Issue 49

A. Guillalet alii, Frattura ed Integrità Strutturale, 49 (2019) 341-349; DOI: 10.3221/IGF-ESIS.49.34 348 Variables Description μ cov Std Distribution a 1 Crack depth (mm) 4 / / Fixed c 1 Crack length (mm) 4 / / Fixed a 2 Crack depth (mm) 4 / / Fixed c 2 Crack length (mm) 20 / / Fixed D Outside diameter (mm) 1219,2 0.0 0,0121 Normal t Pipe thickness (mm) 12,1 0,605 Normal P Maximum operating pressure (MPa) 8 Fixed ∆σ applied pipeline wall stress range (MPa) 150 Fixed K Ic Fracture toughness (MPa.m1/2) 137 13.7 Normal C a =C c Fatigue crack growth rate (mm/cycle) 5*10 -9 Fixed m Fatigue exponent 3.1 Fixed Table 2 : Input parameters for fatigue calculations. A target reliability index of 3.1 is fixed for comparison purpose. Calculations are made using the same reliability method and input variables presented in Tab. 2. Variation of reliability index is plotted against number of experienced cycles for both configurations of crack. According to Fig. 7, crack with initial a/c = 0.2 and a/c = 1 have a fatigue life time of 5*10 4 and >2.4*10 5 respectively. This results can be interpreted according to Fig. 5, where β for shape ratio a/c = 0.5 is less than β for a/c = 1, so the first case reach the target reliability index before the second. Also, these results seems to be in difference with those in [11] where the authors fixed the aspect ratio for all fatigue life. He found that semicircular crack had a shorter life time then shallow crack. This disagreement may be attributed to the assumption of fixed shape factor. Stress intensity factor is estimated with less accuracy. The authors in [11] made the rest of his calculations with constant shape factor a/c=0.62 following recommendations in [18] in order to not shadow the influence of others parameters of study. Figure 7 : Fatigue life time of cracks with different shape factor C ONCLUSION n this article, effect of crack shape factor in reliability analysis of axially cracked pipeline was illustrated. Shape and depth crack ratio were varied to cover different crack shape and growth ranges. Reliability index was plotted against different configurations of shape and depth ratio. According to the variation of reliability index calculated in deep point, this later is greatly influenced by the crack shape and depth ratio. Shallow cracks have lower reliability index followed by deep cracks .As consequences, it seems that shallow cracks are more susceptible to cause fracture then deep cracks. Reliability index calculated in deep point decrease with increasing of depth ratio for all crack shape. The reliability I