Fully Plastic Failure Stresses and Allowable Crack Sizes for Circumferentially Surface Cracked Pipes Subjected to Tensile Loading

Fully plastic failure stresses for circumferentially surface cracked pipes subjected to tensile loading can be estimated by means of limit load criteria based on the net-section stress approach. Limit load criteria of the first type (labelled LLC-1) were derived from the balance of uniaxial forces. Limit load criteria of the second type are given in Section XI of the ASME (American Society of Mechanical Engineering) Code, and were derived from the balance of bending moment and axial force. These are labelled LLC-2. Fully plastic failure stresses estimated by using LLC-1 and LLC-2 were compared. The stresses estimated by LLC-1 are always larger than those estimated by LLC-2. From the literature survey of experimental data, failure stresses obtained by both types of LLC were compared with the experimental data. It can be stated that failure stresses calculated by LLC-1 are better than those calculated by LLC-2 for shallow cracks. On the contrary, for deep cracks, LLC-2 predictions of failure stresses are fairly close to the experimental data. Furthermore, allowable circumferential crack sizes obtained by LLC-1 were compared with the sizes given in Section XI of the ASME Code. The allowable crack sizes obtained by LLC-1 are larger than those obtained by LLC-2. It can be stated that the allowable crack size for tensile stress depends on the condition of constraint of the pipe, and the allowable cracks given in Section XI of the ASME Code are conservative.

A Finite Element Simulation Study on Reducers Based on Ultrasonic Guided Waves

Due to its special structural form, the reducers have common corrosion defects such as corrosion thinning, cracks and so on after being strongly impacted by the medium inside the pipes. However, there are few studies on the detection of reducers with the help of ultrasonic guided waves at present. In order to explore the effect of reducers to ultrasonic guided waves detection, the propagation characteristics of ultrasonic guided waves in reducer were simulated by using ABQUES. In the simulation, L (0, 2) mode was used to detect the reducer by two set of experiments. The first set of simulation experiment, L (0, 2) mode was excited from the large end of the pipe to detect the pipe with axial crack, circumferential crack and no crack respectively. The second set of simulation experiment, L (0, 2) mode was excited from the small end of the pipe to detect the pipe with axial crack, circumferential crack and no crack. The ability to detect defect was assessed by the time domain waveform of reflected echo, and the dynamic stress distribution cloud map is used to visually explain the propagation characteristics of the guided waves passing through the reducer, and the difference of detection capability between the two ends of excitation is judged by comparing the detection results. The conclusion shows that it is more sensitive to detect defect when ultrasonic guided waves are excited at the larger end, the ability to detect defects is weak when ultrasonic guided waves are excited at the small end. The mode conversion occurs and generate F (1, 1) mode, when the guided waves pass through the reducer from both ends. This paper provides effective technical guidance when ultrasonic guided waves detect defect on reducers.

Finite element method investigation of geometrical influences of adhesive and patch in the safety for 90° elbow piping system

Piping system elbow study is the most important part in all fields of hydrocarbons transportation which presents the behaviour of circumferential crack at elbow extrados. The effect of geometry of adhesive and patch in the crack elbow is important in pipeline safety. This study shows the details for along the direction of the circumferential elbow crack by three dimensional (FEM) which is used to determine the stress intensity factor at 90° elbow for two cases: firstly, without patch and secondly, repaired with composite patch. This method allows to predict the behaviour of cracked elbow through the analysis of crack propagation under the internal different pressures taking into consideration the operating conditions. The geometry and nature of composite patch proved that the increase of patch thickness leads to decrease the SIF from 7 MPa.m1/2 to 6.15 MPa.m1/2. It can be concluded that the repairing by composite materials leads to reduce the stress intensity factor with patch which not only can augment the lifetime of pipeline but also decreasing the costs and the pollution.