Crack Paths 2012

above result, the R C Fproperty of the material with artificial defects simulating stringer

type inclusions is evaluated.

It is known that synchrotron radiation micro computed tomography (SR micro CT)

imaging enables detection of inclusions[5] and small cracks. Moreover, an SRmicro C T

could be used in the observation of R C F crack initiation and propagation from

inclusions. The present authors succeeded in detecting small cracks below a flaking

damage and around an artificial defect by SRmicro CT[6].

The objectives of the present paper are to clarify the R C Fcrack path of high strength

steel with artificial defects by SR micro C T imaging and to discuss the mechanism of

R C Fcrack propagation by finite element (FE) analysis. As a result, two types of cracks,

that is, a vertical crack and a horizontal crack were inspected around the artificial defect

by SR micro CT. SIF of horizontal cracks, calculated by FE analysis, became large due

to vertical cracks. The interaction of both cracks would appear to dominate R C Fcrack

propagation originated from artificial defects.

R O L L I NCGO N T A CF TA T I G UTEE S T

Materials and specimens

Ball-on-disc type R C Ftests were conducted under oil lubrication. The material of the

disc specimen and ball specimen were induction-hardened 0.55% carbon steel. Table 1

shows the chemical composition of the disc specimen material. The disc specimen was

6 0 m min diameter and 5 m min thickness. The diameter of ball specimen was 9.525mm.

A ring-shaped region with 35-45mmin diameter on one side of the surface of the disc

specimen was induction-hardened. Vicker’s hardness of induction-hardened region is

HV720. A ball rolling track in the R C Ftest was located in a ring-shaped region. An

artificial defect of 1 5 min diameter and 2 0 0 min depth was introduced on the ball

rolling track of disc specimen by electro-discharge machining. The defect can simulate

a stringer type inclusion (e.g. MnS).

Table 1. Chemical composition of material for disc specimen(mass %)

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(a) Disc specimen

(c) Sample for S Rmicro C T

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Figure 1. Schematic illustration of disc specimen and sample for SRmicro CT.

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