Issue 49

E. Abdelouahed et alii, Frattura ed Integrità Strutturale, 49 (2019) 690-697; DOI: 10.3221/IGF-ESIS.49.62 693 The virtual crack closure technique is based on the energy balance proposed by Irwin. In this technique, the stress intensity factors are obtained for the three fracture modes using the equation: G i =K i ²/E (1) where G i is the energy release rate for mode i, K i is the stress intensity factor for mode i, and E is the modulus of elasticity (I, II and III). In numerical elemental computations, the mesh choice results in the number of nodes and their arrangement that characterize the element and its density in the mesh structure. A series of calculation launching is opted in the first places in order to reach the convergence of computation with an optimization in the density of mesh, three different types of mesh were used: C3D20R: (A 20-node quadratic brig), (C3D15 : A 15-node quadratic triangular prism), (C3D10: A 10-node quadratic tetrahedron). Stabilization of the result such as the stress intensity factor expresses the optimal choice of number and type of elements. It is also important to refine the mesh at the crack levels in order to better capture the value of the stress intensity factor. The number of elements used in the structure is 21414 elements in the pipeline, 2176 in the patch and 2176 in the adhesive. Fig. 3 shows the architecture of the mesh used for the calculations. Figure 3 : Overall mesh of the sample R ESULTS AND DISCUSSION Effect of internal pressure ig. 4 shows the variation of the stress intensity factor in the cracked and repaired pipeline by three types of composite patch (boron / epoxy, glass / epoxy and carbon / epoxy) as a function of the change in internal pressure in the pipeline rift. The stress intensity factor (KI) is calculated in the direction of the fiber (parallel to the direction of the applied load). We can see that the adhesive curing process involves a relatively high level of SIF at the crack front in the pipeline. This means that the adhesive and the patch are in tension. The pressurized pipeline exerts the tensile stresses. The intensity of the constraints in the patch is less significant compared to the constraint in the pipeline. This is because the coefficient of thermal expansion for steel is greater than that of (boron/epoxy, carbon/epoxy and glass/epoxy). However, the stresses in the composite are relatively large because there is a transfer of stresses from the steel pipeline to the composite part through the adhesive layer. On the other hand, it can also be said that the best composite for repair under high pressures is boron/epoxy. This linear variation of the stress intensity factor as a function of the internal pressure clearly reflects the linear behavior of the composite patch, the more rigid the composite, the greater the repair. Temperature effect Fig.5 uses the variation of the stress intensity factor in the cracked and repaired pipeline by three different types of composite patches (boron / epoxy, glass / epoxy and carbon / epoxy) as a function of the temperature in the external environment of the composite cracked pipeline. We notice that the stress intensity factor is higher when the temperature increases; it means that the increase of the temperature reduces the quality of the repair. From the graph (Fig. 4), it can be said that among the F