Issue 36

Lj. L. Vulićević et alii, Frattura ed Integrità Strutturale, 36 (2016) 46-54; DOI: 10.3221/IGF-ESIS.36.05 52 Figure 9 : Step 64 when “inner” and “outer” crack going to be equal (crack length is 200 mm) and stresses around the crack. Figure 10 : The 3D chart of “inner” and “outer” crack growing. The prediction of crack growth rate and residual strength of pipe demands accurate calculation of stress intensity factors (SIFs), which determine the appropriate crack growth increment for the crack. This calculation was performed 100 times in order to simulate incremental crack growth. The obtained relationship between equivalent stress intensity factor K eq and crack length a, Fig. 11, shows tendency of increasing K eq with increased crack length a, while the crack was reached up to 210 mm. The fastest increase of K eq , as expected, was before the seventh step, when crack penetrated the pipe wall. The chart in Fig. 12 shows the obtained relationship between steps and number of cycles. After the 7th step, when the crack penetrates the pipe wall, the number of cycles becomes significantly lower and remains at about the same values until the final step. The chart in the Fig. 13 shows the obtained relationship between the crack length a [mm] and the total number of cycles N. For crack growth from initial crack length until final length of 209.42 mm, 10548 cycles are necessary. Obviously, the most of them occur until the seventh step, in which the crack becomes through-wall crack (8606 cycles), while the further cracks growth requires a very small number of cycles.