PSI - Issue 35

Kpemou Apou Martial et al. / Procedia Structural Integrity 35 (2022) 254–260 Kpemou A. M. et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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pipe steel and the same specimen type. This leads us to also conclude that there is a thickness dependence on the shift of DBTT after HE. 5.2. Fracture mechanism induced by HE To understand the fracture mechanism is the Charpy specimen submitted to HE, it is firstly essential to state to precise following points: • Fracture emanating from notch like in Charpy specimen is ruled by a local fracture criterion applies at notch root . This criterion assumed that the average stress or effective stress in the fracture process volume reaches a critical value [19]. • Hydrogen concentration at notch root decrease with distance from notch tip where for elastic behavior, the stress triaxiality is maximum. • According to Capelle [20] fracture resistance under HE decreases rapidly when hydrogen concentration is higher than a critical one. The volume where the hydrogen concentration is higher than critical value is called critical hydrogen volume Two situations are considered: Capelle [21] has shown that for pipe made in API 5L X52 submitted to an internal pressure of hydrogen of 60 bars for 72 hours, critical hydrogen volume at tip of a V notch is less than the effective volume. Fractographic investigation indicates that cleavage at notch tip extends only on a very short distance considerably less than the critical effective distance which is of the order of 1mm. To check the situations for the present study, fractographic investigations were carried out for ½ Mini-Charpy with and without HE. The evaluation points are indicated in Figure 3a. They are reported in figures 4a and 4b. As it can be seen, the fracture surface is ductile at the notch tip in both cases. This confirms that in our case the critical effective volume is greater than the critical hydrogen one. [ ] H H ef V V V V [ ] or ef c c (5)

Fig.4. (a) Fracture surface of ½ Mini-Charpy tested at 135K with HE; (b) Fracture surface of ½ Mini-Charpy tested at 130K without HE. 6. Conclusion Hydrogen embrittlement is a phenomenon that affects pipe steel mechanical proprieties towards several mechanisms. Ductile to brittle transition temperature can also be affected by HE. Results present in this paper show a