Issue 49

B. G. N. Muthanna et alii, Frattura ed Integrità Strutturale, 49 (2019) 463-477; DOI: 10.3221/IGF-ESIS.49.44 464 semi-elliptical crack angle orientation was studied at the critical position to estimate the critical angle. In the last part, the failure assessment diagram (FAD) was used to show the critical crack depth ratios at a critical position and a critical angle. K EYWORDS . Elbow, Pipe, Semi-elliptical crack, Stress Intensity Factors (SIFs), Failure Assessment Diagram (FAD). and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION ipelines are very important tool to transport hydrocarbons with high safety properties over long distances [1-3]. Indeed, the piping systems including the elbows are subjected to severe factors which may menace the integrity and safety of the pipe [4-5]. Due to the importance of elbows in the petroleum industries including their geometry and location, they are considered to be as critical parts in piping systems [6-7]. Due to their geometry, stress amplification occurs which promote failure and leak. Moreover, their stiffness is less in comparison with straight pipes having the same cross- section and material properties, inducing significantly higher stresses and deformations which may lead to failure. The corrosion phenomenon of elbow is one of the major damage problems in the energy transportation due to unsuitable factors such as fluid nature, operating conditions and the material of pipeline [8-11]. This situation may affect the energy transport significantly and increase the possibility of corrosion to occur. Many authors have studied the interaction between fluid and solid or solid-solid for two types of damage such as erosion and corrosion of steel pipelines [12-15]. El-Gammal et al. [16] have presented the flow accelerated corrosion of a 90-degree elbow by computational fluid dynamics (CFD) simulation. They have proved that the wear maximum value at elbow extrados is approximately 37% less than the maximum value along intrados. Muthanna et al. [17] have investigated the erosion corrosion in the internal surface of pipe elbow. They have found that the small sand particles and the attack of fluid flow on critical positions were responsible for this serious problem. Tian et al. [18] have examined the effect of flow velocity on the corrosion behavior of AZ91D magnesium alloy at pipe elbow to analyze the interaction between fluid and internal wall of elbow in the critical zones. Their results have showed that the corrosion rate augment with the increasing of flow velocity. The erosion-corrosion phenomenon occurs in difficult situations where the interaction is a complex field which leads to make a degradation for wall thickness of pipeline steels [19-21]. Barros et al. [22] have studied the repairing of a real corrosion defect by composite sleeve at welded joints. Moreover, many researchers [23-26] have studied the circumferential stress in the pipe elbow by analytical models, numerical methods and/or experimental tests. They proved that the maximal circumferential stress located in the intrados section of elbow. In addition, other authors [27-31] have used the Failure Assessment Diagram (FAD) to classify the grade of safety of pipelines or elbows. This work aimed to study the safety of a pipe elbow under the effect of gas pressure with change of radius bending, crack position, crack angle orientation, and semi elliptical crack depth ratios. To evaluate the influence of the flow in the internal wall of the critical position of a pipe elbow, a semi-elliptic crack was used. The results of the stress intensity factor (mode I) can give the solutions to avoid corrosion phenomena due to the nature of liquid and the material quality of pipe elbow. This study was conducted to improve the durability and the performance of elbow steel. N UMERICAL STUDY AND SOFTWARE ANALYSIS orrosion and erosion induce the most common defects occurring to pipeline. An example for a corrosion case of a pipeline is shown in Fig. 1. We have performed a numerical study in order to understand, relief and avoid the occurrence of such corrosion problems as much as possible. In this numerical study, the interaction between elbow and natural gas analyses was carried out by ANSYS software [32]. 90° pipe elbow made of API X52 steel was investigated and the pipe and its dimensions are shown in Fig. 1. Tab. 1 and 2 present the chemical composition and mechanical properties of API 5L X52 pipeline steel, respectively. Tab. 3 shows the parameters of the natural gas flow properties. The geometry of the elbow is shown in Fig. 1b. Dimensions are as follow: internal radius R i =298.45 mm, wall thickness t = 12.7 mm and the length L=1000 mm.  is the elbow bending radius radius. P C