PSI - Issue 71

A. Syed et al. / Procedia Structural Integrity 71 (2025) 82–89

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not modelled in the FE simulation, the results begin to diverge from the experimental data after a displacement of 15 mm. As the displacement and load increase, the specimen opens further in the experiment, leading to crack growth, which is not captured in the FE model. The coefficient of friction between the mandrel and specimen was varied between 0.05 and 0.2, and the friction value that best matched the load-displacement behaviour between the experiment and FE analysis was identified. This friction value was found to be 0.1 for the specimen tested at 25 °C and 0.14 for the specimen tested at 300 °C. These friction values are used in subsequent sections to determine the J-integral at various crack growth values.

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Test Temperature - 300°C Experiment FE analysis

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Fig. 5: Comparison of load-displacement response obtained from experiment and FE analysis at temperature (a) 25°C; (b) 300°C.

4.3. Quantification of stress triaxiality at different temperature The stress triaxiality is evaluated to understand the crack initiation and propagation toughness at different temperatures. The J- h approach is used for evaluating the stress triaxiality where ‘h’ is given in Eq. (1) ℎ= (1) where is the hydrostatic stress and is the von Mises stress. The hydrostatic and von Mises stress are obtained at a normalised distance along the crack tip. Normalis ed distance is defined as rσ/J, where 'r' represents the distance from the crack tip, 'σ' is the material's yield strength, and 'J' is the value of the J -integral at a specific displacement. For this analysis, the J-integral value is set at 100 kJ/m². The concept of normalised distance was introduced by Dodds et al. (1993) to compare crack-tip constraints across different specimen geometries and material properties, and the same approach is applied in this study. It is observed that the triaxiality at the crack tip is 0.77 at 300 °C, while it increases to 0.8 at 25 °C. This suggests lower crack initiation toughness at lower temperatures under higher stress triaxiality conditions. Furthermore, the stress triaxiality curve at 300 °C is lower than that at 25 °C, as shown in Fig. 6(b). The von Mises stress contour at a displacement of 10 mm, corresponding to a J-integral value of 100 kJ/m², is shown in Fig. 6(a). High stresses are concentrated at the crack tip, and a zone of elevated stresses forms along the hoop direction due to the specimen's cylindrical geometry. This differs from standard fracture specimens, where the plastic zone forms ahead of the crack tip. The maximum von Mises stress, approximately 928 MPa, occurs at the crack tip of the specimen.