13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 0.0001 0.01 1 100 10000 0 50 100 150 200 Shear stress [MPa] Hydrogen free With hydrogen 300 K (CH=0.49 /nm) 28 81 0 50 100 150 200 Shear stress [MPa] Hydrogen free With hydrogen 300 K (CH=1.24/nm) 10-12 1 10-9 10-6 10-3 103 Figure 2. Relationship between dislocation velocity and shear stress at 300 K; (a) low hydrogen concentration condition(CH = 0.49 /nm), (b) high hydrogen concentration condition(CH = 1.24 /nm) 3.3. Dislocation Emission from Crack Tip The activation stress of dislocation source to emit a new dislocation is determined as follows: s IIC s x K π τ = , (10) where, KIIC is the critical stress intensity factor for edge dislocation emission from a mode II crack tip. The atomistic analyses showed that increasing the hydrogen concentration, the KIIC value decrease in alpha iron[12]. We employ these relationships and the critical stress intensity factors used in this study are shown in Table 2. It clearly shows that the dislocation emission is enhanced in the presence of hydrogen. Table 2. Critical stress intensity factor for dislocation emission Hydrogen free CH = 0.49 [/nm] CH = 1.24 [/nm] KIIC [MPam1/2] 0.443 0.440 0.430 4. Analysis Results of Dislocation Dynamics 4.1. Low Hydrogen Concentration (CH = 0.49 /nm) The correlation between applied stress intensity factor rate ( a K π τ &= & MPam1/2/s) and the distance of leading dislocation (first emitted dislocation) from a crack tip (i.e. the size of the plastic zone) at the specific stress intensity factor (K = 0.589 MPam1/2) is shown in Figure 3 (a). It reveals that the size of the plastic zone becomes large in the presence of hydrogen, which is considered to attributes the increase of dislocation velocity ( τ < 29 MPa in Figure 2), and the enhancement of dislocation emission (in Table 2). On the other hand, at the high applied stress intensity factor rate conditions, the size of plastic zone becomes small compared with that in the presence of hydrogen. It is considered to attributes the increase of effective stress exerted on each dislocation near the crack tip ( τ > 29 MPa in Figure 2). Figure 3 (b) shows the number of emitted dislocations. This figure shows (a) (b)
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