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J. Tong et alii, Frattura ed Integrità Strutturale, 25 (2013) 44-49; DOI: 10.3221/IGF-ESIS.25.07 46 Figure 2 : The random speckle pattern on the specimen and a typical displacement map extracted from the DIC measurement. The locations of R 2 and R 4 are illustrated with respect to the crack tip. FINITE ELEMENT ANALYSES inite element analyses were carried out on the CT specimen using ABAQUS [9] under plane stress loading conditions to obtain the near-tip strain distribution on the specimen surface, and the results were compared with those measured by the DIC. Material model The material model by Lemaitre and Chaboche [10] was adopted where both isotropic and nonlinear kinematic hardening rules were used to describe the monotonic and the cyclic behaviour of SS316L [11, 12]. The equivalent von Mises stress is defined as:       3 : 2 dev dev f S S         (1) where  dev is the deviatoric part of the back stress and S is the deviatoric stress tensor. The size of the yield surface is defined using a simple exponential law:   0 0 1 pl b Q e         (2) where 0  is the size of the yield surface at zero plastic strain, Q  and b are isotropic hardening material parameters. The equivalent plastic strain is given by:       2 2 2 1 2 2 3 3 1 1 1 1 2 pl v                   (3) The evolution of the kinematic hardening component is defined as:   0 1 1 pl pl C C C                  (4) where  is the back stress, and are material parameters, and is the rate of change of C with respect to temperature and field variables. A total of five materials parameters are required to run the FE analyses: Initial yield stress  0 ; kinematic hardening parameters C and  and isotropic hardening parameters Q ∞ and b . The values of these parameters for SS316L were obtained using the experimental data for a strain range 4.0%    [13], and are summarised in Tab. 1. F