Strengthening of Misaligned Welded Pipes with Outer Circumferentially Crack Using FRP Bandage Finite Element Analysis

Abstract

Finite element analysis is used to investigate the effectiveness of Fiber Reinforced Polymers (FRP) bandage in rehabilitating misaligned welded pipes with outer circumferential surface crack. It is assumed that the pipes are subjected to an axial tensile stress. Linear Elastic Fracture Mechanics (LEFM) regime is adopted to estimate the stress intensity factor (SIF). ANSYS software is used in the study through 3D-axisymmetric finite element analysis. The results demonstrated that stiffening with FRP bandage has a great influence on the reduction of the stress intensity factor at the crack tip improving the fracture integrity of the misaligned welded pipes. The most common solution of the rehabilitation process for damaged pipes fabricated from metals is very important mater. The rehabilitation technique for pipes is needed because of external erosion and corrosion for pipes or due to external damages .Usually such process requires using heavy machines and skilled workers in addition to the time needed. By using Fiber Reinforced Polymers (FRP) techniques to increase the strength for existing pipes, it is very important for the corroded and damaged pipes to increase the resistance to the surrounding environment. The conventional techniques for repairing damaged pipes need operation of cutting, welding and inspecting in addition. This may also require stopping the system during the repair period. Furthermore, for many cases the repair process is undesirable because of the residual stress induced in the pipe during the welding process which tends to decrease the fatigue performance of the pipe material in addition to the weakness of the pipe wall due to any of corrosion types such as pitting. Therefore, the final rehabilitation cost using FRP is low compared with the traditionally usual trends using welding techniques. Pipeline welding contractors are very often used in mechanized and semi-automatic welding processes. It is generally accepted that the most welded structures enter service containing tiny cracks or flaws (1). Moreover, these flaws may extend to a size where the safety of the structure becomes dangerous. It is therefore essential that engineers and designers have methods of assessing the significance of flaws detected during manufacture or after the structure has entered its service life, especially the structures that are connected with the human safety, such as pipes. Generally, cracks in piping system often occur at welding joint and around it (2). Most of these cracks are located in circumferential direction. For this reason circumferential cracks play an important role in the safety analysis of pipes (3). Strengthening techniques have drawn attention by many researchers to investigate the effectiveness of the stiffeners in the rehabilitation processes through using experimental as well as finite element methods. Ahmed et al. (4-6) investigated the effect of using FRP strengthening techniques to rehabilitate structural members under numerous loading conditions through FEA. Mourad et al. (7-18) have studied the effect of stiffening on crack growth through three-point bending TPB, compact tension CT and compact shear CS specimens. Chang-Young Oh et al. (19) investigated existence methods to estimate stress intensity factors due to welding residual stresses and compared with finite element (FE) solution for welding residual stress profiles, generated by simulating repair welds and compared with analytical solution. On the other hand, finite element analysis (FEA) was carried out to obtain the elastic stress distribution at cylinder to cylinder junction in pressurized shell structures that have applications in space vehicle design (20). The peak stress value is found to reduce due to filleted butt joint as expected and also confirmed through test results. Pipes with a flawed girth weld for two common girth weld tension tests: the curved wide plate test and the full scale pressurized pipe tension test, and subjected to global plastic deformation were studied by finite element models (21). Xiaohui Chen et. al. (22) presented an overview of recent progresses in experimental investigation and finite element analysis (FEA) of ratcheting behaviour of pressurized piping. The stress-intensity factor for the circumferential semi-elliptical surface cracks in a hollow cylinder's cross