PSI - Issue 60

Prakash Bharadwaj et al. / Procedia Structural Integrity 60 (2024) 655–664 Prakash Bharadwaj / StructuralIntegrity Procedia 00 (2019) 000 – 000

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loop is suppressed. For each of the curves in Fig. 7(a), the CPZ were evaluated as per procedure described in section 4.3. Here for most of the straight-line fittings, the value of R 2 is greater than 0.99. This suggests that the approximation provided by the straight-line fitting for the observed data points is satisfactory. These fitting lines are shown in Fig. 7(b). The minimum distance between two straight lines is found to be more than 0.1% at a point located within the cyclic point zone (CPZ). The slope of the straight lines decreases as we move away from the crack tip. Over eighty percent of the observed points in the loading group have an ordinate value that is more than 0.2% von-Mises equivalent strain. This clearly shows that the location lies within the MPZ. At lower load levels, a crack-closing effect is observed. As a result, at lower load levels, reverse plastic strain remains nearly constant. This observation is consistent with the observation in the literature (Zhang et al. 2011) The cyclic plastic zone size ahead of the crack tip is determined for various crack sizes using the DIC system, and the results are presented in table 1. The computed cyclic plastic zone size is compared to the results obtained via finite element (FE) analysis, as well as to Irwin's expression of CPZ. Equation (6) represents the mathematical expression proposed by Irwin (1960) for the calculation of the size of the cyclic plastic zone (CPZ). 2 0 = 1 ( 2 0 ) 2 (6) As the DIC system measured the CPZ on specimen surface, two-dimensional plane stress FE analyses of the full CT specimen were conducted for the same loadings and crack sizes. A Chaboche nonlinear kinematic hardening model validated for the current material is employed in the finite element analyses. The specific information pertaining to the finite element mesh, material parameters, and loading pattern is provided by Bharadwaj et al. (2023)

Fig. 7. (a) Load-strain curve for different points in different regions; (b) Straight-line fitting of observed data point

Both the back stress variation-based approach (Hosseini et al. 2020) and the 0.1% average load-strain loop opening criteria can be employed to quantify the size of the CPZ in finite element (FE) analysis. In order to ensure consistency with the experimental methodology, the average load-strain loop opening criteria of 0.1% is employed in FE analysis to determine the size of the CPZ which is reported in table 1. The CPZ obtained using Irwin’s expression was roughly 2.9 to 3.7 times of the size obtained using DIC evaluated experimental CPZ. It may be noted that the Irwin's analytical expression is based on an elastic perfectly plastic material and does not take into account the material's hardening response. This results in a higher estimate of CPZ by Irwin's expression. The difference in