PSI - Issue 60

K. Mariappan et al. / Procedia Structural Integrity 60 (2024) 444–455 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

451

8

Hollomon, Swift, Ludwigson and Voce are listed in the Table 1. Low  2 value indicates a better fit with the experimental data. Holloman relation (Eq.1) followed by Swift relation (Eq. 2) yielded higher  2 values for all the test conditions thereby indicating their inadequacy to describe the flow behaviour of 316L(N) SS. At 300 K, the flow curves of all the specimens i.e. 0 N f , 0.05 N f , 0.1 N f , 0.3 N f and 0.5N f and that of the as-received specimen, i.e. 0 N f , at 823 and 873 K were well described by the Ludwigson equation. Whereas, at the elevated temperatures of 823 and 873 K for the specimen with prior fatigue damage, i.e. 0.05 N f , 0.1 N f , 0.3 N f and 0.5N f , Voce equation gave better description of the flow behaviour. Figure 8 show the typical fit of Ludwigson and Voce relation applied to the as-received specimen tested at 300, 823 and 873 K. It may be observed that both Ludwigson and Voce relations may be used to describe the flow behavior closely. Figures 9 and 10 show the variation of work hardening parameters obtained for the Ludwigson and Voce equations, respectively with varying prior fatigue deformation. Since the  2 values obtained for Voce relation at 300 K were high, the parameters obtained at 300 K were not considered in the analysis and Fig. 10 shows the variation of Voce parameters at 823 and 873 K only.

Table 1. Typical  2 values obtained for different flow relationships for 316L(N) SS subjected to prior strain cycling up to various fractions of fatigue life at three different temperatures. Temperature (K) Prior strain Cycles (%)  2 values Holloman Swift Ludwigson Voce

300

0 5

927

18.9 15.5 15.6 14.6 18.1 17.9 13.3 12.3 11.5 10.8 12.1 16.2 14.3 11.5 12.3

10.2 11.4

11.3 12.2 13.7 10.7 12.3 11.2 10.5 9.2 9.1 7.3 9.7 8.6 8.3 7.9 10.7

1505 1685 1464 1579

10 30 50

7.8 7.6 6.0

823

0 5

133 389 321 122 431 346 259 124 91

10.8 12.1 11.9

10 30 50

9.4 8.9 8.9

873

0 5

11.6 10.8

10 30 50

9.2 9.7

78

3.3.1. Effect of Prior Fatigue Damage on Ludwigson Parameters Figures 9a-d show the variation of Ludwigson parameters K 1 , n 1 , K 2 and n 2 as a function of prior fatigue damage up to various fatigue life fractions. From the Figs. 9a, 9c and 9d, it is observed that the strength coefficient, K 1 and K 2 and the ductility constants n 2 in general, increased from the ‘ as received ’ condition to reach a peak value and further decreased with increase in the amount of prior fatigue damage at all the test temperatures. The strain hardening coefficient, n 1 , exhibited a continuous decreasing trend with increasing prior fatigue damage. The reduction in the value of K 1 , K 2 and n 2 , and increase in the n 1 values are more pronounced at the higher temperatures of 823 and 873 K. The Ludwigson parameter K 1 is the work hardening coefficient which is an index of strength and found to exhibit variation similar to cyclic stress response (Fig. 5b). Reduction in the strength coefficient K 1 has been reported by the Samuel et al (2006) from their simulation studies of Ludwigson curves with induced prior plastic strains. Vijayanand et al. (2011) have reported similar variation with prior plastic deformation i.e. prior cold worked 316L(N) SS. Girish et al (2007) have attributed the sudden fall of strain hardening exponent, n 1 , at 823 and 923 K to the predominance of thermally activated dislocation motion over dislocation retarding effects due to DSA which enhances the dislocation dislocation interaction induced recovery. The parameter K 2 is the measure of deviation in the true stress-true strain plot from Holloman relation. The observed variations in K 2 with varying prior strain cyclic damage were similar to that exhibited by the peak cyclic stress at all the test temperatures. The exponent n 2 , which is negative, expresses the rate at which the ratio between the short-range and long-range stresses decreases when the plastic strain increases [Soussan et al (1991)]. Higher absolute values of n 2 denotes the influence of long-range stress and also implies that