Issue 50

R. Boutelidja et alii, Frattura ed Integrità Strutturale, 50 (2019) 98-111; DOI: 10.3221/IGF-ESIS.50.10 101 ( ) [ ] ( ) γC log 273+TC exp O =f 6 3 C 5 4 C 2 2 (3) where O 2 is oxygen concentration in ppm, T is temperature in degrees centigrade, and γ is water conductivity in μs/cm. The loading term f 3 is considered to be a function of stress. For constant applied load case, f 3 is given by ( ) σC=f 7 9 C C 8 3 (4) where σ is stress in MPa. C 1 to C 9 are constants whose values depend on type of material. Values for these constants are presented in Tab.1. Constant C 1 C 2 C 3 C 4 C 5 C 6 C 7 C 8 C 9 Value 23.0 0.51 0.18 −1123.0 8.7096 0.35 0.55 2.21 ×10 −15 6.0 Table 1 : Numerical values of constants C i used for predicting the initiation and propagation of SCC for AISI304[15] The time to initiation t I for a given D sigma is considered as a random variable following lognormal distribution. The mean and standard deviation of Log (t I ) are given by: Mean value of ( ) ( ) sigma I D log 4.21 - 3.10 -=t Log (5) Standard deviation of ( ) 3081 .0=t Log I Crack size at initiation In pc-PRAISE, shape of surface crack initiated due to IGSCC is considered to be semi-elliptical (Fig.2), which is also consistent with shapes of stress corrosion cracks reported by Helie [33] and by Lu [34]. Surface length of initiated cracks, (l = 2b), is assumed to be log normally distributed with a median value of 3.175 mm and a shape parameter of 0.85 [15]. Depth of initiated crack is taken to be 0.0254 mm. Figure 2: Geometry of the part-through circumferential crack considered Crack Growth Model The growth of very small cracks that have just initiated cannot be treated from a fracture mechanics standpoint [1]. Therefore, an initiation velocity is assigned to newly initiated cracks ( ) ( ) sigma 1 D logG+J=ν Log (6) where J is normally distributed and G is a constant. It can be noted that Eqn. (6) is similar in form (power law) as that proposed by Helie [33], based on experimental observations. For AISI 304 austenitic stainless steel, J has a mean of 2.551 and standard deviation of 0.4269, and G =