Issue 29

L. Zhao et alii, Frattura ed Integrità Strutturale, 29 (2014) 410-418; DOI: 10.3221/IGF-ESIS.29.36 416 crack front is the highest. The strain rate increments significantly decrease with the increasing crack depth, which indicates that the deep surface crack growing slow down. The J -integral is not increasing with crack length (from 7.5 mm to 10 mm) in Fig. 11 because the hoop stress that causes the crack opening is decreasing (normalized distance from 0.09 to 0.125) in Fig. 4 with crack length (from 7.5 mm to 10 mm). That is, the loading conditions (mainly the RS) are different when the crack length is 7.5 mm and 10 mm. The increase of J -integral should be offset by the decrease of crack driving force. The distributions of strain rate and J -integral ahead of deeper surface crack front are quite different in the interfacial region of Alloy 182 buttering and Alloy 182 weld. The strain rate decreases due to the constraint of material with high hardness while J -integral increases in the interfacial region. Some researchers discovered that the mechanical properties of welded joints had many distinct characteristics compared with homogeneous materials, especially for the fracture properties. Path dependence of J -integral exists because of the finite deformation in the vicinity of crack tip and the heterogeneous mechanical properties of welded joint. Therefore, there are some doubts on the applications of J -integral in heterogeneous DMW joints [17]. The plastic strain rate should be a more reliable fracture parameter to evaluate the SCC behavior of DMW joint than J -integral. 0 30 60 90 120 150 180 0.000 0.003 0.006 0.009 0.012 0.015 0.018 0.021 a=5mm a=7.5mm a=10mm Crack angle,  ( o ) Normal plastic strain rate, d  p /da(1/mm) 0 30 60 90 120 150 180 0 1 2 3 4 5 6 7 8 9 a=5mm a=7.5mm a=10mm Crack angle,  ( o ) J-integral, J (N/mm) Figure 10 : Normal plastic strain rate ahead of crack fronts ( r 0 =60μm). Figure 11 : J -integral ahead of crack fronts. SCC growth rate In order to predict the amount of time required before leakage occurs in normal PWR, a detailed prediction on stress corrosion cracking growth rate of DMW joints are performed by many researchers. Generally, stress corrosion cracking growth law for Alloy 82/182 weld metals in PWR primary water environment has been described as a function of applied stress intensity factor K I and thermal activation corrosion term: temperature, based upon the extensive SCC material testing data [18]. These correlations provide the flaw evaluation guidelines for Ni-based alloy materials in PWR environment. The stress intensity factor K I along the crack front in the elastic-plastic material is converted by the value of J -integral. However, there are some doubts on the applications of J -integral along interface crack front in heterogeneous DMW joints. Combined with EPFEM, a pre-analytical method is used to predict SCC growth rate of DMW joints in PWR environment. The stress corrosion cracking growth rate can be written as: 1 ( )       m p m a d da dt da (3) where the oxidation rate constant   a is 7.478×10 -7 , the exponent of current decay curve m is 0.5 [19]. Fig. 10 shows the normal plastic strain rate at a characteristic distance ( r 0 =60μm) ahead of the crack fronts. Based on the normal plastic strain rates and the formula, SCC growth rate evaluations were performed for Alloy182 weld metal conservatively at a service temperature of 345°C. The disposition curves of SCC growth rate are shown in Fig. 12. The units describe crack growth rate in terms of millimeter per second. Note that for the axial crack case, because low alloy