PSI - Issue 36

Robert Kosturek et al. / Procedia Structural Integrity 36 (2022) 153–158 Robert Kosturek, Janusz Mierzyński, Marcin Wachowski et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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The obtained plots allow to state that friction stir welded joints of AZ31 are characterized by three stages of fatigue life and for all investigated total strain amplitudes we can distinguish: a very short period of cyclic hardening (up to 100 cycles) followed by cyclic stabilization period and a relatively long period of cyclic softening until failure (Fig. 1a-c). The analysis of the hysteresis loops (Fig. 1d) indicates that dissipation of energy at ε=0.5% is on a similar level for all investigated joints. Data from stabilized hysteresis loops allowed to establish the relationship of stress amplitude versus plastic strain amplitude with appropriate equations describing the curves for each joint (Fig. 2).

Fig. 2. Stress amplitude versus plastic strain amplitude with established equations for obtained curves.

The obtained curves (Fig. 2) can be described by the power function: σ a =k ′(ε ap ) n ′

where σ a is the stress amplitude [MPa], ε ap is the plastic strain amplitude [mm/mm], k ′ is the cyclic strength coefficient [MPa], and n ′ is the cyclic strain hardening exponent. The values of k’ and n’ are established directly from function describing plots in Figure 2 and presented in Table 4. Tab. 4. The established low-cycle fatigue parameters for the tested samples. Sample Cyclic strength coefficient, k ′ Cyclic strain hardening exponent, n ′ Coefficient of determination, R 2 M1 979.12 MPa 0.2523 0.9983 M2 1171 MPa 0.2804 0.9514 M3 1176 MPa 0.2844 0.961 M4 897.24 MPa 0.2347 1 The established values of k′ and n′ confirm that the low cycle fatigue behavior of the samples for all used welding velocities is very similar (Fig. 2, Tab. 4). The SEM images of selected fractured samples are presented in Fig. 3.