PSI - Issue 71

Prasanna Dupare et al. / Procedia Structural Integrity 71 (2025) 118–125

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duration. For a 1-minute hold test, stress relaxation occurred rapidly. Maximum stress relaxed during a 1-minute hold was found to be approximately 50 MPa. For 10 minutes hold, stress relaxation occurred in two stages with two different stress relaxation rates. Stress relaxation with higher relaxation rate during the initial 10% hold time was followed by relaxation with diminishing rates. For the initial 10% (approx. 60 sec) of hold time, approximately 60 MPa relaxation in stress occurred with a stress relaxation rate of 1 MPa/ sec, whereas for the remaining 90% (approx. 540 sec) of hold time, 40 MPa stress was relaxed with a rate of 0.074 MPa/sec. Maximum stress relaxation during 10 minutes hold was found to be 100 MPa. The initial relaxation with high rates corresponds to matrix detefaol.r, m19a9ti3o)n. Iwnehlearsetaics srterlaaixnartaioten cinansebceocnadlcsutlaagteedwuitshinsgl oewq.2rate associated with c reep damage (Rodriguez . = . + . =0…………………………………………(1) = − = − 1 …………………………………..(2) Where, ε t is total strain, ε c is creep strain, ε e is elastic strain, E is modulus of material For 1 minute hold test, creep strain rate was 4.73x10 -6 s -1 . For 10 minutes hold test, creep strain rate during initial relaxation stage was 5.68x10 -6 s -1 and second stage was 4.20x10 -7 s -1 . Accumulation of plastic strain during the hold time was observed, which was linked to the creep damage. The formation of creep cavities during the hold, interacting with fatigue cracks, and was found to enhance the fatigue crack growth and ultimately lead to reduction in fatigue life(Goyal et al., 2018b)

Fig. 5 Stress Relaxation Curves for 1 minute and 10 minute hold CFI test

3.4 Fractography Fracture surface of specimen tested without hold time i.e. 0 minute (LCF/continuous cycling), with hold time of 1 minute and 10 minutes at strain amplitude of ±0.25% and temperature 650 °C were examined under scanning electron microscope (SEM). Fig.6 (a) shows fracture characteristics of without hold test (Continuous cycle LCF test). It depicts fatigue crack initiation, crack propagation, and final fracture. A single crack initiation site was observed in without hold test and growth of this primary crack was responsible for final fracture. Fig. 6 (b) shows well defined striation for LCF specimen which was an indication of transgranular fracture. There were no intergranular cracks observed for without hold specimen indicating fracture is completely associated with matrix damage.