Issue 50

A. Sarkar et alii, Frattura ed Integrità Strutturale, 50 (2019) 86-97; DOI: 10.3221/IGF-ESIS.50.09

Figure 2 : Block loading sequence under strain control used in the crack growth experiments.

under strain control with  t

Figure 3 : Fatigue crack propagation behavior under LCF-HCF interaction with different B s

/ 2 LCF

: ±0.6%

and  t

/ 2 HCF

: ±0.1% (Equivalent Δσ HCF

: 150 MPa) T: 573 K

R ESULTS AND DISCUSSION

Establishing a critical criterion for occurrence of LCF-HCF interaction ig.3 presents the variation of crack length with the number of loading blocks at 573 K, under a loading sequence given in Fig. 2. It is observed that the crack propagation was similar irrespective of the B s till a ‘critical’ crack length (a cr ) was attained. However, once the crack length extends beyond a cr , significant acceleration in the rate of crack growth was noted, depending on the B s . The rate of crack propagation is found to be highest for a higher B s of 200 compared to the lower B s of 10 and 1 (only LCF cycling). This resulted in the shortest fatigue life in the former case (B s : 200) amongst all the three B s used in the present study. It may be noted that B s is an important variable in Fig. 3. The concept of “critical crack length” is based on the fact that the crack growth data coincides in a similar manner irrespective of the B s , till a particular crack length is reached, beyond which significant difference in crack growth data is observed with variation in B s . According to Miner’s rule, the damage under the HCF cycles in the block is considered as zero when the stress amplitude in HCF is much below the fatigue limit and hence cannot impart any significant damage. In the present case, since the HCF strain amplitude is very low (±0.1%), equivalent σ HCF is calculated using elastic modulus at 573 K for nuclear grade 316LN SS (0.07wt.%). At 573 K, E value for nuclear grade 316LN SS (0.07wt.%) was reported as 1.5 GPa [9]. Using a strain amplitude of ±0.1%, equivalent σ HCF is estimated as 150 MPa which was less than the fatigue limit [10 11]. The strain-ratio ( R ) during the HCF cycling is 0.71. However, the crack is found to propagate even during the HCF cycles, once a cr is reached, as reflected from the change in crack growth behavior with respect to B s (Fig. 3) indicating that Miner’s hypothesis may be untrue for the present experimental results. This suggests that even though the HCF stress is far below the fatigue limit, HCF cycling contributes significantly to the crack growth, once the length of the crack (which may be nucleated from the LCF cycle) reaches the critical value of a cr . In other words, strong LCF-HCF interaction prevails beyond a cr . It may be possible that crack-growth data overlap at low number of cycles (blocks). However, it may be noted that the present tests are carried out at fixed LCF strain amplitude of ±0.6%, varying only the HCF cycles superimposed on it ( B s ). This suggests that crack growth data should be similar for all the cases unless the HCF cycling imparts considerable influence on the crack growth. Hence, the difference incurred in the crack growth data is accounted F

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