PSI - Issue 19

Martin Nesládek et al. / Procedia Structural Integrity 19 (2019) 231–237 Nesládek et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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3

2. Experimental campaign

The material under investigation is a CrMo rotor steel. DOA is calibrated by high strain rate isothermal LCF tests. The assumed temperature range is 20 – 600 °C, from which four levels of temperatures (20, 200, 400 and 600 °C) for the isothermal LCF and static tensile tests were selected. From the LCF experiments, the isothermal cyclic stress strain curves (CSSCs) at saturation may be derived and used to calibrate the material constitutive model. The Chaboche non-linear kinematic hardening procedure (Chaboche, 1989) has been used along with viscoplastic correction according to Nagode et al. (2012). Dwell period strain-controlled tests were carried out under 450, 550 and 600 °C to obtain stress relaxation curves required to calibrate the viscoplastic correction. More details on the experimental campaign, especially with regards to calibration of the temperature dependent material properties have been introduced recently by Nesládek et al. (2018). TMF tests were carried out under the conditions summarized in Tab. 1. Uniaxial strain-controlled push-pull loading was applied along with two ranges of temperatures. The tests under 100-600 °C are referred to as “cold start” tests, since these conditions are close to the ST operating regime that occurs after a longer shutdown period. The other tests, under 450-600 °C temperature range, are referred to as the “hot - start” experiments. These conditions are expected to be the most frequent in the turbine flexible operation with daily period. However, in the TMF experiments, both the mechanical loading and temperature were applied in the triangular waveforms with 240 s and 120 s period for cold-start and hot-start regimes, respectively. The plots in Fig. 1 show samples of the applied strain and temperature in the two assumed types of waveforms - the in-phase and out-of-phase. Thin-walled tubular specimens (Fig. 2) were manufactured from a forged CrMo alloy semi-finished product. Geometry with internal diameter d = 7 mm conforms to ASTM E 2207 standard. However, to prevent buckling of specimen gauge section under in-phase mechanical and thermal conditions, the internal diameter had to be reduced to d = 5.5 mm for this type of tests. Servohydraulic actuator is utilized to induce the axial strain. Resistive heating by water-cooled grips is applied and actual temperature in specimen gauge section is controlled by a welded thermocouple. The digital image correlation method (DIC) is employed to online axial strain control. A virtual extensometer is formed by two points in specimen gauge section where a random speckle pattern from a ceramic spray is applied. Before the start of the thermo-mechanical test block, the temperature expansion of the specimen gauge section is measured while controlling zero axial force. The acquired data are used for separating the mechanical strain from the total strain, which is detected primarily by the DIC system. 2.1. Thermo-mechanical fatigue tests

Tab. 1. TMF test matrix and the obtained specimen lifetimes.

Temp. [°C]

Axial strain [-]

Phase shift

Ød [mm]

Test No.

R [-]

N [-]

Lower

Upper

1 2 3 4 5 6 7 8

0.007 0.007 0.009 0.009 0.005 0.005

-1 -1 -1 -1 -1 -1 -1 -1

100 100 100 100 450 450 450 450

600 600 600 600 600 600 600 600

0

5.5

704

"Cold start"

400, 430 414, 436 270, 278

180

7

0

5.5

180

7

0

5.5

702

520, 487

180 180 180

7 7

"Hot start"

990, 916, 1092

0.0035 0.0030

7 1609, 1268, 1627