PSI - Issue 13

Yinghao Cui et al. / Procedia Structural Integrity 13 (2018) 1291–1296 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

1293

3

global model mesh is 18424, as shown in Fig.1 (b). The element type of the sub-model is consistent with whole model, and the minimum size of the element is about 0.1 μm in sub-model.

σ

b

a

a

2L

W

σ

Fig.1. Finite element mesh and geometric model (a) Geometric model (b) Global mesh model

3.2. Material model

Nickel-based alloy 600 is a power hardening material, and its material mechanical property is described by Ramberg-Osgood equation in this numerical simulation [13]:

n

0   = +     0

      

(4)

0

where ε is the total strain, including elastic and plastic strain; σ is the total stress; ε 0 is the material yield strain ; σ 0 is the material yield stress ; α is the offset coefficient and n is the strain hardening exponent. The creep rate of alloy 600 is calculated using the time hardening model model [14] in the FEM simulation: n m cr A t   =  (5) where cr   is the creep strain rate. is the applied stress; n is the creep exponent; A is the power law multiplier; m is time constant. Material properties of alloy 600 and its surface oxide are given in Table 1.

Table 1 Mechanical and creep properties of alloy 600 and its oxide formed in PWR primary water [15-16] Material E /GPa   0 /MPa  n A n m A lloy 600 190 0.286 436 1 5.29 0.39×10 -14 3.969 - 0.6801 Oxide 19 0.3 -- -- -- -- -- --

3.3. Loading and boundary conditions Working loading are divided into two parts, one is external load, including residual stress and working pressure, the other is internal load produced by formation of oxide film. When crack length is 2 mm and stress intensity factor K reaches 5 MPa·m 0.5 , the corresponding external load σ is applied as an external load, as shown in Fig.1. Oxide film growth process was simulated by expansion, and its coefficient β was assumed to be 0.001 and temperature differenc e ΔT was assumed to be 6 °C, Thus wedging stress was obtained as σ P =30 MPa [17-18]. To determine the main mechanical factors affecting creep strain at crack tip in full life cycle, a typical FEA model was calculated under the external load, wedging stress and a joint action of external load and wedging stress based on the wide plate tensile specimen.