PSI - Issue 28

Avanish Kumar et al. / Procedia Structural Integrity 28 (2020) 93–100 Avanish et al. / Structural Integrity Procedia 00 (2019) 000–000

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Table 2. Summary of mechanical properties of produced steels obtained from various mechanical tests performed at room temperature. YS – Yield strength, UTS –Ultimate tensile strength, TE – Total % elongation (Kumar and Singh, 2018b)

Charpy impact Energy (J)

Ultimate tensile strength (MPa)

Fracture toughness, K 1C (MPam -1/2 )

Yield strength (MPa)

Total elongation (%)

Specimen

NB250

1560±32

1807±156

7.2±0.16

29.1±1.2

6.5±0.7

NB300

1382±20

1676±7

14.1±2

37.1±2.8

11±1.4

NB350

1028±52

1285±27

25.7±3.65

45.6±1.8

14.75±0.35

In order to get a microscopically sharp crack ahead of the machined chevron notch, fatigue pre-cracking was done under ∆ K decreasing (force shedding) test method to get a nominal crack size of 10.5 mm (Kumar and Singh, 2019). In the next step to determine the threshold stress intensity factor range ∆ K th , load was shed with an increase in crack length in a continuous manner with the normalized K -gradient, C = - 0.05 mm -1 using an automated technique and for a few specimen force shedding in steps was done manually. It was done such that the reduction in P max for any of the two steps was less than 20% and measurable crack extension was allowed before proceeding to the next step (Conshohocken, 2016). The ∆ K th of three steels for a fatigue crack growth rate of ~ 10 -9 m/cycle is given in Table 3 (Kumar and Singh, 2019). This step of fatigue test has given the stage-I of the typical d a /d N – Δ K plot. The other two stages of d a /d N – Δ K plot were obtained by conducting the fatigue crack growth rate test under a constant load range. Finally a comprehensive d a /d N – Δ K plot as shown in Fig. 2 was obtained by combining the results of all the three stages of crack growth.

Fig. 2. Fatigue crack growth rate (d a /d N ) versus stress intensity factor range ( ∆ K ) for produced steels

The ∆ K th values of three steels as given in Table 3 suggest that the steel with coarser microstructure and higher RA content offers higher resistance to crack growth near the threshold regime. Since nano-structured bainitic steels with a range of microstructure were tested at a low stress ratio of 0.1, roughness-induced and phase-transformation-induced crack closure may play an important role in retarding the fatigue crack growth near the threshold (Suresh, 1998). Fig. 3 shows the 3-D image of fractured surface near the threshold region of crack growth. The S a values shown on the image represent the surface roughness of the respective sample. It indicates that the steel with finer microstructure has