PSI - Issue 28

Jan Seyda et al. / Procedia Structural Integrity 28 (2020) 1458–1466 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 7. Evolution of small cracks under torsional loading at ε eq = 0.008: (a) at 35% of real life, (b) failure.

In case of the out-of-phase loading, small cracks could be observed in many directions, since principal axes, so also the shear strain vector, rotate (Fig. 8). The most developed cracks were visible in the direction normal to specimen axis. It corresponds to the plane of maximum shear and normal strain. On the loading level corresponding to similar fatigue life as the highest level of axial loading, a very high density of small cracks was observed. At the end of specimen’s fatigue life, the coalescence of small cracks occurred.

Fig. 8. Evolution of small cracks under out-of-phase loading at ε equ = 0.008: (a) at 35% of real life, (b) failure.

The small cracks initiated in many directions, forming a dense net. It can be clearly seen on Fig. 8 (b) that the preferred direction of cracking is a horizontal plane, on which maximum shear and normal strain act. Differences in cracking behavior occurred at the lowest level of out-of-phase loading (Fig. 9). In which much less density of small cracks was observed. All cracks propagated in the horizontal direction, but only after 50% of fatigue life they could be observed on the replica. The main crack formed in the last few cycles of loading, from four cracks only, with an average length of 2 mm (Fig. 10). In Fig. 9 (d) and Fig. 10 (b) branching of cracks can be seen, forming due to the local change of the stress at the crack tip of suitable length. The shades at the bottom right on Fig. 10 (b) are the edge of replica tape.