycle up to 5 Â 10 9 . With a sufficient stress (above 88 MPa), PSBs in the ferrite grain was observed by optic microscope after 10 8 cycles loading. Investigation with the SEM shows that the PSB can appear in the body-centered cubic crystal in the gigacycle fatigue regime. Because of the grain boundary, however, the local PSB did not continually progress to the grain beside even after 10 9 cycles when the stress remained at the low level.

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

“…On the other hand, the dislocation slips characteristics in polycrystalline copper (fcc) under VHCF conditions have been well studied . Lately, studies for polycrystalline alpha iron (bcc) have been reported . For those two representative polycrystalline metals, the crack initiates from the damaged volume close to the surface where intensive localized irreversible slip bands were found in the interior .…”

Osobennosti mekhanicheskogo povedeniya armko-zheleza pri ispytanii v rezhime gigatsiklovoy ustalosti

“…Дальнейшее развитие и верификация предложенных моделей требуют детального экспериментального исследования процессов развития дефектов в материале на завершающей стадии циклического деформирования с малыми амплитудами. Результаты исследования эволюции структуры армко-железа в [12] показали, что в области гигацикловой усталости в чистом железе могут формироваться как полосы устойчивого сдвига на поверхности образца (аналогично меди или никелю), так и характерные подповерхностные структуры типа «рыбьего глаза».…”

Investigation of mechanical properties of Armco-iron during fatigue test

“…The study on structural evolution of armco-iron was done in (Wang et al 2012). Here it has been shown that in the region of gigacyclic fatigue, the cyclic deformation of pure iron can lead both to the formation of a stable shear band on the sample surface (by analogy with copper or nickel), and characteristic subsurface structures of the "fish eye" type.…”