Issue 49

E. Abdelouahed et alii, Frattura ed Integrità Strutturale, 49 (2019) 690-697; DOI: 10.3221/IGF-ESIS.49.62 691 For pipeline repair, a number of technical design issues must be considered that may be important depending on the repair circumstances. The application of a bonded repair to a pipeline poses a number of technical design problems with materials that may be important depending on the circumstances of repair. [5] One example is the development of residual stresses when a high cure temperature adhesive is used to bond a repair patch to a substrate with a different coefficient of thermal expansion. [6] The main drawback of the use of gr / ep (graphite / epoxy), b / ep (boron / epoxy) and g / p (glass / epoxy) results from a discrepancy in the coefficients of thermal expansion between the composite and the metal. [7] Residual thermal stresses are particularly important when high temperature curing adhesives are used to bond the patch. For example, in a typical repair applied to pipeline structures, the reinforced region is initially heated to a temperature of about 100-120 ° C under pressure for about 1 hour and then cooled to room temperature after curing of the adhesive. 8] Due to the differences between the elastic properties and the thermal expansion properties of the composite patch and the metallic pipeline, residual thermal stresses may occur. It has long been recognized that in some cases, residual heat stress is a serious problem for the effectiveness of composite patch repair. [9] If the repair material is different from the substrate, the residual stress level must be calculated during the design process. [10] Numerous attempts have been made in recent years to estimate the values of these residual stresses. Köpple et al. [11] used an analytical method based on linear elastic fracture mechanics and finite element method and considered a steel pipeline with a through-wall defect repaired by a composite material. Zarrinzadeh et al. [12] used the ABAQUS software to better simulate the experimental results of cracked aluminum pipe with patch repair under fatigue load. T. Nateche et al. [13] used a composite patch to repair damaged pipelines. It is a fast and economical method without interruption of service. Mhamdia et al. [14] analyzed the effects of residual thermal stress on the variation of the stress intensity factor in an aluminum plate repaired with a composite boron / epoxy patch. According to Albedah et al. [15] the residual thermal stress caused the reduction of the fatigue life of repaired aeronautical structures. To mitigate the effects of residual heat stress on repair performance, cure temperature and adhesive properties must be optimized. Lam et al. [16] also considered a circular tube that is repaired by a composite patch to study the effect of the patch on reducing stress intensity factors. The purpose of this study is to analyze the effect of the variation of the stress stress factor of repaired crack in steel pipelines with aadhesived composite patch. The novelties of this work are the parametric studies of the effects of the geometric, thermal and mechanical properties of the composite on the variation of the stress intensity factor at the end of cracks repaired with aadhesived composite patch. F INITE ELEMENT MODEL he cracked steel pipeline that is repaired by a composite patch under an internal pressure load as shown in Fig. 1. The API 5L X70 elastic cylindrical steel pipeline having the following dimensions: D pipe =304,8mm, L pipe =1000 mm, e pipe =4mm. A transverse crack of length 2a to the pipe axis is assumed to exist in the pipeline. This crack is repaired with a composite patch of boron / epoxy 2 mm thick, and length 4a. The adhesive used for bonding is FM-73 with a thickness e a = 0.2 mm, inside diameter equal to the outside diameter of composite patch. The inside diameter of the composite patch is equal to the outside diameter of the pipeline. The adhesive is cured at 120° C and the ambient temperature is considered to be 20° C. The elastic and thermal properties of the pipeline and the composite are given in Tab.1. Analysis by the finite element method of the configuration illustrated in Fig. 1 is performed using the calculation code ABAQUS [17]. In order to avoid the degrees of complexity in the finite element model of this three-dimensional structure, we have formulated some simplifying assumptions that allowed us to capture the essential characteristics of the response. These assumptions are: - each part of the model is considered as a three-dimensional structure. no bonding property at the interface between the adhesive and the two other structures (pipe, patch). at the interface, each mesh node is common between the adjacent structures. - The adhesive is homogeneous elastic and isotropic [18]. - The deformation of the adhesive is under the effect of shearing and peeling. - The patch is unidirectional composite oriented perpendicular to the crack. The adhesive is modeled as a third layer. The nodes are common between the patch/adhesive and adhesive/pipe interfaces for continuity of deformation and stress. The advantage is to be able to capture the necessary characteristics of the adhesive such as the transfer of charges between the pipe and the patch. The finite element model consists of three subsections; cracked pipeline, adhesive and composite patch. Due to symmetry, only half of the repaired pipeline is considered. To generate the crack front, a number of elements originally created around the crack tip are replaced by a "crack block". This crack block is meshed with quadratic elements, which are mapped in the space of the original elements and merged with a quadratic mesh. The mesh was refined near the crack end zone using at T