PSI - Issue 7

Ludvík Kunz et al. / Procedia Structural Integrity 7 (2017) 44–49 Ludvík Kunz / Structural Integrity Procedia 00 ( 201 7) 000–000

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3. Experiments Creep/fatigue tests were conducted on a modified resonant pulsator Amsler. The specimen was heated in an electric resistance furnace on air. The temperature gradient 0.2°C/mm on the specimen gauge length was determined by means of three thermocouples placed in the middle and on the ends of the specimen gauge length. During the fatigue test the thermocouple which was attached to the specimen in the middle of the gauge length was used for the temperature measurement and the second one placed on the lower end of the gauge length was used for temperature control during the test. The long-term stability of specimen temperature during the testing was better than ± 1 °C. The desired temperature of testing was reached at controlled zero load. The mean stress was applied 1 h after reaching the desired temperature from the reason of temperature stabilization of the whole loading system. Then the cyclic stress amplitude was applied during a ramp of the length of some thousands of cycles. The frequency of cyclic loading was about 120 Hz. The elongation of the specimen during the test was measured on machine crossbar. 4. Results Results of the experimental determination of the dependences of time to fracture on the high-frequency stress amplitude superimposed on mean stress are shown in Fig. 3. Fig. 3(a) shows the behavior of IN 713LC superalloy loaded at the temperature of 800 °C and Fig. 3(b) the behavior of MAR-M 247 at the temperature of 900 °C. The general trend of the influence of increasing stress amplitude on the time to fracture, which is indicated by full curves in Fig. 3, is the same for both the superalloys. Superposition of small stress amplitudes does not influence the

a

b

1000

1000

mean stress 300 MPa 350 MPa 450 MPa

mean stress 450 MPa 500 MPa 600 MPa

T = 800 °C

T = 900°C

100

10 Time to fracture t f [h] 100

10 Time to fracture t f [h]

1

0 40 80 120 160 200 σ a [MPa]

0

40

80 120 160

Stress amplitude [MPa]

Fig. 3. Influence of superimposed stress amplitude on the mean stress on the time to fracture of (a) IN 713LC, (b) MAR-M 247.

c

b

a

C

Fig. 4. Fracture surfaces of (a) IN 713LC failed at mean stress 600 MPa, (b) IN 713LC failed at mean stress 600 MPa with superimposed stress amplitude 40 MPa, (c) MAR-M 247 failed at mean stress 300 MPa and stress amplitude 40 MPa.