Issue 49

H. Berrekia et alii, Frattura ed Integrità Strutturale, 49 (2019) 643-654; DOI: 10.3221/IGF-ESIS.49.58 648 0,0000 0,0002 0,0004 0,0006 0,0008 0,0010 0,0012 0,0014 0 50 100 150 200 250 300 350 400 450 500 550 600 650 Equivalent stress [MPa] Deformation Déf-5 Déf-10 Déf-20 Déf-25 Déf-35 Déf-45 Déf-60 Déf-65 Déf-75 Déf-95 Figure 6 : Behavior of corroded pipes during hydrostatic test. The internal pressure of the gas fluctuates according to the gas demand (see Figure 7), therefore undergoes variations which causes the phenomenon of fatigue. Fatigue is an insidious phenomenon because of its hidden nature can cause breaks for stress levels well below the limit of elasticity. 0 5 10 15 20 0 1000 2000 3000 4000 5000 6000 7000 Pressure (Mpa) Number of days Figure 7 : Variation in service pressure as a function of time [11]. Note that during service the pipe can be damaged this is explained by the fact that the phenomenon of fatigue is insidious because of its hidden nature. The rupture can occur for stress values well below the yield point since the fatigue limit is always below the yield point. Fatigue is a phenomenon characterized by a strong micro plasticity in the vicinity of micro defects (micro voids, inclusions, precipitates) that are potential sources of damage. B EHAVIOR – D AMAGE C OUPLING t present, it is well established that the damage affects the elastic characteristics of the material, because of the reduction of the effective section of the force-resistant volume element. In damage mechanics, the damage variable is defined by a surface density of microcracks [12]. A