PSI - Issue 65

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

207

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

8

a

b

0 2 4 6 8

200 250 350 450 550 650 Stress, MPa

100 1000 Cycles Number to Failure,N f

100 1000 Cycles Number to Failure, N f

Fig.9. Fatigue curves of the soft and hard low-cycle loading of steel 22k in stress (a) and strain (b) strain conditions at 270 0 C.

Quasi-static failure occurred. At the number of cycles to failure in the 2010 cycles, the strain from the initial cycle to the time of failure decreased from 3.15% to 0.34% (i.e., almost 10 times, and the accumulated strain was only 2.8%. Representation of the rigid loading curve in stresses shows that strain aging is accompanied by a significant strengthening of the material with increasing number of loading cycles, reaching maximum values at states equal to 650 MPa (at the initial cycle stress of 394 MPa) at a durability of 88 cycles and 460 MPa (at the initial stress of 245 MPa) at 1672 cycles of failure. In both cases, the ultimate stresses exceeded in the first case by 2.7 times and the stress at rupture was at the level of the tensile strength, in the second case by 1.9 times. A comparison of the fatigue curves of soft and hard loading shows that, for practically equal durability (86 and 88 cycles), the initial stress under soft loading is 1.5 times higher than under hard loading. At the same time, under hard loading, the limiting stresses reach the initial soft-loading stress (Fig.8,b). The stresses under soft loading change insignificantly at the moment of failure (Fig.4,a). A comparison of the durability of the 2010 soft loading and 1672 cycles of hard loading shows that the initial stress of soft loading is 2 times higher than the initial value of hard loading. At the same time, the initial strain of soft loading exceeded the initial strain of rigid loading by 3.3 times. In the entire durability range, the initial strains and stresses of soft loading significantly exceeded those of rigid loading (Fig.9). Along with this, it should be considered that strain aging, by strengthening the material, promotes the occurrence of brittle states. Thus, the experiment shows that at a given load amplitude (soft loading), the effect of strain aging is manifested as a continuous decrease in the width of the hysteresis loop, reaching minimum (critical) values at the moment of the limit state (Fig.8,a). At a given range of elastic-plastic deformations (rigid loading), the effect is associated with a significant increase in the stress with increasing number of loading cycles (Fig.8,b). Maximum (critical) stresses are observed when the material exhausts its bearing capacity (limit state). 1. Intensive strain aging actively affects the redistribution of strain damage in the half-cycles of tensile, accumulated, and elastic strains. 2. Strengthening due to strain aging significantly reduces the damage caused by accumulated strain and promotes the development of deformation processes in the uniform region. 3. In strain aging, the state of macro crack formation and final brittle fracture. 4. Quasistatic fracture is observed only at durations 100 cycles. 5. The strain-kinetic criterion of fatigue failure can be used to assess the levels of accumulated damage and limit states in the presence of deformation aging processes. 6. The most significant effect of strain aging was observed in the initial half-cycle of loading. 7. Due to the decrease in plasticity resulting from strain aging, damage due to elastic deformation increases with increasing durability. 8. Under low-cycle loading, strain-aged materials had better resist fracture in the soft deformation mode. 9. The bearing capacity of a structural material is evaluated by its true strength and corresponding plastic deformation. 4. Conclusions.