PSI - Issue 48

Štěpán Major / Procedia Structural Integrity 48 (2023) 230 – 237 Major / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Model based micro or nano-void coalescence. Second model analyzed in this article is working with assumption that dominant process at start of fracture is micro void coalescence, see Fig. 3. In this in model, the primary function of hydrogen in the embrittlement process is envisioned to be the stabilization and promotion of vacancy agglomeration. This hydrogen accumulation can be accentuated by dislocation-assisted convection.

Fig. 3. Stages of hydrogen induced fracture process. Stage1: The plastically deformed zone at the crack tip is affected by hydrogen. This deformed volume at crack tip is the source dislocations.; Stage 2: Hydrogen accumulates in this area and at the same time the number of dislocations increases, which is related to the plastic deformation localized here. Along with the increase in the number of dislocations, the number of vacancies also increases.; Stage 3: The emergence of hydrogen – vacancy complexes.; Stage 4: Coalescence of voids a growth of the final crack. In case of this, we will assume that the intensity with which the hydrogen concentration increases will be proportional to the number of micro-voids or hydrogen-vacancy complexes. Like the previous model, this model also works with data obtained from the studied quarry, so it is a model more suitable for the reconstruction of the quarry process than for predicting service life. In this case we will use the method known as COD to determine the J- integral. The acronym COD is abbreviation for critical crack tip opening displacement, see Sabirov (2005). Measured COD i values is connected with the value of J -integral by well-known relation, see Sabirov (2005):

1

(11)

J

COD







i

y

i

d

N

Where d N is dimensionless constant, σ y is the yield strength and COD i are obtained by comparison of two opposites profiles on two fracture surfaces. Subsequently, the stress tensor is calculated, see Sabirov (2005):

1

   

   

E J 

1

N







(12)

,

N











ij

ij

Hard

n y N 

I r 

  

This calculation is based on the theory of HRR field around the crack tip. Name of this theory is derived from the initials of the authors of HRR method which was proposed by Hutchinson, Rice and Rosengren in 1967-1968, see Rice (1968) and Sun (1993). In this equation I N are dimensionless constants. Value of this constant as well as value of d N strongly depend on strain hardening coefficient N Hard . Quantities Θ and r have a geometric meaning and correspond to the polar coordinates (angle Θ and radius r ) of the studied point in a situation where the origin of the coordinate system corresponds to the position at the crack front. 4. Model based on continuum mechanics The third approach is based on a method proposed by trio of authors Ananand, Mao, Talamini (2019). This method is based on continuum mechanics. The approach of these authors is based on the facts that:  Fracture occurs primarily perpendicular to the direction of the maximum principal tensile stress.  Fracture is transgranular in nature and exhibits “‘quasi - cleavage”, “flat”, and “nanovoid” features on the fracture surfaces.