Issue34

K.L. Yuan et alii, Frattura ed Integrità Strutturale, 34 (2015) 476-486; DOI: 10.3221/IGF-ESIS.34.53 484 proposed by Glinka [27] in the vicinity of the weld toe taking K t so obtained by Eq.6 into account, while that apart from the weld toe is calculated by elastic finite element analysis. The symbol used for Eq. 7 and the combined stress distributions are shown in Fig.13, where σ n represents the nominal stress. a) b) Figure 13 : (a) Symbol and coordinate used in Eq.7, (b) Assumed stress concentration near the weld toe. The stress intensity factors K I are calculated by 1/2 I 0 [ ( ) ( , ) ] [sec( )] a x c a K y m y a dy B T      (8) which is based on the weight function method in code API [28]. Here, the uniform stress distributions in the specimen width direction are assumed, and m ( y , a ): the weight function which is standardized by the API, σ x (y): the stress distributions, y: distance from surface, a : surface crack depth, c : half crack length, and B : width of specimen. The crack growth rate of the surface crack at the weld toe is given by eff eff th / [( ) -( ) ] m m da dN C K K    (9) based on modified Paris-Elber law using the effective stress intensity range, ᇞ K eff , and its threshold value, ( ᇞ K eff ) th . The total and effective stress intensity ranges, ᇞ K and ᇞ K eff , are calculated by total total min res eff max min total total max res / (1.5 ) 0.5 for , with = , 0.5 K R R K K K K K K R K R K K                (10) where material constants C =1.45E-11, m =2.75, ( ᇞ K eff ) th = 2.45 MPa m according to WES2805[29], R total is the total stress ratio, K max , K min and K res are stress intensity factors for maximum, minimum applied stress and residual stress, respectively. A single initial semi-circular crack of depth 0.2 mm and final crack depth of 0.8 T is assumed to propagate at the weld toe in the center of specimen width, referring to the WES recommendation. Fig. 14 shows a comparison of the results of estimations, experiments [3, 20] and recommended FAT-values [4]. These results demonstrate that the proposed fatigue life prediction method clearly distinguishes the difference of fatigue strengths of as-weld and UIT weld joints, by taking the stress concentration and residual stress near the weld toe into account. It should be noted that since multi initial crack followed by crack coalescence are not considered in this work, the present simulation may lead to a slight over-estimation at high applied stress range. In addition, the predicted surface crack shapes are also compared in Fig.14, in which we can clearly observe that the aspect ratio of the UIT case is relatively higher than that of as-weld case because of the compressive residual stress combined with a lower stress concentration factor near the plate surface.