numero25

G. Qian et alii, Frattura ed Integrità Strutturale, 25 (2013) 7-14; DOI: 10.3221/IGF-ESIS.25.02 11 F RACTOGRAPHY FOR SPECIMENS TESTED IN ENVIRONMENTAL MEDIAS or the case of fatigue tested in water and 3.5% NaCl aqueous solution, the crack origination observed by SEM is mainly the surface related initiation. Additionally, unlike the single crack origin for specimens tested in air, multiple fatigue crack origins were observed [8], and the fracture surface morphology for fatigue crack steady growth zone is predominantly intergranular, as shown in Fig. 5, where the secondary cracks along grain boundaries and cross section of the specimens are observed, which is the phenomenon of grain boundary embrittlement due to the aqueous environmental effect. The presence of widespread secondary cracks is the damage characteristics of the material subjected to aqueous environmental medias. Figure 5 : (a) Cracking surface for the specimen tested in water, at σ max =156 MPa and N f =8  10 7 , (b) secondary cracking (SC) in the cross section of the specimen tested in 3.5% NaCl, at σ max =41.3 MPa and N f =9  10 7 . M ODEL FOR S-N CURVE PREDICTION IN DIFFERENT ENVIRONMENTAL MEDIAS t is known that crack initiation related to nonmetallic inclusions (subsurface crack initiation) is attributed to the weak cohesive state between inclusion and matrix. Under cyclic loading, a crack may easily form due to the interface debonding and grow into the matrix. In such a case, the subsurface crack initiation cycle N i is [19] i i i 4 lW N U   (5) where W i is the surface energy related to subsurface crack initiation and Δ U i is the unit increment of energy for subsurface crack initiation. W i and Δ U i are functions of grain radius l , inclusion radius r ( ψ = r / l ), stress amplitude Δ σ and the resistance of dislocation movement k ( φ =0.5Δ σ / k ). N  is defined to normalize i N , such that   2 2 i AW N k l   (6) where   2 1 A      (7) with  being the shear modulus and ν being Poisson’s ratio. Thus, the normalized N i is denoted as i n i i 2 4 N n N U       (8) where   U is the dimensionless unit increment of energy for subsurface crack initiation. F I 20  m (a) S C 20  m (b)