PSI - Issue 14

Taslim D. Shikalgar et al. / Procedia Structural Integrity 14 (2019) 529–536 T.D.Shikalgar et al./ Structural Integrity Procedia 00 (2018) 000–000

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3.2. Mesh convergence study A mesh convergence study is also carried out to make sure that numerical load v/s punch displacement data is mesh independent. Three FE models of a pre-cracked specimen are prepared for this purpose. In three models, size of elements in front of crack tip and in thickness directions are 0.025, 0.035, 0.05 mm respectively and corresponding number of elements/nodes are 62557/69075, 46965/52125 and 30545/35361 respectively. Computed load v/s displacement curves for all the cases are shown in Fig. 6. It may be seen that there is no change in the results for all the three cases. Therefore, it may be concluded that numerical load v/s punch displacement curve is independent of the finite element mesh size.

Fig.6. Comparison of numerical LD data for 3 different element size

Fig.7. Computed J-integral v/s contour no. for different displacements

3.3. Calculations of J-integral The computed stress and strain data near the crack tip are then used to calculate applied J-integral. The domain integral method to calculate J-integral suggested by C. F. Shih (1986) is employed for this purpose. J-integral is calculated for the first twenty contours for specimen surrounding crack tip. It is observed that J-integral is approximately constant from contour no. 16 to 20 for p-SPT specimens. Fig. 7 shows a variation of calculated J integral v/s contour integral number for six punch displacement values. Fig. 8 shows a plot of average J-integral as a function of punch displacement. In case, it is assumed that crack initiates at the point of experimental peak load, the value of J-initiation is calculated as 246.78 kJ/m 2 and 344.02 kJ/m 2 of material 20MnMoNi55 and T91 respectively. These values are significantly higher than the reported value in literature (M. S. E1-Fadale et al. (1995), Contract report (2005)) for this material measured using standard CT specimens. Accordingly, micromechanics damage mechanics analysis using GTN model has been attempted to accurately predict the crack initiation load and hence the initiation fracture toughness. The next section describes this analysis.

Fig. 8. Estimation of crack initiation toughness using average J-integral v/s punch displacement of (a) 20MnMoNi55, (b) T91.