Issue 36

Lj. L. Vulićević et alii, Frattura ed Integrità Strutturale, 36 (2016) 46-54; DOI: 10.3221/IGF-ESIS.36.05 50 Chart in Fig. 5 shows very similar behavior of experimentally tested specimen and its 3D numerical simulation. Beyond 10 6 cycles very small number of cycles to collapse is left, and in this area two curves are exactly the same. Generally speaking, one can say that experimental and numerical results agree well, with some differences which require further investigation. 0 1 2 3 4 5 6 7 8 0 200000 400000 600000 800000 1000000 1200000 number of cycles N crack length a [mm] XFEM test Figure 5 : Comparing curves from experiment and 3D simulation. F ATIGUE LIFE PREDICTIONS OF PIPES WITH AXIAL SURFACE CRACK he main technical characteristics of the oil rigs from where the observed pipe are as follows, [4]:  layer pressure (Kp-31): maximum=10.01 [MPa], minimum=7.89 [MPa],  layer temperature: T=65 [°C],  number of strokes of pump rod: n PR =9.6 [min -1 ]. The geometry used in simulations is pipe with axial surface crack in the base metal (BM), Fig. 6. The pipe is made of API J55. On the outer surface of the pipe there is an initial axial surface crack with dimensions: a=3.5 mm and 2c=200 mm. The wall thickness is 6.98 mm. Figure 6 : Geometry of pipe. A finite element model of the pipe was created using the Abaqus software. The initial crack length used in the analysis was 200 mm, and it was 3.5 mm deep (the wall thickness is 6.98 mm). Mesh was refined around the initial crack, and a uniform template of elements was used. The growing crack was incremented at steps of 0.2 mm. The first step was crack opening, and after that, the crack was growing through the inner side of wall, in radial and axial direction, while, in 7th step, it becomes through-wall crack. Fig. 7 shows the crack growth at beginning (1 st step - crack opening), whereas Fig. 8 shows crack growth after 7 th step when the crack “breaks” through the wall. Afterwards, crack grows in axial direction only T