Issue 41

A.S. Chernyatin et alii, Frattura ed Integrità Strutturale, 41 (2017) 293-298; DOI: 10.3221/IGF-ESIS.41.39 296 Internal surface crack propagation in weld joint of a pipeline Let consider a welded zone of the pipe with outer diameter 1420 mm and wall thickness 18.7 mm. The pipeline internal pressure is 10.6 MPa and it leads to the active axial and circumference stress 200 MPa and 400 MPa, respectively. Note that axial direction is perpendicular to the plane of the weld joint. In operation, there are additional axial stresses that occur because of bending and thermal loading. The axial stresses in conjunction with the pressure pulsation lead to cyclic loading. The following parameters of the loading cycle are accepted in the work: average stress is 200 MPa, the minimum and maximum values are 133 MPa and 267 MPa, respectively. In addition, there are residual stresses in the zone of welding. Distribution of axial residual stresses (RS) along the thickness was accepted in accordance with the results of well-known experimental study of a full-scale pipeline (Fig. 2). a) b) Figure 2 : Loading condition of the pipe (a) and the axial residual stress distribution along pipe thickness (b) Taking into account the small curvature of the pipe, the region with the crack is considered as thick plate. The initial front geometry of the crack located in the weld joint plane on the inner pipe surface is assumed in form of a semicircle with radius of 4 mm. Ten terms in the Williams expansion are kept to estimate the SIF and the T-stress at points of the fatigue crack front after discrete cycles Δ N = 10 5 . The total number of cycles, at which a calculation stop was occurred, (if previously SIF did not reach the critical value) is N =10 7 . The results of calculations in the form of finite configurations of the crack front at various conditions are shown in Fig. 3. Figure 3 : Final configuration of the crack front (half) under different loading conditions. It should be noted that independently of the initial crack geometry, final configuration of the crack front is close to the semi-elliptical. As expected, the stress T>0 leads to decreasing of the crack growth rate. This fact allows concluding about the conservatism of classic Paris and Forman formulas (not taking into account the effect of constraint and the residual stress). Otherwise, the residual stress (RS) affects on the rate of crack growth. Stress intensity factor distribution along the crack front strives for a constant value during crack size growing in absent of the RS. Presence of the RS leads to quite complex distribution of the SIF along the crack front. In contrast to the SIF, distribution and values of the T-stress along the crack front are weakly dependent on the RS.