PSI - Issue 7

6

Taizo Makino et Al./ Structural Integrity Procedia 00 (2017) 000–000

Taizo Makino et al. / Procedia Structural Integrity 7 (2017) 468–475

473

Φ 15 μm Circular hole defect Softinclusion ( E =100GPa ) W /O defect( E =205.8GPa )

Ba ll

200μm

Disc

x

Symmetry surface

y

Infinite element

z

(c) Soft inclusion or W /O defect

(b) Circular hole defect

(a ) Entire model

Fig.4. FEA model simulating RCF test.

3.3. Stress states around defect obtained by FEA In this section, the crack initiation and propagation of the vertical cracks are discussed based on the FEA result. We focussed on stress states at the surface edge of the defect where vertical cracks observed at the early stage of RCF and 0.1 mm in depth of defect because vertical cracks were propagated along the defect in the depth direction. Fig.5 presents the stress variation during rolling contact obtained from FEA. The maximum value of stress in the rolling direction at the surface edge of defect σ y , the range of σ y , i.e., Δσ y , the maximum value of shear stress in the rolling- depth direction during rolling contact τ yz , and the range of τ yz , i.e., Δτ yz were compared between circular hole defect, soft inclusion with bonded interface and W/O defect. All four parameters were the highest in the circular hole defect, and the second highest in the soft inclusion with bonded interface, and the lowest in the W/O defect. Separation of the interface of soft inclusion does not vary the compressive value of σ y , but increases the tensile value of σ y and all τ yz to the same values as those of the circular hole defect. This is because separation of the interface makes inclusion not support tensile or shear stress except compressive stress. The above-mentioned FEA results suggest following three phenomena regarding the vertical crack initiation. • Circular hole defects indicate earlier initiation of vertical cracks due to higher stress at the surface edge of a defect. • Soft inclusions with a bonded interface indicate later initiation of vertical cracks due to lower stress. • Separation of the interface of soft inclusions increases stress to the same as that of hole defects and facilitates vertical crack initiation. Treating hole defects and soft inclusions as artificial defects and sulphide inclusions respectively, we can notice that the above three phenomena are consistent with the experimental results described in 3.1. The phenomena demonstrate the reason for the difference of damage behaviour originating from between artificial defects and sulphide inclusions.