PSI - Issue 23

Tomáš Babinský et al. / Procedia Structural Integrity 23 (2019) 523–528 Babinský & Polák / Structural Integrity Procedia 00 (2019) 000 – 000

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5

b)

a)

Fig. 3. SEM micrographs of Rene 41 cycled at 0. 8 % at 23 °C (SE contrast): (a) Structure (slightly etched in OPS), (b) fracture surface in detail.

b)

a)

2

2

1st cycle 10th cycle 30th cycle 100th cycle 2000th cycle

1st cycle 10th cycle 30th cycle 100th cycle 2000th cycle

-2/E eff 2 · d 2 σ r /d ε r 2 ∙10 3 [MPa -1 ]

1,5

1,5

-2/E eff 2 · d 2 σ r /d ε r 2 ∙10 3 [MPa -1 ]

1

1

0,5

0,5

0

0

0

400

800

1200

1600

0

400

800

1200

1600

Fictive Stress, ε r E eff /2 [MPa]

Fictive Stress, ε r E eff /2 [MPa]

Fig. 4. Second derivatives of Rene 41 alloy. (a) Tensile half-loops, (b) compressive half-loops.

4.3. Evaluation of effective stress Effective stress is a function of temperature and strain rate (Polák (1991)) . Thus, saturated effective stress is a material constant for given temperature and strain rate and should not change during cycling. However, in real cycling it is impossible to keep those variables constant. According to Petrenec et al. (2013), the first minimum in the plot corresponds to the “rapid relaxation of the plastic strain in all plasticized volumes under decreasing effective stress when the direction of the straining has been reversed.” That being said, effective stress develops over time during cycling. To evaluate effective stress, we selected only first cycles so that we could capture the initial state of the material as precisely as possible. Effective stress was evaluated with the help of the linear approximation of the steep line leading to the peak of second derivative. Data are summarized in Table 2. Unlike 713LC alloy, the study on 738LC alloy by Petrenec et al. (2013) clearly distinguished the share of both γ and γ’ phases and, moreover, the cyclic response is shifted towards smaller fictive stress. That may be caused by relatively smaller fraction of precipitates in 738LC alloy leaving more space for dislocation slip in γ matrix. Thus,