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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curves ahead of the crack tip distinguishing the CPZ is possible. The DIC system provides strain measurement corresponding to overall/global load. Due to a lack of suitable constitutive relations, it is hard to obtain the local stress value in experiments through DIC measured strains. However, it is observed that load-strain hysteresis also has similar characteristic features as shown in Fig. 4(b). This has also been observed by Pan et al. (2009). Hence, the load-strain hysteresis information in-lieu of stress-strain is used for cyclic plastic zone identification ahead of the crack tip.

Fig. 4. (a) Typical representation of plastic zones; (b) Typical stress-strain behaviour at selected points

4.3 Evaluation of Cyclic Plastic Zone (CPZ) The equivalent strain value of 0.2%, is used to distinguish between the elastic and plastic zones. Once the plastic and elastic regions have been distinguished, the monotonic plastic zone and cyclic plastic zone have to be differentiated using analysis of load-strain cycle data. A complete cycle of recorded load-strain data points is divided into loading and unloading groups at any location ahead of the crack tip. For a load-strain cycle at any position within the plastic zone, the strain value of each data point in the loading group is subtracted from the corresponding strain value in the unloading group. If the average difference (AD) of the strains is more than 0.1%, the place is in a cyclic plastic zone; otherwise, it is in a monotonic plastic zone. A similar method has been implemented by Gao et al. (2022) to get the CPZ. Fitting the two straight lines using the observed data points of the loading and unloading groups is one technique to obtain an AD greater than 0.1%. The R 2 value of the fitting is more than 0.98, indicating that the straight-line fitting is good. The minimum distance (in terms of equivalent strain) between two straight lines is larger than 0.1%, indicating that the AD value is greater than 0.1%.