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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lower surface roughness whereas steel with coarser microstructure and higher content of RA has higher surface roughness. This indicates higher extent of roughness-induced crack closure and hence higher value of ∆ K th in steel transformed at higher austempering temperature. Therefore, it is evident that the coarser microstructure with plenty of RA can deflect the crack path and hence decreases the Mode-I driving force for crack growth as well as reduce the effective crack length in the macroscopic crack growth direction. The higher crack path tortuosity(macroscopic) with an increase in austempering temperature also makes the effective crack growth slower (Kumar and Singh, 2019). Moreover, prior work on near-threshold fatigue crack growth behavior also showed that coarser-grained materials showed higher fatigue crack growth threshold (Suresh, 1998).

Table 3. Summary of fatigue crack growth rate test results

Crack length to catastrophic failure, mm

Maximum stress intensity factor at failure K max , MPam -1/2

Paris-Erdogan law coefficients

Specimen

∆ K th , MPam

-1/2

c

m

R-squared

NB250

8.75

9.83×10 -10

4.08

0.9592

18.5

30.17

NB300

9.6

1.62×10 -9

3.74

0.9722

19.4

38.54

NB350

10

1.95×10 -9

3.64

0.9873

20.5

47.2

Fig. 3. 3-D image of fractured surface for roughness measurement near the threshold region of (a) NB250, (b) NB300 and (c) NB350 steel

Huo et al. (Huo and Gao, 2005) reported that there is development of compressive stress due to the volume expansion when RA transforms to martensite in plastic zone at the fatigue crack tip. This compressive stress assists in crack closure and thus slows down the crack growth. Since the steel transformed at higher austempering temperature has higher content of RA between BF laths, the probability of crack to encounter RA lath and time the crack spends in that phase increases. Moreover, lower carbon content in coarser RA at higher austempering temperature makes is mechanically and chemically more metastable to transform to martensite and thus higher phase-transformation induced crack closure effects. The stable fatigue crack growth region which follows the Paris-Erdogan law defined by: d a /d N = CΔ K m , where C and m are the intercept and slope of the regression line fitted to linear part of the d a /d N -Δ K plot of three steels are clearly distinguishable. Paris’ law coefficients are given in Table 3. It is very obvious from the plots that the rate of fatigue crack growth is higher for steel transformed at a lower austempering temperature. The Paris law regime of NB250 steel is smaller and it quickly enters the stage-III where the crack growth becomes unstable. On the other side, NB300 and NB350 steels show larger regions of stable crack growth approximately up to a Δ K value of 32 and 40 MPam -1/2 respectively and eventually fails once the crack length becomes critical crack length determined by the mode-I fracture toughness of the steel. It is known that the crack growth rate in stage-II is less sensitive to microstructure compared to near threshold crack growth because the cyclic plastic zone size becomes several order of grain size and hence continuum mechanism of crack growth follows (Ritchie, 1979; Suresh, 1998). However, Paris’ law coefficients show that the steel with the coarser microstructure shows marginally higher resistance to crack