PSI - Issue 68

A.F Perez et al. / Procedia Structural Integrity 68 (2025) 439–445 A. F. Perez et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 4. Strain field – DIC analysis – 600°C.

4. Discussion 4.1. !" values – Comparison between models and physical measurements

Once the CT samples had been fully opened by fatigue, the final crack extension was measured. If the standard models corroborated the physical measurements, the same value of final crack extension should be obtained. However, there is a large discrepancy between the two sets of data. Models gave a final extension around three times greater than that measured physically. To match reality, a new approach was used to plot the J-R curves, which consists of using the relationship that links the crack length at time of the test with the corresponding PD voltage: % = & + % − & ' − & + ' − & , (1) The index 0 corresponds to the initial state and the index corresponds to the final state. & and ' are the crack lengths measured on the samples. This equation gives a new method for determining Δ and Figure 5 shows the three new J-R curves that have been drawn to match the value of the final crack extension. The results are far from those obtained initially using the standards. The average value of !" was around 320 kPa·m, whereas Figure 5 shows that for Test 1, !" is around 600 kPa·m. For tests 2 and 3, the !" value seems to tend towards something even greater. Using the three final crack extensions and adopting a power law of the type = (Δ ) # , a final J-R curve can be plotted. The value of !" thus obtained is around 800 kPa·m, well above the 321 kPa·m estimated by the methods based on standards. There are two main reasons for this large discrepancy. • On the one hand, the specimens tested had a huge contraction zone visible to the naked eye (Figure 1). At 600°C, the material likely enters a regime where creep mechanisms become dominant. Creep can lead to significant viscoplastic flow in the material. This flow causes stress to concentrate at the crack tip, resulting in an observable "denting" effect (Figure 1). This indentation is indicative of the material's reduced stiffness and diminished resistance to plastic deformation at high temperatures, where the material begins to deform more readily before any substantial crack propagation occurs (Tvergaard & Needleman, 1884). The shear banding observed is likely amplified by the increased ductility of the material at 600°C. At this temperature, the material’s microstructure may become more susceptible to localized deformations, particularly in regions where microstructural interfaces, grain boundaries, or pre-existing defects act as sites for stress concentration. These localized areas yield more easily