PSI - Issue 13

Takehiro Shimada et al. / Procedia Structural Integrity 13 (2018) 1873–1878 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

1876

4

calculated using the representative saturated cycle (N=50) data was within 1%. Therefore, in this paper, the creep fatigue damage is calculated using saturated cycle (N=50) data. Figure 4 shows schematic hysteresis loop of creep- fatigue tests (note that in Fig. 4, 1 means the start time to apply tensile stress, 2 is the start time to hold the strain, 3 is the end time to hold the strain, 4 is the start time to apply compressive stress , 5 is the time to reach the maximum compressive stress applied. The number 1-5 is used in equation (8)-(12) as the subscript to express the time).

Fig. 3. Comparison of experimental data and simulation results; (a) hysteresis loop, (b) relaxation curve during hold time. The experiment and simulation conditions are; mechanical strain: 0.4% and 1.0%, hold time: 10min, mechanical strain rate: 0.1%/sec

Fig. 4. The scheme of modified ductile exhaustion method explained by the typical hysteresis loop of creep-fatigue tests.

4.1. Time fraction rule method Fatigue damage per cycle is estimated by low cycle fatigue test result organized by inelastic strain range written in equation (8), and the time fraction rule for the creep damage per cycle is calculated by equation (9) where von Mises stress is normally used.

2 3

p   =

2 5 in in

) p m −

− ε ε

( f d B  = 

,

(8)

t

( r mises dt

3 =  

d

(9)

c

t 

)

t

2

4.2. Ductile Exhaustion method Creep damage can be also estimated by the ductility exhaustion method as described in equation (10). It is known that the ductile elongation decreases under low strain rate or low stress level (Hales, 1983). However, in this creep-