PSI - Issue 13

Gyo Geun Youn et al. / Procedia Structural Integrity 13 (2018) 1297–1304 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

1298

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This research is executed as a basic study to quantify the ageing effect and seismic loading effect of primary pressure components made of CF8A. In this research, a method to quantify thermal ageing effect and cyclic loading effect is proposed. To quantify the thermal ageing effect of CF8A, monotonic tensile test and C(T) test are done. Then, a concept of thermal ageing constant “ C ” is introduced to quantify the fracture toughness of aged CF8A. Cyclic tensile test and C(T) test are performed to evaluate the effect of cyclic loading. Two assumptions are made to determine the fracture criteria under cyclic loading. Then fracture toughness predicted from the proposed method is compared with the test data.

Nomenclature A, B, ψ material constants in multi-axial fracture strain energy locus C thermal ageing constant J J -integral P max maximum load in cyclic loading P min minimum load in cyclic loading R load ratio W f p multi-axial fracture strain energy W p equivalent plastic strain energy σ m , σ e mean normal stress and equivalent stress ∆ ω , ω incremental damage and accumulated damage ω c critical damage

2. Experiments 2.1. Chemical Composition and Ageing Procedure

CF8A cast stainless steel is chosen for this study. The chemical composition of CF8A is shown in Table 1 [Chopra et al]. CF8A has good mechanical strength and weld ability, so it is used for primary pressure boundary components of pressurized water nuclear reactors. However, CF8A is easily exposed to thermal ageing which causes the increase of hardness and decrease of toughness and ductility. To get the thermal aged specimen, accelerated testing was conducted. The test time for ageing was calculated by Eq. 1 which was proposed by Chopra et al. In the equation, t age is ageing time, E is activation energy, T s is ageing temperature and F is ageing factor. In this research, the ageing procedure is performed in 400 o C for 4,189 hours to target 40 years ageing in real condition (32EFPY). ( ) 1000 1 1 log 19.143 273 673 age s E F t T   = − −    +  (1)

Table 1. An example of a table. Chemical composition of CF8A (wt%) [Chopra et al] C Mn P S Si Ni

Cr

Mo 0.5

Co

Fe

0.08

1.50

0.040

0.040

1.50

8.0~11.0

18.0~21.0

-

Balance

2.2. Tensile Test Results Tensile tests of unaged and aged CF8A were conducted at room temperature under both monotonic and cyclic loading conditions. Test was performed with R&B UNITECH (Universal Testing Machine). The specimen has the diameter of 5mm and the gauge length of 25mm. The specimen for monotonic tensile test is designed by ASTM E8-09 and specimen for cyclic tensile test is designed by ASTM E606-92. Fig. 1 shows tensile test results of unaged and aged CF8A under monotonic condition. It shows that yield and tensile strength increase but ductility decreases due to thermal ageing. Tensile test is also performed under cyclic loading condition and the hysteresis loop is shown in Fig 2. The result shows that the maximum stress slightly increases as ageing proceeds because the growth of hardness occurs.