Cyclic plastic deformation and damage in 304LN stainless steel

1. Cyclic plastic deformation and damage in 304LN stainless steel --Surajit Kumar Paul et al. Reporter: Yong Wang Supervisor: Professor Xu Chen

2. Highlights • LCF and ratcheting response, ratcheting–LCF interaction, and LCF and ratcheting damage evolution in 304LN stainless steel are experimentally investigated. • True stress controlled ratcheting test procedure is adopted in this investigation to assess the materials ratcheting performance.

3. Experiment • AISI 304LN austenitic stainless steel was available in the form of pipe with 320mm outer diameter and 25mm thickness. • Cylindrical specimens of 7mm gauge diameter and 13mm gauge length were machined from the pipe in such a way that the loading axes of the specimens were parallel to the pipe axis.

4. Low cycle fatigue • LCF tests were performed at the total strain ranges of ±0.2%, ±0.5%, ±0.7%, ±0.85%, ±1.0%, ±1.2%, ±1.4%, ±1.6%, ±1.8% and ±2.0% with strain rate of 1×10−3 /s. • below the strain amplitude of 0.5%, the material displayed negligible initial cyclic hardening followed by progressive cyclic softening throughout its life • at strain amplitude of 0.7–1.2%,the initial pronounced cyclic hardening and followed by mild progressive cyclic softening is exhibited • at strain amplitude larger than 1.4%, the material exhibited very rapid cyclic hardening in initial few cycles and followed by mild gradual cyclic hardening almost without reaching its saturated value till final fracture

5. Ratcheting • True stress controlled ratcheting responses of 304LN stainless steel are examined in this investigation.

6. Ratcheting

7. Ratcheting plastic strain amplitude and plastic strain energy are damaging parameters in cyclic plastic deformation, therefore damage growth is delayed in the presence of mean stress and hence the ratcheting life is prolonged.

8. Effect of pre-ratcheting on subsequent LCF behavior • Pre-ratcheting tests are performed at 0.1%, 0.25% and 0.5% of its ratcheting life, i.e. interrupted at 198, 450 and 990 cycles. After pre-ratcheting, LCF tests are conducted with 0.7% strain amplitude. • Reason behind choosing 0.7% strain amplitude is that average stress amplitude in LCF is close to the ratcheting stress amplitude, so that other effect in cyclic plastic deformation (i.e. strain range effect, loading dependent cyclic hardening/softening, etc.) can be avoided.

9. Effect of pre-ratcheting on subsequent LCF behavior • total cyclic plastic damage evolution path in LCF with pre-ratcheting • remaining LCF life fraction versus ratcheting strain accumulated during pre-ratcheting

10. Effect of pre-LCF on subsequent tensile behavior • Pre-LCF experiments with 1.0% strain amplitude are interrupted at 15, 200, 510 and 800 cycles, i.e. 0.015, 0.2, 0.5 and 0.78 of LCF life fractions. LCF life fractions can be determined by the ratio of current number of cycles and number of cycles to failure (for 1.0% strain amplitude is 1017). • Those pre-LCF damaged specimens are then used for tensile test.

11. Effect of pre-LCF on subsequent tensile behavior • yield stress, ultimate tensile stress and uniform elongation variation with LCF life fractions

12. Effect of pre-LCF on subsequent tensile behavior Factographs and image processed factographs in pre-LCF (strain amplitude 1.0%) followed by tensile test: a1, b1, c1, d1 and e1 are factographs; a2, b2, c2, d2 and e2 are image processed factographs at 0, 15, 200, 510 and 800 cycles pre-LCF.

13. Effect of pre-LCF on subsequent tensile behavior • average dimple size versus number of cycles • dimple number in unit area versus number of cycles

14. Effect of pre-ratcheting on subsequent tensile behavior • Pre-ratcheting experiments with mean stress of 120MPa and stress amplitude of 420MPa are interrupted at 50, 450 and 1500 cycles, i.e. 0.025, 0.23 and 0.76 of ratcheting life fractions

15. Effect of pre-ratcheting on subsequent tensile behavior • yield stress, ultimate tensile stress and uniform elongation variation with ratcheting strain fractions • yield stress in tensile test of pre-ratcheting specimen, plastic strain amplitude and hysteresis loop area variation with number of cycles

16. Effect of pre-ratcheting on subsequent tensile behavior • Factographs and image processed factographs in pre-ratcheting (mean stress 120MPa and stress amplitude 420MPa) followed by tensile test: a1, b1, c1 and d1 are factographs; a2, b2, c2 and d2 are image processed factographs at 0, 50, 450 and 1500 cycles pre-ratcheting.

17. Effect of pre-ratcheting on subsequent tensile behavior • average dimple size versus number of cycles in pre-ratcheting • dimple number in unit area versus number of cycles

18. Conclusions • Ratcheting life improves with increasing mean stress. Mean stress dependent hardening, i.e. decreasing hysteresis loop area and plastic strain amplitude with increasing mean stress, thought to be responsible for the improvement in ratcheting life. • Pre-ratcheting significantly reduce subsequent LCF life. Permanent ratcheting strain accumulation can be the root cause of LCF life reduction. • Pre-ratcheting alter the hardening/softening behavior in succeeding LCF tests • Tensile properties, i.e. yield stress, ultimate tensile stress and uniform elongation alter in a systematic manner with LCF and ratcheting damage. • Average dimple size increases and dimple number in unit area decreases with LCF damage whereas, average dimple size decreases and dimple number in unit area increases with ratcheting damage.

19. Review • Effect of pre-ratcheting on subsequent LCF behavior • Effect of pre-LCF on subsequent tensile behavior • Effect of pre-ratcheting on subsequent tensile behavior Plan • Effect of pre-tension(strain control and stress control) on subsequent LCF behavior • Effect of pre-tension(strain control and stress control) on subsequent ratcheting • Effect of pre-LCF on subsequent ratcheting

20. Thank you