PSI - Issue 75

D. Tousse Tchamassi et al. / Procedia Structural Integrity 75 (2025) 450–456 Tousse / Structural Integrity Procedia (2025)

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Indent (extensometer)

Crack deviations

Crack

PSBs

20 µm

100 µm

(a)

(b)

Fig. 7. Side surface views of a specimen of expanded material, fatigue tested at ±0.8%. (a) Surface crack and extensometer attachment; (b) enlarged view showing PSBs (bright lines) and crack deviation at high-angle boundaries of martensite. 4. Conclusions The effects of prior plastic deformation on the low cycle fatigue behavior of a high strength martensitic steel used for pipeline connections were examined using both a macroscopic approach and an examination of damage and fracture development, leading to the following results: • Whatever the amount of prior applied strain, a similar fatigue strength behavior was found, with a Manson-Coffin behavior associated to an exponent of -0.7; • Cyclic softening tended to yield a unique relationship between the current stress amplitude and the current number of applied cycles; the amount of cumulative plastic strain at fracture correlated with the number of cycles at failure in an approximately linear relationship, with a slope close to 2.5×10 -3 ; • A slight reduction in fatigue lifetime (by about 10%) was observed whatever the prior deformation path, with respect to the undeformed material, for an amount of prior applied strain between 2% and 4%; 2% prior tensile strain was shown to satisfactorily mimic the expansion process, regarding the low cycle fatigue lifetime; • Several crack initiation and propagation events from persistent slip bands were found in each specimen, whatever the testing conditions. As expected, the low cycle fatigue lifetime was driven by the crack propagation kinetics. Acknowledgements The authors wish to warmly thank Dr Patrick Aimedieu (Navier Laboratory, France) for his help regarding X-ray tomography data acquisition, Mr Abdennour Meddour and Mr Régis Clément (Mines Paris – PSL) for their invaluable help in setting up low cycle fatigue and tensile tests. Ms Asmae Elochi (Mines Paris – PSL) is gratefully acknowledged for fruitful discussions. References Guguloth K., Sivaprasad S., Chakrabarti D., Tarafder S., Low-cyclic fatigue behavior of modified 9Cr – 1Mo steel at elevated temperature, Mater. Sci. Eng., vol A604, pp. 196 – 206, 2014 Hagiwara N., Masuda T., Oguchi N., Effects of Pre-strain on Fracture Toughness and Fatigue-Crack Growth of Line Pipe Steels, J. Pressure Vessel Technol, 123(3), pp. 355-361, 2001 Moço R., Gonçalves F., Donato G., Effects of manufacturing plastic prestrains found on calendered and UOE pipes and pressure vessels on structural integrity assessments regarding fatigue crack growth and LBB, Procedia structural integrity, vol 13, pp. 1915-1923, 2018 Munier R., Doudard C., Calloch S., Weber B., Towards a faster determination of high cycle fatigue properties taking into account the influence of a plastic pre-strain from self-heating measurements, Procedia engineering, vol 2, pp. 1741-1750, 2010 Sivaprasad S., Tarafder S., Ranganath R., Effect of pre-strain on fracture toughness of HSLA steels, Materials Science and Engineering, vol 284, pp. 195-201, 2000