PSI - Issue 14

Rakesh Kumar et al. / Procedia Structural Integrity 14 (2019) 668–675 Author name / Structural Integrity Procedia 00 (2018) 000–000

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these sites will lead to fatigue crack nucleation when the FIP reaches a critical value. The fatigue cracks emanated from these sites may propagate towards next high FIP value zones, similar to the coalescences of nucleated micro cracks to transit from a short fatigue crack to long crack.

(a) (b) Fig.2: (a) The 16 grain RVE with periodic boundary conditions; (b) Stress strain behaviour of hydrogen charged and uncharged polycrystalline nickel samples after few initial fatigue cycles.

Table 1: Values of various parameters used for computation

1

Parameter

Symbol Magnitude

Units

Number of solute atoms per lattice site Number of lattice sites

- � 1.�2 � 10 �� mol/mm � ∆ � �1.0 � 10 � Nmm/mol � �.0 � 10 �� mm � / �� 2.3� � 10 �� mol/mm � � �.0� � 10 �� mol/mm � �� �� � �.�� � 10 � mm �� | | 2.�� � 10 �� mm � 0.1/b - -

Trap binding energy Lattice diffusivity

Initial hydrogen concentration

Reference hydrogen concentration Initial SSD density Burger vector length SSD creation parameter

SSD annihilation parameter � 10 Taylor hardening parameter � 0.3 Reference stress � 20 Fig. 3. The emergence of critical value of FIP suggesting the potential sites for fatigue crack nucleation. The FIP is a function of accumulated plastic strain, grain boundary normal stress and total hydrogen concentration. 5. Conclusion and outlook In this work, a modeling framework is presented that can explain the underlying mechanism of fatigue crack initiation in metals under hydrogen environment. Critical parameters responsible for low cycle fatigue crack nucleation are local elastic and plastic anisotropy, grain boundary normal stress and hydrogen concentration. The combined effect of these parameters is represented as single fatigue indicator parameter. The critical value of FIP defines the number of cycles required for the crack nucleation. Using such a framework, role of microstructural features such as grain size, special grain boundaries and triple junctions on hydrogen induced fatigue failure can be well understood. In addition, the simulation framework can be extended to include other trapping sites e.g. voids and interaction of hydrogen with the dislocation structure e.g. persistent slip bands and dislocation pileups. A work in this direction is undergoing presently. -