PSI - Issue 19

Wolfgang Schneller et al. / Procedia Structural Integrity 19 (2019) 556–565 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Table 5. High cycle fatigue test results

Scatter Band LLR

Comparing to M-AB

Difference M to UP

Slope FLR

Condition

 f(50%)

M-AB UP-AB M-HIP UP-HIP M-SA UP-SA

0.253 0.087 0.288 0.109 0.268 0.109

Basis

Basis

12.99

1:1.14 1:1.57 1:1.04 1:1.43 1:1.07 1:1.43

-65.6% +13.8% -56.9% +5.9% -56.9%

-65.6%

5.20

Basis

19.37

-62.2%

4.30 8.17 4.54

Basis -59.3

3.5. Fracture surface analysis A fracture surface analysis for each condition and each specimen is carried out. The outcome of this investigation is that there are different failure modes for the unprocessed and machined condition. For each machined condition, an internal imperfection, such as pores (observed for the AB condition, as shown in Figure 5.(a)) or microstructural inhomogeneities like Si-rich phases (observed for the HIP and SA condition) is responsible for failure. Regarding the machined HIP specimens, solely the debonding of Si-crystals provoke the failure, see Figure 5.(b). For the machined SA condition, pores, as well as Si-rich phases are found within the crack initiation site and therefore, shows a combined failure mode.

Fig. 5. (a) Fracture surface of one as-built specimen; (b) Failure origin of one HIP specimen

On the contrary, for each unprocessed condition the failure is caused by a specific surface roughness feature, exemplary depicted in Figure 6. No signs of pores or microstructural inhomogeneities are found at the failure origin. A certain combination of notch depth and notch root radius, which results in the highest stress concentration, is responsible for failure. The substantive effect of the surface roughness regarding fatigue is also reported in [27,31,32]. Summarizing, one can say that if the as-built surface is present, internal imperfections are negligible and roughness characteristics are decisive for the fatigue strength and dominate all other imperfections [33,17] what leads to significantly reduced cyclic material properties.