PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 736–743 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

740

5

Fractographic analysis of the samples by scanning electron microscopy (Fig. 4) showed that fracture always initiates at a region where the fracture mode may be classified as TTS ( tearing topography surface ), a term coined by Thompson and Chesnutt (1979) and Costa and Thompson (1982). The TTS domain is the hydrogen-affected zone or hydrogen-assisted micro-damage (HAMD) region in pearlitic microstructures, as discussed by Toribio and Vasseur (1997), Toribio (1997c) and Toribio (2012).

Fig. 4. Tearing topography surface (TTS).

5. Formulation of a kinematic fracture criterion The hydrogen embrittlement results plotted in Fig. 3 show quite clearly that although performed at the same global displacement rate (crosshead speed), the failure load in a hydrogen environment depends on the notch geometry. If, as is suspected, local strain at the notch tip is a more relevant variable, a plot of failure loads versus local strain rate should give a single curve. Because fracture loads are also influenced by stress triaxiality –a function of notch geometry– it would be better to apply a more general fracture criterion on the basis of the stress tensor. A simple general criterion can be formulated on the basis of the distortional part of the strain energy density, or accordingly, the effective or equivalent stress in the Von Mises sense, since the latter is directly related to the former. This criterion can be formulated in the following way: Fracture will take place when the distortional part of the strain energy density (or, accordingly, the effective or equivalent stress in the von Mises sense) reaches a critical value over a critical region. Such a criterion has been valid for notched samples of high strength pearlitic steel tested in air (Toribio, 1997d). According to this criterion, fracture will take place when the equivalent stress in the Von Mises sense reaches a critical value over a critical region characteristic of the microstructure of the material. The fracture criterion in hydrogen environment is a generalization of that in inert environment (air), taking into account that now both the critical equivalent stress of the material and the critical region are affected by the ingress of hydrogen. The critical equivalent stress depends on the amount of aggressive element (hydrogen) that penetrates the sample during the test, and the critical zone in hydrogen environment is the TTS region, the zone microscopically affected by the hydrogen. The geometry might be represented by only one variable x S , the depth of the maximum hydrostatic stress point (measured from the notch tip). This point is a characteristic of each specific geometry, independent of the loading process, as demonstrated by Toribio (2012). Its role in hydrogen embrittlement processes is so relevant that the depth of the TTS region in the quasi-static tests reaches exactly the position of the maximum hydrostatic stress point (the quasi-static tests are those performed at the minimum strain rate, slow enough to reach the steady-state solution for the hydrogen diffusion problem) cf. Toribio (2012).