PSI - Issue 23

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

272

4

Fig. 2 a) PSBs on the surface of sample cycled with constant total equivalent strain amplitude 0.44 % b) FIB cut through the same PSBs.

3.2. Microstructural changes in cycling TEM micrograph of specimen cycled in torsional mode with ε a,eq = 0.3 % is presented in Fig. 3. It was found that these changes in microstructure occurred during the cycling:  the dislocation density increased and dislocations arranged into characteristic structures formed by parallel thin band with high dislocation density separated by layers with low dislocation density;

 islands of α´ martensite are formed;  thin deformation twins appeared.

Since α´ martensite is a magnetic phase, its presence can be identified and measured by magnetic measurement. The results of TEM observations and of magnetic measurements suggest that phase transformation from austenite to α´ martensite is much more intense in case of pure torsional cyclic loading than in the other two modes.

Fig. 3 TEM micrograph of structure of specimen cycled in torsion with equivalent amplitude 0.3 %.

Fig. 4 Fatigue life curves of different loading modes.

3.3. Crack growth kinetics

The fatigue life in all three modes is presented in Fig. 4 as a function of total equivalent strain amplitude. Cycling in pure torsional mode leads to the longest fatigue lives while axial cycling results in the shortest lives.