PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 198–205 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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Another approach is to study the influence of maximum hydrostatic stress (  max ) on diffusivity. According to Table 3,  max seems to be relevant in the quasi-static tests . However, this variable does change as the load applied on the sample increases during the test, making difficult a simple numerical approach. It would be useful to consider a parameter related to the maximum hydrostatic stress but remaining constant during the test. In this conceptual frame, the stress triaxiality (ratio of the hydrostatic to the equivalent stress) seems to be adequate, since it is almost constant during the tests (during the elastic regime the triaxiality does not change, whereas in the plastic regime there is a small change). The SSRT are between both values. A triaxiality factor T can be defined as the maximum value of the stress triaxiality in the sample:

T = Sup (  /  eq )

()



where  is the hydrostatic stress and  eq the equivalent stress (in the Von Mises sense) and  the domain. The triaxiality factor T was computed from the elastic distribution, and the resulting values for geometries A, B, C and D were 0.9, 1.4, 0.5 and 0.4 respectively. Fig. 5 offers the failure load in the quasi-static tests versus the triaxiality factor of each geometry. It is possible to observe a monotonic decreasing dependence, which emphasises the role of maximum hydrostatic stress in hydrogen diffusion.

Fig. 5. Relationship between the asymptotic value of the failure load in hydrogen environment for quasi-static tests and the triaxiality factor (maximum value of the triaxiality in the sample, a characteristic of the geometry).

Therefore, the higher the specimen triaxiality, the more elevated the hydrostatic stress levels in such a geometry, in such a manner that the analyzed notched geometries can be ranked according to their triaxiality levels (and related values of maximum hydrostatic stress) in the following order: B, A, C, D, a series that resembles the last name (BACH) of Johan n Sebastian Bach. Fig. 6 shows the Bach’s image by Elias Gottlob Haussmann, the only authentic portrait of Johann Sebastian Bach (that in which the composer has the canon triplex in his hands). 6. Conclusions 1. Hydrogen transport by diffusion is stress-assisted, and the hydrogen moves not only to the minimum concentration sites, but also towards the maximum hydrostatic stress locations. 2. The tearing topography surface (TTS), a new microscopic fracture mode, is associated with hydrogen effects, and it seems to consist of micro-damage or micro-tearing produced by the hydrogen. 3. The hydrostatic stress plays a relevant role in accelerating the diffusion of hydrogen in the sample, both increasing the boundary concentration and enhancing the hydrogen flux associated with stress gradients.