PSI - Issue 68

5

S. Ghosh et al. / Procedia Structural Integrity 68 (2025) 1329–1336 S. Ghosh et al. / Structural Integrity Procedia 00 (2025) 000–000

1333

(R P0.2 ) of H-Si and M-Si DQP steels were estimated to be 926 and 918 MPa, respectively. The corresponding average ultimate tensile strengths (R m ) were 2137 and 2086 MPa, respectively.

Table 3. Tensile properties of the investigated DQP steels (Ghosh et al. 2022). Steel RA (%) C RA (%) HV5 R p0.2 (MPa) R p1.0 (MPa) R m (MPa) A (%) H-Si DQP 15 0.63 603 926 ± 45 1318 ± 28 2137 ± 49 12 ± 0.4 M-Si DQP 13 0.59 598 918 ± 36 1261 ± 33 2086 ± 44 13.6 ± 0.5

2,01E-03

(b)

(a)

1,51E-03

! ! # "

1,01E-03

5,10E-04

M602 M-Si

1,00E-05 !!#" , %/'(')*

M601 H-Si

50 ∆,,-." %

20

30

40

60

70

ΔK, Mpa.√m

Fig. 2. a) FCGR test specimen, b) fatigue crack growth rate (da/dN) vs. stress intensity factor range ∆K for DQP steels.

Figure 2a represents the specimen geometry used for FCGR tests, and Figure 2b depicts corresponding results. Crack growth rates are slower in the case of M-Si DQP steel compared to H-Si DQP steel. The high FCG resistance in M-Si DQP steel is attributed to the synergistic effects of plasticity-induced crack closure, crack deflection mechanisms and TRIP of RA. In M-Si steels, the presence of bainite together with RA helps induce plasticity around the crack tip, which helps to close the crack under lower stress cycles and slows further crack propagation. Figure 3 shows the evolution of the cyclic and monotonic plastic zone sizes for both the steels, calculated according to Eq.1 and Eq.2, respectively. 2 % = & ! + " ! ' " , ( (1) 2 ) = & ! + ( ∆ ' " " , ( (2) where, % = maximum stress intensity factor at a given crack growth rate, + = yield strength , ∆ = stress intensity factor range, % = monotonic plastic zone, and ) = cyclic plastic zone. As the crack propagates with increase in ∆ , . . , both the cyclic and monotonic plastic zone sizes increase, which means that a larger amount of the DQP steel microstructure shall be plastically deformed. Cheng et al. (2008) have shown that the cyclic plastic zone is the zone where most of the phase transformation (including TRIP) shall take place, whereas the monotonic plastic zone implies the effect of overall plasticity of the microstructure controlling the crack propagation. As the crack propagates, an increase in the cyclic plastic zone size indicates incremental increase in the amount of RA transformed. Although the growth of the plastic zones was quantitatively comparable in both the steels, the M-Si DQP steel with its multiphase microstructure comprising austenite-bainite-martensite microstructure shall lead to greater plasticity due