PSI - Issue 60

174 10

Chitresh Chandra et al. / Procedia Structural Integrity 60 (2024) 165–176 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

layer is function of expose time, more the exposure, more the oxide amount. As evident from the Fig. 9 the exposure time in all the three stages for 600 sec hold time (both force ratio) is higher as compare to the 10 sec hold, therefore the amount of oxide is more in this case. Also, the amount of oxide is highest for the initial stages as compared to other two and least for the final stage Therefore the features at the final stages are visible. During the initial stage of the creep fatigue crack for 10 sec hold time for both force ratio the fracture surface is relatively flat whereas for the hold time of 600 sec the surface is not uniform. The sample tested with 0.5 force ratio and 600 sec hold time; dimples covered with oxide layers are visible. During the midterm stage the non-uniformity of surface increase for all the cases. The final stage comprises fast fracture consisting of dimples because of the high crack growth rate fracture at the end of test. For the sample tested at 0.5 force ratio and 600 sec holds, the micron sized voids are clearly visible near the dimples. Therefore, we can say that during the initial stage the creep is prominent damage mechanism for the high force ratio and hold time. 4. Conclusions Creep-fatigue crack growth tests were conducted on P91 steel specimens under different conditions of hold times and force ratios. The following conclusions were made - • The crack growth life is consistently lower for samples subjected to higher force ratio. • The (da/dt) avg vs (C t ) avg plot shows that creep is dominant damage mechanism at higher force ratio and hold time. • P91 has been proven to be creep ductile on the basis of ΔV e /ΔV vs N/N f plots, hence (C t ) avg is deemed to be apt for quantifying crack growth behaviour under creep conditions. • In spite of being oxidized, the fracture surface exhibits different features with the progress in the crack growth.

Acknowledgements The authors would like to acknowledge Dr. G. Sasikala formerly Head, MDTD and Dr. Shaju K. Albert, formerly Director MMG and Chairman AUSC PEC, IGCAR Kalpakkam, India, for support of this work.

Appendix A. List of Symbols

a

Crack length

a o a f

Notch length

a i B

Initial crack length Specimen thickness

Final crack length

B N

Net specimen thickness

W N f

Specimen width

N

Cycle

Final Cycle

N 0.2

Initial crack growth cycle

R

Force Ratio (P min /P max )

t h

Dwell period Force Range

P

Applied force

∆ P

P max

Maximum Force applied Average C-t parameter

P min ∆ K

Minimum Force applied Stress Intensity factor Range Time dependent crack growth rate

(C t ) avg da/dN

Cycle dependent crack growth rate

(da/dt) avg

U

Measured voltage corresponding to crack length ‘ a ’ Measured voltage corresponding to crack length ‘ a i ’

U f

Measured voltage corresponding to crack length ‘ a f ’ Total measured change in load line displacement Measured load line displacement during dwell period

U i

∆ V

∆ V e

instantaneous elastic part of ∆ V Effective Young’s Modulus

∆ V c

E’

E

Young’s Modulus