PSI - Issue 13

Mehul Lukhi et al. / Procedia Structural Integrity 13 (2018) 607–612 M. Lukhi et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 4: Macroscopic stress vs. cycles, volumetric strain vs. cycles (Lukhi et al., 2018)

Fig. 3: Micromechanical cell model of NCI (Lukhi et al., 2018)

3. Results

3.1. Mechanism

The macroscopic stress response of the cell model under uni-axial loading is shown in Fig. 4. The macroscopic stresses in Fig. 4a exhibit initially a regime of cyclic hardening before the stress drops suddenly within a few cycles. The sudden drop in the macroscopic stress is related to the cyclic necking and void coalescence. When the macroscopic stress experiences the sudden drop, it is considered as a failure point in the simulation. Such a criterion is common to define failure in the fatigue experiments, too. Point A in Fig. 4 represents the cell model condition at the beginning of the simulation. Point B is the intermediate configuration of cell model during the simulation and point C defines the failure point. The void ratchetting process is shown using the cell model mesh conditions at point A, B and C in (Lukhi et al., 2018). The evolution of the macroscopic volumetric strain, which is due to the growth of the void volume, is shown in Fig. 4b. With increasing number of cycles, the volumetric strain increases steadily due the void ratchetting mechanism. When cyclic necking initiates at point C, the volumetric strain increases rapidly. The initial shape of the graphite particle (void) can be quantified by shape factor S . The value of S can vary between 0 to 1 (oblate particle to perfect sphere). Simulated strain-life curves for di ff erent values of S are shown in Fig. 5 together with experimental data for ferritic NCIs from literature. It is evident from Fig. 5 that the shape of graphite particle has a strong influence on the predicted fatigue life. Values S ≈ 0 . 70 are realistic for these grades of NCI. The figure shows that simulated strain-life curve for S ≈ 0 . 70 is in qualitative and quantitative agreement with the experimental data. For strain amplitude less than 1%, no void ratchetting is observed, and it is proposed that at lower strain amplitudes the failure is controlled by incipient crack formation (Lukhi et al., 2018). 3.2. E ff ect of shape of graphite particle

3.3. Load sequence e ff ects

Most structural components are subjected to cyclic loads with di ff erent amplitudes. Classically, loads with di ff erent amplitudes are evaluated by means of the hypothesis of linear damage accumulation (Miner-Palmgren rule). Devia tions from the Miner-Palmgren rule are termed as load-sequence e ff ects . The present micromechanics model allows to simulate LCF failure for arbitrary loading histories. In order to assess load-sequence e ff ects, simulations with block