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J. Tong et alii, Frattura ed Integrità Strutturale, 25 (2013) 44-49; DOI: 10.3221/IGF-ESIS.25.07 47 E [GPa]  0 [MPa] Kinematic hardening Isotropic hardening C [MPa]  Q ∞ [MPa] b 193 100 60000 280 200 6 Table 1 : The material parameters for SS316L [13]. The finite element model The compact tension specimen as used in the experiment was modelled using the finite element method (ABAQUS). Due to symmetry of the geometry and the loading, only half of the specimen was meshed with 4-noded quadrilateral plane stress elements, as shown in Fig. 3. Finer elements were generated around the crack-tip area (  ≈ 1.8 µm) and a rigid line was attached along the symmetry line to prevent the potential penetration of the crack flanks due to crack closure under cyclic loading. A mesh convergence study was carried out, and a mesh size of 3.6 µm was found to be adequate such that the effect of mesh size on the stress-strain responses became negligible. The loading pin (marked in red in Fig. 3) was modelled as elastic and no slip was allowed between the loading pin and the specimen [14]. Cyclic tensile loading was applied at the centre of the pin according to the loading scheme (Fig. 1). A sequential crack tip node-release technique was applied to model the micro-crack growth, where nodes were released incrementally and set to the micro-crack growth length detected, otherwise the crack was assumed stationery. The strain values were calculated at the integration points and the average strain values were obtained for R 2 and R 4 over a square of 25 µm x 25 µm, similar to those obtained by the DIC method. Figure 3 : The FE mesh for the half-model of a CT-specimen, where a finer mesh (  ≈ 1.8 µm) was used in the crack tip region. R ESULTS AND DISCUSSION he strain evolutions with cycles are shown in Fig. 4 at R 2 and R 4 for the four test series (as shown in Fig. 1) from both the DIC and the FE analyses. It is evident from both analyses that tensile strains increase with the number of cycles in all load cases, although the FE analyses appear to predict higher responses than those from the DIC. A closer agreement between the two is achieved at R 2, as opposed to R 4 , indicating that grain size might serve as a suitable candidate as a “critical distance”. Micro-crack growths were detected at some of the early test series, such that ratchetting behaviour could not be monitored on these occasions. These may be due to the pre-cracking load history or the notch effect. Further work is being carried out on controlled crack growths so that the relation between the ratchetting strain and the micro-crack growth may be explored. T Rigid line