Crack Paths 2012

Growthand Paths of Small Cracks Initiated FromInclusions

in Ultra High Cycle Fatigue

A. Roiko1 and J. Solin2

1 V T TTechnical Research Centre of Finland, P.O.Box 1000, FI-20044 V T T(Espoo),

Finland, andrew.roiko@vtt.fi

2 jussi.solin@vtt.fi

ABSTRACT.In ultra-high cycle fatigue (UHCF)the early growth mechanisms, paths

and rates of the micro-structurally small cracks are not well known. Growth of

subsurface – and undetected – cracks cannot be monitored in real time and post

fracture fractography is complicated because of the statistical nature of local

microstructure. Furthermore, hammering or sliding often damages the surface before

the test is stopped. W eaddressed this challenge with two approaches. This paper deals

with fractographic details, such as crack growth on adjacent planes and their

coalescence, as well as formation of the “optically dark areas”. A sub-set of data

consisting of 11 bearing steel specimens loaded at the same stress amplitude to lives in

the range of 10·106 < Nf < 650·106 are studied and discussed. The second project

revealed that very early initiation and crack arrest are typical for small notches in the

34CrNiMo6Q T steel. The relation of crack paths and microstructures will be discussed.

I N T R O D U C T I O N

Fatigue crack nucleation and growth from surface as well as subsurface non-metallic

inclusions in high strength steels is of importance in many industries and has been the

focus of manystudies and as well as a book by Prof. Murakami. [1] There still remains

however many questions relating to the characterization and behaviour of the initiation

and growth of microstucurally small cracks in high strength steels. Studies have been

done in the past using focused ion beam (FIB) milling to cut cross-sections of metals

and then use FIB imaging to create ion channeling contrast pictures of the

microstrucure. [2, 3] This imaging technique has been applied to small cracks in steels

to study the profile in depth. [4, 5] Researches have recently used these FIB tools to

show how the size and crystal orientation of the grains affect the growth and direction

of small cracks from notchs and inclusions in the steel. [6-8] In this paper similar

techniques are used to study the role the martensitic subgrain microstrucre has on the

initiation and growth of small cracks in high strength steels from inclusions and notchs.

Recently a group of Finnish investigators studied the role of nonmetallic inclusions

in hard bearing steels (100Cr6) and a quenched and tempered steels (34CrNiMo6) [9].

The inclusions promoted subsurface initiation, which makes direct observations on

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