PSI - Issue 65

A.N. Romanov / Procedia Structural Integrity 65 (2024) 200–208

205

A.N. Romanov / Structural Integrity Procedia 00 (2024) 000–000

6

a

b

700

600

500

400

1

10 100 1000 Cycles Number, N

Fig.4. True stresses in the tensile half-cycle (a) and elastic deformation damage (b) of 22k steel at 270 0 C in the soft loading regime.

a

b

c

d

Fig.5. Strain damage (a)-(c) in half-cycle tensile (square), accumulated (triangle), elastic strain (rhombus), total damage for each specimen (black circle), and total damage for three specimen samples (d) of 22k steel at 270 0 C

As shown in Fig.5, in the investigated durability range, the main damage occurred due to reversible plastic deformation (strain in half-cycle tensile) under mild loading conditions. Damage from unilateral strain accumulated in the first loading cycle remained constant for most of the durability and only at quasi-static failure (Fig.5,a) amounted to a significant value (13%). The increase in durability (Fig.5,a-c) was accompanied by a continuous increase in the damage fraction due to elastic deformation from 0.068 (6.8%) at N f = 86 cycles to 0.287 (28.7%) at N f = 2010 cycles, i.e. a 4-fold increase. A further increase in the damaging role of elastic deformation is observed with increasing durability, and this deformation becomes the main one in multicycle fatigue. The kinetics of the final damage calculated from Equation (6) are presented in Fig.5,d. Under loading in a given range of elastic-plastic deformation, hardening from strain aging promotes active stress growth (Fig.6,a), especially after a transient period (100 cycles of loading). Damage due to elastic deformation was not greater than 20% (Fig.6,b).