PSI - Issue 13

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

1292

2

Owing to the oxide film formed on its surface, the nickel base alloy is a strong anti-corrosion material. SCC propagation process is a film rupture, electrochemical reaction and film reproduced at the crack tip [5], which including surface scratch, micro crack state, small crack state and long crack state. Therefore, it is very important to obtain accurate creep crack propagation rate for quantitative prediction nuclear structure service life [6]. Considering the long period of SCC and the operation temperature of reactor and steam plant components is 288 ℃ ~340 ℃ , and this temperature is below than 0.35T m (T m is melting point of material), and crack tip front also belongs to high stain zone, thus it is more likely to occur low temperature creep (LTC) in crack tip process zone [3,7]. Many research results have demonstrated that creep plays an important role in SCC crack growth, and crack tip strain rate could be instead by crack creep rate was also proposed [7-8]. Since creep strain at crack tip has great influence on the full life cycle of the key structural materials in nuclear power plants, it is necessary to study the main mechanical factors affecting creep strain at crack tip in the whole SCC life cycle. The finite element model of SCC full life cycle was established in this paper, and mechanical factor affecting creep strain at crack tip was studied.

2. Theoretical Basis

2.1. Experimental materials and methods To detailedly explain the complex phenomena related to SCC in different material and environments. Ford and Andresen proposed a predicted model of SCC growth rate according to slip-dissolution/oxidation mode [8], which considers the crack tip film will rupture if the crack tip strain reaching the rupture strain of oxide film, and the SCC growth rate is expressed as follows [10-11]:

m

   

   



i

 

dt da

z F M   

(1)

( ) 0

m t

=







ct

0

m

1

−

f

where, M and ρ are atomic weight and density of the metal, respectively. Z is the charge due to the oxidation process, F is Faraday’s constant, and i 0 is the oxide current density in bare metal surface, t 0 is the initial time of current decay, ε f is the oxide film rupture strain at crack tip and ε · ct is the crack tip strain rate. The rupture of oxide film at the crack tip is characterized by the creep strain reaching the rupture strain at the crack tip, the crack tip strain rate could be instead by crack tip strain rate approximately, which is written as follows [12]: (2) where  cr the creep strain rate at the crack tip, and substituting Eq (2) into Eq (1), the SCC propagation rate can be written as Eq (3), which creep strain at crack tip is an important mechanical parameter to determine crack growth rate. ct cr   =

m

   

   



dt da

Z F M   

i



(3)

( ) 0

m t

=







cr

0

m

1

f 

−

3. Finite element model 3.1. Specimen model

Numerical simulation is conducted using EPFEM. A finite wide plate, tensile specimen with a single edge crack is selected to simulate this process. Plate width is 80 mm and its length is 160 mm, as shown in Fig.1(a). The SCC propagation is a full life cycle process, which can be divided into surface scratch, micro crack stage, small crack stage and long crack stage. To research the main factors affecting creep strain close to the crack tip and its effect on the crack growth rate in full life cycle, a small size blunt notch is designed at the crack tip, and the crack length a i s assumed as surface scratch, 5μm, 50μm, 0.2 mm, 2 mm, respectively. To improve the analytical precision at the crack tip, the sub-model technique is employed to calculate distribution of the creep strain at crack tip. The 8-node biquadratic plane strain element is adopted in ABAQUS, with total number of the