Search

In the study, experimental and theoretical studies were carried out to assess the influence of the shapes of dents in the tank wall on the stress-strain state of the defect zone. By testing fragments of a cylindrical tank, it was found that the most appropriate expression is (5), which could take into account the leaching of the tank wall, resulting in a decrease in the stress concentration index. At the same time, during theoretical studies in this paper, it was found that polynomials determined the stress concentration coefficient, where the obtained analytical expression data were compared with the data determined numerically in the ANSYS program, and it was found that the spread was from 2% to 10%. According to the results of a numerical study of the stress-strain state of the dent zone in the tank wall, graphical dependences of the stress concentration coefficient on the dimensionless depth of the dent for various values of the dimensionless radius of the dents and do not exceed 2% of the indicators that are obtained. At the conclusion of the experimental and numerical studies, a conclusion was made about the degree of influence of the geometric dimensions of the dents on the stress concentration index. [ABSTRACT FROM AUTHOR]

Ground explosions are one of the threats to the structural integrities of buried pipelines. To investigate failure modes of buried pipelines subjected to ground explosions, numerical calculation models of buried pipelines in soil and rock layers were established. The stress, strain, plastic deformation and failure modes of the buried pipeline were investigated. The effects of the explosion magnitude, diameter-thickness ratio, burial depth, explosion height and explosion offset on the failure behaviour of the pipeline in the rock layer were studied. The results showed that the pipeline deformation process only lasted for 0.03 s. The high stress zone and plastic strain zone were present in the upper part of the pipeline, and a dent appears. A buried pipeline in a rock layer is more dangerous than that in a soil layer due to the reflection of the explosion wave in the rock layer. The deformation, plastic strain and total energy of the pipeline increase gradually in the rock layer with increasing explosion magnitudes or diameter-thickness ratios, but these indices decrease with the increased burial depths, explosion heights and explosion distances. Empirical formulas can be used to predict the strain and dent rates of the buried pipeline. These results can provide the theoretical basis and references for laying pipeline, as well as for their safety evaluations and maintenance, etc. [ABSTRACT FROM AUTHOR]

The aim of this work is to analyse the local and the global structural behaviour of rectangular steel plates with a local dent. Nonlinear finite element analyses have been performed to explore the effect of different dent depths on the ultimate strength and the post-collapse behaviour. The post-collapse modes are discussed and the change of the buckling mode for different plate thickness ranges is categorized. The behaviour in post-collapse regime is analysed using the defined stress–strain rate. Two relationships have been developed to estimate the ultimate strength reduction as a function of the plate slenderness and dent ratio. Based on the existing ultimate strength design formula and a new developed factor, a new formulation for ultimate strength of damaged plate has been introduced accounting for the effect of local dent. The new developed formulation is capable to account for an initial global imperfection, residual stresses, openings, corrosion deterioration and existence of dents. [ABSTRACT FROM AUTHOR]

Changes in the internal pressure and the temperature in a steel pipeline generates cycles of high axial compression/relaxing in the line. This makes the line vulnerable to cyclic creep and progressive plastic straining (ratcheting). On the other hand, offshore pipelines are usually laid unburied, so they might become disposed to heavy accidental transverse loads, such as those caused by impacts from trawl gears, anchors or other dropped objects. This paper deals with the ratcheting behavior of dented steel offshore pipelines under uniaxial cyclic loading. The experimental study were conducted on small scale specimens from X80 steel pipes with an outer diameter of D=44mm and D/t of 22. Effects of some key parameters such as the initial pre-strain, the cyclic stress amplitude and the dent depth magnitude were investigated. It was found that while the initial pre-strain and the cyclic stress amplitude had strong effects on the ratcheting response of both dented and intact tubes, the strain ratcheting rate was radically increasing by the presence of a dent. The dent was also significantly affecting the number of stress cycles which could be tolerated by the pipe. When the initial pre-strain was low the changes in the ratcheting behavior of the dented pipes and the intact pipe were less significant. With higher initial pre-strains, even very small dents were resulting in remarkable degradation of the pipe ratcheting response. [ABSTRACT FROM AUTHOR]

Highlights: [•] Modeling of cutting forces while applying a slicing motion to the push-cutting process. [•] Analysis of state of stress at the cutting edge. [•] Stress Intensity Factors for cutting with a symmetrical and asymmetrical cutting knife. [•] Fracture toughness of the paper sheet material is determined through DENT test. [•] The new model is verified by sideways push-slice cutting stacked thin paper material. [Copyright &y& Elsevier]

Highlights: [•] Experimental burst tests have been performed on vessels containing a dent+gouge defect. [•] The gouge dimensions were identical for all the vessels. [•] The influence of the dent depth on burst pressure was analysed. [•] The most of time, failure occured on other place than that of the defect. [•] A criterion based on stress triaxiality was proposed to evaluate the burst pressure and failure location. [Copyright &y& Elsevier]

Abstract: The origin of more than 90% in service pipeline failures is attributed to the presence of stress concentration areas such as defects in forms of gouges, dents, or when these defects are present together. These defects are provoked by external factors interferences, for instance by excavation machines. The influence of the dent depth on the burst pressure of the pipeline was evaluated. An approach based on a simple local strain criterion has been proposed to predict the damage and the failure of evaluated dented pipe. To validate a purely empirical rule, which specifies that a critical depth dent is equal to 10% of outside diameter of the pipeline, a series of mechanical tests (denting tests and burst tests) were conducted. Finite element analysis used in order to compare the numerical and experimental results. Experimental results showed that a single dent did not affect the burst pressure of the investigated specimens; therefore the rule of a critical depth of dent equal to 10% was found very conservative. [Copyright &y& Elsevier]

Abstract: A new mechanistic approach is used to characterize resistance of polyethylene to deformation and fracture in double-edge-notched tensile test. The new approach considers all three mechanisms involved in the fracture process, i.e. for fracture surface formation, shear plastic deformation, and necking, and can be used to determine values of specific energy consumption for each mechanism. This is different from the conventional approach, known as essential work of fracture (EWF), which does not consider the difference between shear plastic deformation and necking. Results from the new approach for a polyethylene copolymer show that specific energy density for fracture surface formation is about half of that determined from the EWF approach, and specific energy density for necking is very close to that determined from simple tensile test. The latter provides some support for validity of the new approach in characterizing fracture behaviour of polyethylene when accompanied by large deformation and necking. The paper also points out crack growth conditions that have to be met for valid application of the EWF approach and shows that such conditions are not met when deformation and necking occur in polyethylene. [ABSTRACT FROM AUTHOR]

Abstract: Application of the crack compliance method to the analysis of thin-walled rings with a radial crack has two features: a crack is considered as a concentrated angular compliance and the deformation of all other sections of the rings is calculated as for a curvilinear beam. The latter can be most conveniently found by the method of initial parameters where the values of generalized forces and displacements at the end of some zone are determined as a linear combination of their values at the beginning of the zone. The goal of the study is to derive and apply the method of initial parameters equations taking into account the influence of circumferential stresses on the ring curvature. As far as the authors know, this is the first time that the stress intensity factor has been derived for an elastic thin-walled pipe with a radial crack in a geometrically nonlinear formulation. Here, an increase in pressure leads to a somewhat slowed increase in the stress intensity factor. In addition, a number of problems for dents are considered. The effect of the dent shape on the stress–strain state is analyzed. An expression for the stress intensity factor for a complex defect, a crack emanating from the dent apex, is presented. [Copyright &y& Elsevier]

To calculate stress intensity factors in elastic pipes with long flaws, the known Cheng-Finnie method is used. A universal description of the mode of behavior of beam is proposed, for which equations of the method of initial parameters with allowance for geometric nonlinearity (effect of axial force on lateral deformation) have been derived and are employed. This allowed us to determine, apparently for the first time, the values of stress intensity factors in thin-walled pipe in a geometrically nonlinear formulation, when increase in pressure leads to their somewhat “ retarded” increase. The results have been generalized for a more complex flaw-crack starting from the dent tip. [ABSTRACT FROM AUTHOR]

Dents have been identified as among the most common deformation defects in oil and gas pipelines. In this study, the multidimensional mechanical responses, springback, and rerounding behaviors of dents as well as the interaction of multiple dents on pipelines and their effect on pipe integrity were investigated through the three-dimensional finite element method and full-scale burst tests. The burst test demonstrates that a plain dent has a minimal effect on the pipeline burst pressure, and the burst failure position is not located in the dented area. Generally, when a dented pipeline that is subjected to monotonically increasing internal pressure bursts, the dent does not fully reround, and a convex protuberance forms in the axial shoulders on both sides of the dent. During pressurization, the maximum equivalent plastic strain in the dented pipe always occurs on the inner surface near the dent center in which no necking is found in the radial direction of the wall thickness. However, a larger equivalent stress is observed around the edge of the dent center, whereas the stress at the center is relatively small, resulting in the formation of a plastic hinge in the dented area. If internal pressure fluctuations occur, this can easily lead to fatigue failure. Finally, the interaction among multiple longitudinally aligned dents was investigated, and the critical spacing affecting the integrity of the pipeline was determined. The investigation reveals that when the dent spacing exceeds the pipe diameter, the interaction between two dents may be ignored, and the dent effect on the pipe integrity may be separately evaluated. However, if the dent spacing is less than 0.5 times the pipe diameter, then the dents must be treated as a combined dent. Further, if the spacing is 0.5–1.0 times the pipe diameter, then dent interaction must be considered. • Developed a finite element model to assess the integrity of pipelines with multiple longitudinally aligned dents. • The edges of the dent center show highly strain sensitivity to the variations of internal pressure. • API 579 suggests the empirical value of rebound coefficient at 1.43 may lead to non-conservative result. • A circle of plastic hinge was observed around the dent which may cause fatigue of pipe material. • The critical spacing distance of interaction between multiple longitudinally aligned dents on pipelines was determined. [ABSTRACT FROM AUTHOR]