PSI - Issue 42

Mihaela Iordachescu et al. / Procedia Structural Integrity 42 (2022) 602–607 Mihaela Iordachescu et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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from the crack extension that produced the final ruptures in air. These differ in size and in the shape of the cracking front, but together allow an understanding of the initiation and further evolution of the assisted cracking during the slow strain rate phase of the tests. According to Fig. 2a, Fig. 2b and Fig. 2e, the load level reached in the cases of P8, P3 and P6 specimens before interrupting the SSRT-FIP test was enough for subcritical cracking initiation and growth from the notch front. The subsequent fracture testing in air was preceded by heat tinting, which involved the loss of the hydrogen charged during the exposition to FIP medium. The final collapse of the specimens occurred by shear failure of the remaining ligament along two symmetrical planes at 45° (Fig. 3a, Fig. 3b and Fig. 3c), even though prior to it stable critical cracking took place with signs of ductile tearing and also with the change of the crack configuration from elliptic to triangular. This triangular crack extension produced the plastic exhaustion of the resistant ligament with no apparent previous plastic deformation for P3 and P6 specimens, while a high level of previous plastic deformation was required for P8 specimen. The critical crack extension of triangular shape could be explained by the transition from the plane stress to plane strain condition that occurs between the outer and the deepest points of the crack front. The higher stress triaxiality under plane strain condition requires higher levels of plastic deformation to reduce the stress concentration at the crack front and restrains ductile fracture.

Fig. 4. SEM images showing: a) the SSRT-FIP cracking and the triangular crack extension in air; b) the fracture surface morphology of SSRT FIP cracking; c) the longitudinal cut profile of the SSRT-FIP crack; d) the fracture surface morphology of the stable crack extension in air; e) the longitudinal cut profile of the stable crack extension. 3.3. Fracture micro-mechanisms Fig. 4 shows representative fractographic images related to the damage micro-mechanisms involved in subcritical cracking under SSRT-FIP and stable critical crack extension that triggers the tensile collapse. The fracture profiles of both subcritical and critical cracking are closely determined by the lath martensite microstructure of the steel.