PSI - Issue 14

Sachin Bandgar et al. / Procedia Structural Integrity 14 (2019) 330–336 Sachin V Bandgar/ Structural Integrity Procedia 00 (2018) 000–000

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limiting value of both Kmax and ΔK must be met as per the stated unified approach. Hence based on the results and the range of data obtained in our work, plots of two significant parameters ΔK vs Kmax was plotted for various crack growth da/dN for Steel A and Steel B Fig. 6. From the plot in Fig.6 (a), in the case of steel A, it is evident that limiting Kmax value for crack to grow at da/dN = 2x 10 -4 mm/c with a load ratio of R=0.1 is 33.06 MPam 1/2 and R=0.5 is 50.834 MPam 1/2 . This indicates the limiting value of Kmax significantly increases with increasing R ratio. From the experimental results, limiting Kmax values for other crack growth rates viz. 3x 10 -4 mm/c and 4x 10 -4 mm/c are also shown in the fig.6. Similar behaviour of increased Kmax with increasing R-ratio has been observed for Steel B as well. 3.5 Fractographic analysis Scanning Electron Microscopy (SEM) of fractured surface is done at different ∆K to study the effect of stress intensity factor on crack morphology. It was found that secondary cracks are predominant in steel A at a load ratio of R=0.1 as compared to R=0.5. However for steel B, secondary cracks were found at both the load ratios. A representative fractograph of steel A and steel B at ∆K of 35 MPa*m 0.5 is shown in Fig 7 and 8.

Fig 7. (a) Steel A at R=0.1at ∆ K=35 MPa*m 0.5 ; (b) R=0.5 at ∆K=35 MPa*m 0.5

Fig 8. (a) Steel B at R=0.1 for ∆ K=35 MPa*m 0.5 ; (b) at R=0.5 for ∆K=35 MPa*m 0.5

4. Conclusion  Results indicated that there was increase in Paris slope 'm' and decrease in Y intercept 'C' with increase in load ratio for both steel A and B.  In respect of fracture mechanics parameters, the value of 'm' was 70% more and around 50% more than that of