PSI - Issue 23

Ladislav Poczklán et al. / Procedia Structural Integrity 23 (2019) 269–274 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Two types of tests were done in this study:

 Fatigue life test - for studying the mechanism of fatigue crack initiation (and measurement of fatigue behavior);  Crack growth rate test.

Fig. 1 Geometry of the specimen a) smooth specimen b) notched specimen.

Cyclic tests were done in three different loading modes:

 axial tension/compression mode;  pure torsional mode;  axial/torsional in phase mode with the ratio between the axial and the torsional equivalent strain / √3 = 1 . The cyclic loading was done at room temperature on a computer controlled MTS 809 servohydraulic machine. All tests were done in symmetrical mode (R  = -1) with constant total strain amplitude. The strain rate was chosen with consideration of the low thermal conductivity of the material and its value was constant in testing (triangular cycle), equal to 3×10 -3 s -1 . All crack growth tests discussed here, the same total equivalent strain amplitude  a,eq was adopted. It was calculated by Eq. 1. ε a,eq = √ 2 + 2 /3 (1) Since the notch shown in Fig. 1 b) was the initiation site for fatigue crack growth, a light microscope Navitar was placed to see this area and to take micrographs of growing cracks. The measuring of the crack growth was done by regular interruptions of the cycling. During each interruption, the micrographs of the cracks were taken and the crack length was determined as a half of the total crack length on surface. The specimens were also observed in a Field Emission Gun Scanning Electron Microscope (FEGSEM) Tescan Lyra 3. For studying the crack initiation mechanism, the cuts perpendicular to the specimen surface were prepared by Focused Ion Beam (FIB) technique. Changes in microstructure were studied in transmission electron microscope (TEM) JEOL 2100F. 3.1. Fatigue crack initiation In Fig. 2a), SEM micrograph of sample surface cycled in multiaxial in phase mode with ε a,eq = 0.44 % is presented. Several parallel PSBs are visible. The first fatigue cracks initiated along the bands. The coalescence of cracks from two neighbor PSBs is visible in the center of Fig 2a). In Fig. 2b), the FIB cut through the area in Fig. 2a) is shown. It can be seen that fatigue crack initiate in PSB and grew along them. This finding is in agreement with studies of Tanaka and Mura (1981) and Po lák and Man (2014). The same mechanism of fatigue crack initiation was observed in other two modes, too. 3. Results and discussion