PSI - Issue 13

F. Bülbül et al. / Procedia Structural Integrity 13 (2018) 590–595 Fatih Bülbül / Structural Integrity Procedia 00 (2018) 000 – 000

593

4

surface morphology showing a non-elliptical fatigue fracture surface in Fig. 3a (orange line). Figure 3b shows a detail view of an area behind the micro-notch where the fracture surface morphology exhibits very smooth structures and deviates strongly with respect to its direction. In Fig. 4c, faceted structures are clearly depicted.

Fig 3. (a) Fatigue fracture surface of the experiment shown in Fig. 3; (b) view close to the micro-notch; (c) view showing facetted structures. 3.2. Long crack propagation behaviour in vacuum at increased stress amplitudes The fatigue crack propagation behaviour in vacuum was also investigated in detail at increased stress amplitudes. At the beginning of the fatigue experiment shown in Fig. 4, the sample was loaded with a stress amplitude of Δơ/2 = 120 MPa. The stress amplitude was stepwise increased by 10 MPa when the crack advance reached 100-200 µm. After achieving a stress amplitude of Δơ/2 = 140 MPa, the stress amplitude was stepwise increased by 5 MPa in the same manner. In the beginning part at Δơ/2 = 120 MPa , it can be seen that the crack propagated along an activated slip band up to Δơ/2 = 140 MPa on the left as well as right side (Fig. 4a and b).

Fig. 5. Crack propagation in vacuum at Δơ/2 = 150 MPa: (a) crack path; (b) EBSD analysis with calculated glide traces; (c) crack propagation rates of both crack tips.

Fig. 4. Crack propagation in vacuum with increasing stress amplitude: (a) crack path; (b) EBSD analysis with calculated glide traces; (c) crack growth rates of both crack tips.