ABOUT CONICAL CUMULATION IN STORAGE AMPOULES

Conical cumulation in storage ampoules consists of a periodically repeating wave pattern - the formation of an axial high-pressure region as a result of the focusing of oblique waves and its unloading. In this case, the convergence of the oblique wave is accompanied by a loss of stability - protrusions appear at the wave front, the collision of which leads to an increase in pressure. The expansion of the high-pressure region is accompanied by the formation of an axial crack and continues until its pressure becomes lower than the pressure of the incoming oblique waves, after which the flow pattern is repeated.

Structure-Property Relationships of Polyamide 12 Grades Exposed to Rapid Crack Extension

Thermoplastic materials have established a reputation for long-term reliability in low-pressure gas and water distribution pipe systems. However, occasional Slow Crack Growth (SCG) and Rapid Crack Propagation (RCP) failures still occur. SCG may initiate only a small leak, but it has the potential to trigger RCP, which is much rarer but more catastrophic and destructive. RCP can create a long, straight or meandering axial crack path at speeds of up to hundreds of meters per second. It is driven by internal (residual) and external (pressure) loads and resisted by molecular and morphological characteristics of the polymer. The safe installation and operation of a pipe throughout its service lifetime therefore requires knowledge of its resistance to RCP, particularly when using new materials. In this context, the RCP resistance of five different polyamide (PA) 12 grades was investigated using the ISO 13477 Small-Scale Steady State (S4) test. Since these grades differed not only in molecular weight but also in their use of additives (impact modifiers and pigments), structure-property relationships could be deduced from S4 test results. A new method is proposed for correlating these results more efficiently to evaluate each grade using the crack arrest lengths from individual S4 test specimens.

The interaction of fundamental torsional guided waves from axial and oblique defects in pipes

A quantitative study of the interaction of the T(0,1) torsional mode with axial and oblique defects in a pipe is presented in this paper. A mode decomposition technique employing the chirplet transform is used to separate the multimodal signals reflected from the defects. Reflection signals are obtained from experiments on a carbon steel pipe. The influence of the crack length and inclination angle on the reflection is investigated. The reflection from an axial defect is found to consist of a series of wave pulses with gradually decaying amplitude. The results show that the reflection coefficient of an axial crack initially increases with the crack length but finally reaches an oscillating regime. Furthermore, for an oblique crack, it is revealed that the reflection coefficient is linearly dependent on the equivalent circumferential extent of the defect and is independent of the axial length.

Determination of the Critical Value of Material Damage in a Cross Wedge Rolling Test

This study investigates the problem of material fracture in cross wedge rolling (CWR). It was found that this problem could be analysed by means of well-known phenomenological criteria of fracture that are implemented in commercial FEM (Finite Element Method) simulation programs for forming processes. The accuracy of predicting material fracture depends on the critical damage value that is determined by calibration tests in which the modelled and real stresses must be in good agreement. To improve this accuracy, a new calibration test is proposed. The test is based on the CWR process. Owing to the shape of the tools and test piece used in CWR, the forming conditions in this process deteriorate with the distance from the centre of the test piece, which at a certain moment leads to fracture initiation. Knowing the location of axial crack initiation in the specimen, it is possible to determine the critical value of material damage via numerical simulation. The new calibration test is used to determine the critical damage of 42CrMo4 steel subjected to forming in the temperature range of 900–1100 °C. In addition, 12 criteria of ductile fracture are employed in the study. The results show that the critical damage significantly increases with the temperature.

A flexural mode guided wave transducer for pipes based on magnetostrictive effect

The flexural mode guided waves of pipes which are sensitive the axial crack and suitable for wave focused gain more attention recently. In this paper, a non-contact flexural mode guided wave transducer based on magnetostrictive effect is provided for pipes. Based on the magnetostrictive transduction principle and the wave structure of the flexural mode guided wave, the sensing method for generating and receiving the flexural mode guided waves based on magnetostrictive effect is obtained. According to the theoretical analysis, a non-contact magnetostrictive transducer for F (3, m) mode guided waves is given. Six permanent magnets which are evenly distributed in the circumferential direction of the pipe and arranged in opposite polarities are employed to provide the bias magnetic field in the circumferential direction. A solenoid coil is employed to induce the axial alternating magnetic field. The bias magnetic field distribution of the flexural mode guided wave in the pipeline is analyzed by the finite element simulation. The mode of the transduction guided wave in the pipe is verified by experiments based on the dispersion curves.

Experimental verification of a TE01 mode converter to locate a crack in a metal pipe

In this study, a TE01 mode converter was developed to locate an axial crack on the inner surface of a metal pipe. Three-dimensional finite element simulation was conducted to evaluate the effect of inserted coaxial cables on the transmission characteristic of the mode converter. The result showed that the energy of TE mode microwaves leaned to transmit to one side when the cables penetrated with inclination. To demonstrate the effectiveness of the mode converter, experimental verification was conducted. The mode converter was fabricated based on the simulation result. Microwaves were emitted through the pipe wall of the converter to propagate on both sides (‘right side’ and ‘left side’) of a pipe with an artificial slit. Compared with the signals from each side, the reflection from the slit on the right side was more significant than that on the left side. This result is consistent with the numerical simulation result.

Validation of the New Gamma Exponent Model for Axial Crack Assessment in Oil and Gas Pipelines

In the late 1960s and early 1970s, the researchers of the NG-18 committee at the Battelle Institute in Columbus, Ohio completed a seminal study on the failure pressures of flaws in oil and gas pipelines. One of the key developments was the “log-secant” equation for the assessment of axial crack-like flaws. The model was later modified to improve its accuracy and precision. The Gamma Exponent Model (GEM) was recently developed for assessment of axial crack-like flaws in pipelines. The developer recognized that the NG-18 log-secant model was theoretically derived on length, and then empirically corrected for depth. The new GEM was theoretically derived on depth, and then empirically corrected for length. The new model is similar in mathematical form to the original NG-18 log-secant model, but there are some key differences. This work is a validation study of the GEM using axial crack failure pressure data from the industry literature. Laboratory tests with machined flaws, and hydrotest and in-service failures with natural metallurgical flaws, are also considered. The results of the GEM are compared to the equivalent failure predictions using other models. The strengths and limitations of the new model are discussed in the context of improved accuracy and precision for crack assessments.

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.

Research on internal detection technology for axial crack of long-distance oil and gas pipeline based on micromagnetic method

Due to the lack of the efficient and accurate detection method, the axial crack of pipeline seriously threatens the safe operation of oil pipeline, and therefore the application of micromagnetic nondestructive testing technology in the field of pipeline axial crack detection is of practical significance. To study the characterization of axial crack by micromagnetic signal, a numerical model of micromagnetic signal to detect the axial crack of pipeline is established based on the micromagnetic theory in this article. The characteristics of micromagnetic detection signals of cracks with different sizes and directions are calculated. The propagation law of micromagnetic detection signal is analyzed, and the experimental study of X70 pipeline is carried out to verify the correctness of theoretical model and simulation. The results show that at each tip of the axial crack, the axial component of the micromagnetic detection signal has a peak and a valley, and the radial component has an extreme value. The amplitude of the micromagnetic internal detection signal at the axial crack linearly increases as the depth or length of the crack increases. The propagation law of micromagnetic signal conforms to the falling exponential function, specifically, the smaller the crack depth, the faster the signal decays and eventually stabilizes. The micromagnetic detection signal of the probes to detect the crack having a certain angle with the axial direction is arranged with equal displacement spacing.