PSI - Issue 60

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

453

10

At 823 and 873 K the absolute values of n v increased with increase in the amount of prior strain cycling. For the 0 N f specimen, the absolute n v value are smaller. This indicates that the prior fatigue damage accelerates the recovery process. Studies of Gottstein and Argon (1987) reveal that the variation of work hardening rate,  with stress,  , varies linearly for recovery process dominated by cross slip / glide and increases for recovery by climb and grain boundary migration. In view of this, increase in the absolute value of n v suggests a change in recovery mechanism from cross slip to that dominated by climb and sub-boundary migration processes, at higher temperatures and strains.

10

3000

500

Type 316 L(N) SS - 0N f 3x 10 -3 s -1

SS 316 L(N) - 0N f 3x 10 -3 s -1

(a)

(b)

5

2500

400

0

2000

-5

300

 s (MPa)

 i (MPa)

1500

nv

-10

 i

 s

823 K 873 K

200

-15

1000

823 K 873 K

-20

100

500

-25

0

10

20

30

40

50

0

10

20

30

40

50

Fatigue Life Fraction (% N f )

Fatigue Life Fraction (% N f )

Fig. 10 Variation of Voce parameters (a)  i and  s , and (b) n v with varying prior fatigue life fractions for the 316L(N) SS at 300, 823 and 873 K..

0.024

6x10 4

Type 316 L(N) SS 3x 10 -3 s -1

(b)

(a)

Type 316 L(N) SS 3x 10 -3 s -1

5x10 4

0.020

4x10 4

3x10 4

K 1 / 

0.016

2x10 4

300 K 823 K 873 K

0 N f 0.05% N f 0.1% N f 0.3% N f 0.5% N f

1x10 4

300 K 823 K 873 K

Workhardening rate (MPa)

0

0.012

4x10 -1

6x10 -1

8x10 -1

10 0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Plastic strain rate (s -1 )

n 1

Fig. 11 (a) Inter-relation between the strength coefficients and ductility exponents of the work hardening parameters of the Ludwigson and (b) the Universal relation between the work hardening with plastic strain rates for the 316L(N) SS subjected to various degree of prior fatigue damage at 300, 823 and 873 K. 3.3.4. Universal relation between plastic strain rate and work hardening rate As described in the previous section, the variation of Ludwigson parameters exhibited its characteristic behaviour with varying degree of prior fatigue deformation at different temperatures. However, in spite of the wide variation in Ludwigson parameters with temperature and prior deformation conditions, variation of K 1 /  plotted as a function of n 1 exhibited linear relation (Fig.11a). The strength coefficient is normalized with shear modulus,  , in order to compensate the influence of temperature on it. The observed linear relation between K 1 /   which is an index of strength, and the n 1 , which is an index of ductility, points to a universal relation between the evolving stress with the