PSI - Issue 7

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

333

I. Milosˇevic´ / Structural Integrity Procedia 00 (2017) 000–000

7

1.2

1.2

D7.5, conventional D7.5, Miner mod. Haibach

D4, conventional D4, Miner mod. Haibach Subsurface Initiation

1.15

1.15

Subsurface Initiation Poly approach D7.5 Miner elementary

1.1

1.1

Poly approach D4 Miner elementary

1.05

1.05

1

1

0.95

0.95

0.9

0.9

Standardised stresses, -

Standardised stresses, -

0.85

0.85

0.8

0.8

1E+5

1E+6

1E+7

1E+8

1E+9

1E+10

1E+5

1E+6

1E+7

1E+8

1E+9

1E+10

Cycles to failure, -

Cycles to failure, -

Fig. 10. S / N curve D 4 representing di ff erent approaches: k 1 - infinite life (Miner elementary); k 1 - k 2 (Miner mod. Haibach); Exponential projection fitted into double kinked S / N curve.

Fig. 11. S / N curve D 7 . 5 representing di ff erent approaches: k 1 - infinite life (Miner elementary); k 1 - k 2 (Miner mod. Haibach); Exponential projection fitted into double kinked S / N curve.

6. Conclusion

This work showed results from fatigue tests with a high strength material in the TA. Two di ff erent specimen diameters were used under T = 20 ◦ C and T = 350 ◦ C. All specimens were analysed by SEM to identify the fracture origin. One major defect type was found and defect sizes were measured ( √ area ) to obtain defect based fatigue strength. Following statements can be derived from present investigations. • All fatigue tests investigated by SEM showed defects containing Al 2 O 3 , MgO and CaO . No other inclusion type could be found. • At elevated temperatures T = 350 ◦ C internal driven crack initiation could be observed that started earlier respec tively at smaller cycles to failure. Crack initiation was mainly identified to start from the microstructure itself (slip planes). • The fatigue strength (HCF) insignificantly increased about 5% due to a smaller specimen size. Also a distinct temperature influence is observed. • Defect sizes of D 4 and D 7 . 5 couldn’t be compared due to significant size ( √ area ) di ff erences. All inclusions had a very sharp prismatic shape. • According to the presented results a constant amplitude limit can’t be recommended. Actual approaches, which use a second slope k 2 up to 10 9 cycles, showed a very conservative proposal. • Murakami’s √ area approach did not provide a satisfactory result. The defect size can’t be used to express a significant e ff ect on the fatigue strength.

Acknowledgements

The authors would like to acknowledge the financial support of the ”COMET - Competence Centres for Excellent Technologies Programme” of the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT), the Austrian Federal Ministry of Science, Research and Economy (BMWFW) and the Provinces of Styria, Tyrol and Vienna for the K1-Centre LEC EvoLET. The COMET Programme is managed by the Austrian Research Promotion Agency (FFG).

References

ASTM International, 1998. Standard Practice for Statistical Analysis of Linear or Linearized Stress-Life (S-N) and Strain-Life (e-N) Fatigue Data. doi: 10.1520/E0739-91R98 .