PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 728–735 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

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5

Fig. 3. Hydrogen-assisted micro-damage (HAMD) region: tearing topography surface (TTS). This non-conventional microscopic fracture mode can be considered as a process zone, damaged area or region microscopically affected by the hydrogen. Thus the depth of the TTS region x TTS (measured from the crack tip) determines the size of the critical region, its central point being located at a critical distance r c =x TTS /2. 5. Formulation of a kinematic fracture criterion To formulate a kinematic fracture criterion, it is useful to express the experimental results in dimensionless form, using the following variable to indicate the embrittlement effect:

F c F o

K c K IC

 =

=

(6)

where  is the ratio of the failure load in the solution (F c ) to that in air (F o ), cf. Fig. 2, which under LEFM assumptions is exactly the ratio of the critical SIF in the HAC test (K c ) to the fracture toughness in air (K IC ), and therefore represents the ratio of the critical equivalent stress in the HAC test to that in the air test. To represent the strain rate effect, the following dimensionless variable can be defined:

x s 2 D* 

 = << d  CT /dt >>

(7)

where << d  CT /dt >> is the space-time average of the local strain rate in the vicinity of the crack tip (space average over the hydrogen affected region; time average over the tests duration), x s the depth of the point of maximum hydrostatic stress (towards which hydrogen diffuses), and D* the hydrogen diffusion coefficient in the steel. The TTS zone was considered as the critical region (r c =x TTS /2), and the following values were used in the calculations: K IC = 53 MPa m 1/2 , D* = 5x10 –11 m 2 /s,  /4 (Fig. 1) and the experimental results given in Table 1. Table 1. Experimental results of the SSRT ___________________________________________________________________________________ Test Displ. Rate (m/s) t C (s) F C /F O x TTS (m) –––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– 1 8.33x10 –6 600 0.93 10 –6 2 3.33x10 –6 1080 0.78 10 –6 3 8.33x10 –8 25200 0.52 1.7x10 –4 4 5.00x10 –8 34800 0.50 1.7x10 –4 5 3.33x10 –8 42000 0.44 7x10 –4 6 3.33x10 –9 540000 0.41 7x10 –4 7 1.67x10 –9 1080000 0.41 7x10 –4 ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––