PSI - Issue 17

Zizhen Zhao et al. / Procedia Structural Integrity 17 (2019) 555–561 Author name / Structural Integrity Procedia 00 (2019) 000 – 00

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Fig. 6 Fatigue life prediction for CRF tests by (a) LDS rule and (b) modified LDS rule

4. Conclusions

Ratcheting-fatigue tests of 2.25Cr1MoV forge steel were conducted at 455 o C with various hold periods at peak and valley stress in this study. Effects of test parameters, i.e. mean stress, stress amplitude, holding period as well as holding direction on fatigue behavior were investigated. A creep-ratcheting fatigue life prediction model based on the linear damage summation rule was proposed. Main conclusions were drawn as below. (1) The existence of mean stress caused the accumulation of inelastic strain in the tensile direction. The incorporation of peak holding period widened the stress-strain hysteresis loop, while creep strains created in peak and valley holding periods made the hysteresis loop even wider. Stress holding stimulated the accumulation of inelastic strain. (2) The increase of mean stress or stress amplitude would shorten fatigue life, and fatigue life reduced exponentially when hold period was incorporated. Longer hold period resulted in shorter fatigue life. But the fatigue life was similar when the sum of hold period is equal, regardless of the holding direction. (3) Fatigue damage was a bit higher than creep damage at short hold period, but creep damage became dominant when hold period was longer than 5s. The creep damages created during stress holding led to shortened fatigue lives in CRF tests. (4) The linear damage summation rule provided satisfactory prediction results for peak holding mode. A new model was proposed by incorporating stress ratio to the linear damage summation rule, and the prediction for peak/valley hold was improved.

Acknowledgements

The authors gratefully acknowledge financial support for this work from the National Natural Science Foundation of China (No. 51435012) and Ph.D. Programs Foundation of Ministry of Education of China (No. 20130032110018).

References

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