PSI - Issue 7

I. Milošević et al. / Procedia Structural Integrity 7 (2017) 327 –334 I. Milosˇevic´ / Structural Integrity Procedia 00 (2017) 000–000

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4

2 × 10 7 ) slight modifications had to be carried out regarding the relationship between inclusion size and run out load level. For a successfull application of the √ area approach observed inclusion sizes were linked to a estimated run-out level at 10 9 by a polynomial function. The mathematical fit of the exponential function (fit through three points) was done according to the S / N curve ( k 1 and k 2 , finite approach III up to 10 9 ). This method showed a good applicability and was introduced in detail by Garb et al. (2017).

4. Results of fatigue tests

Fatigue tests were examined for two specimen sizes and two di ff erent temperatures. Focus was put on the charac terisation of the TA where di ff erent crack origins were expected. Noticable specimens were selected and analysed by SEM and EDS to identify the crack origin. Based on the crack origin size (only from detected inclusions) Murakami’s √ area approach was applied.

0 . 7 0 . 75 0 . 8 0 . 85 0 . 9 0 . 95 1 1 . 05 1 . 1 1 . 15 1 . 2

1.04

1.55

Data Points Murakami

1.01

1.50

1.45

0.97

1.40

Normalised stress, σ norm , - (log.)

0.93

1.35

90%

0.90

S a,norm , - (log.)

1.30

0.86

S a /(HV+C2), - (log.)

Surface Subsurface None

1.25

0.82

10%

1.20

0.78

1.15

10 4

10 5

10 6

10 7

10 8

5

10

100

Number of cycles, N (log.)

√ area, µm (log.)

Fig. 4. Murakami approach - D 4 data

Fig. 3. S / N data for specimens D 4 at T = 20 ◦ C , R = − 1.

D 4 fatigue tests, standardised stress σ norm is displayed over the cycles to failure, are shown in Figure 3. Three di ff erent symbols are illustrated in the S / N diagrams representing surface initiated cracks (black filled circles), inter nal inclusion initated cracks (green semi filled circles) and no detected crack initiation due to inclusions (red filled triangles). Failure mechanism changed from surface initiated failures to internal initiated failures after 10 6 cycles.

a b Fig. 5. Characteristic subsurface failure of D 4 specimens. The failure origin was detected to contain non-metallic inclusions like various oxides ( Al 2 O 3 , MgO , CaO ). A SEM examination, which is illustrated in Figure 5, confirmed the presence of di ff erent oxides like Al 2 O 3 , MgO and CaO . The presence of these elements is attributed to the refinement within the melting process, which is described by Yang et al. (2010) amongst others. The examined specimen in Figure 5 achieved 4.767.440 cycles to failure accord ing to Table 1. No defects were found at two fractured surfaces beyond 2 × 10 7 cycles. Proven defects were measured ( √ area ) from 9.57 to 53.56 µ m. The average defect size is 28.98 µ m.