PSI - Issue 13

Ghassen Ben Salem et al. / Procedia Structural Integrity 13 (2018) 619–624 Ben Salem Ghassen / 00 (2018) 000–000

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Figure 4. (a) Fracture surface of CT20 specimen tested at − 100 ◦ C (b) SEM examination of the fatigue precrack tip in zone A

Figure 5. Positioning of notch from FL in specimen (a) 760AC (b) 760AM (c) 760M

3.3. Toughness analysis

For further analysis of the toughness values, the specimens were classified based on the proportion of austenite in the fatigue precrack tip (Fig.3(b)). As underlined previously, there is a clear disparity between specimens with a ferritic fatigure precrack tip (0% austenite) and those with a mixed precrack tip (10% austenite, 30% austenite) caused by the di ff erence in fracture mechanisms. The proportion of austenite in the precarck tip does not seem to have a significant influence nevertheless between specimens with 10% or less austenite and 30%, meaning that the fracture initiation is not volume dependant. These results have clearly confirmed that the weakest zone in the SS DMW is the MA interface where the critical fracture mechanism is an intergranular fracture. This brittle mechanism needs to be modeled in order to build a predictive model of the brittle fracture risk for the SS DMW. Toughness values for CT specimens with intergranular fracture seem to hit a plateau at low temperatures (under − 80 ◦ C), which is consistent with the notion of threshold toughness or threshold stress below which brittle fracture cannot occur, already introduced by several authors [9, 10, 11, 12]. Moreover, considering the role played by the carbides and inclusions in the initiation of the intergranular fracture mechanism, a 3 parameters Weibull critical stress based model was selected. To model the brittle behavior of the MA interface, a stress based criterion with a threshold stress σ th was used. It was shown in [13, 14] that a reliable method to determine σ th is to use Notch Tensile (NT) specimens tested at a very low temperature. Semi-circular notched specimens were chosen with an external diameter of 10mm, a notch radius of 1mm and depth of 2mm. In a first step, three tests were performed with the same specimen geometry but with the notch placed at di ff erent positions from the hard layer in order to identify the most suitable configuration for threshold stress calculation (Fig.5). To choose this configuration promoting intergranular initiation at the MA interface, SEM analysis of the fracture surfaces was coupled with numerical simulation of the tests. Tests were carried out at − 170 ◦ C to minimize the plastic deformation in the hard layer and a laser beam measurement system available at CEA Saclay [15] was used to measure local strain during the tests. For the three specimens, fracture occured in the notch near the FL. SEM images of the fracture surfaces (example in Fig.6(a))) indicate the presence of both intergranular and cleavage fracture for the three tests, confirming that the specimens failure occured near the MA interface following two possible scenari : either initiation of the brittle failure at the MA interface and then deviation to the ferritic side or initiation in the ferritic decarburized layer and deviation to the MA interface. In specimen ”760AC”, intergranular fracture seems to have developed in a larger area 4 4. Determination of threshold stress 4.1. Experimental study and geometry validation