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. 5. Evolution of small cracks under axial loading at ε eq = 0.005: (a) at 70% of fatigue life, (b) few cycles before failure.

Fig. 6. Evolution of small cracks under axial loading at ε eq = 0.002: a) at 90% of fatigue life, b) failure.

In both cases, the main crack formed in the very last cycles, that is less than 5% of fatigue life. The identified damage mechanism for axial loading in the applied loading levels range was a coalescence of small cracks. The initiation and growth of small cracks followed a shear damage mechanism. Cracks initiated and grew in directions of maximum shear strain, which are ± 45° for tension-compression. Next, the cracks linked with each other without a change in direction. Cracks initiated and propagated on the whole surface, in directions of maximum shear strain, that is from 0° to 45° to the horizontal axis, up to 15 to 100 μm of length. After that, they coalesced to form the main crack. The same damage mechanism was observed under torsional loading (Fig. 7). Small cracks initiated and grew in the directions of maximum shear strain, that is parallelly and perpendicularly to the specimen axis. Next, the coalescence was observed. For this case of loading, small cracks grew longer, up to 400 μm, and, by contrast, they crossing most often. This was very rarely observed under axial loading. This is the only loading case where the value of normal deformations is zero on the plane of maximum shear strain. Perhaps, this explains the above-described phenomenon because cracks do not interact with each other.