Investigation of plasticity-induced fatigue crack closure

Plasticity induced crack closure and constraint effects due to finite plate thickness are both fundamental aspects in the mechanics of fatigue cracks. Moreover, plasticity induced crack closure provides an effective first-order correction to the crack driving force, as used in the correlation and prediction of fatigue crack growth. The approach developed in this study utilises the distributed dislocation technique to model fatigue cracks growing under constant amplitude loading in finite thickness plates. Numerical results are obtained through the application of Gauss-Chebyshev quadrature and are presented for the crack opening stress ratio. An excellent agreement is observed with previous three-dimensional finite element studies

Effect of wall thinning on deformation and failure of copper‐nickel alloy elbows subjected to low cycle fatigue

Various industrial standards and safety regulations specify the minim number of cycles, which pressure equipment must tolerate during a single seismic event. The compliance with these standards and regulations is necessary in order to ensure safety and strength of critical structural components, eg, pressure pipe system of the primary coolant loop of nuclear reactors or condensed water supply of large hospitals or other large facilities. However, the strength can degrade with time due to flow accelerated corrosion causing wall thinning. Pipe bends are particularly susceptible to the localised corrosion, and, therefore, these structural elements need a special consideration. The current paper studies the effect of localised corrosion on low cycle fatigue behaviour of a copper‐nickel elbows using experiment and finite element method (FEM). The outcomes of the study demonstrate a quite low reduction of low cycle fatigue life even at severe local wall thinning at different locations

Determining openness and compressibility of natural fractures of carbonate reserves in the Logovskoye deposit

The large amounts of hydrocarbons and produced oil in Perm krai are found in carbonate reservoirs. Accurate determination of reservoir properties is currently one of the topical issues. Laboratory studies of core samples do not always allow to evaluate parameters of fractured environment because of the eventual core destruction (along the fractures) at the surface and tiny dimensions of the samples investigated. A method to determine parameters of natural fractures involving tracer injection into the injection wells in the Logovskoye deposit, Perm krai (Tournaisian-Famennian deposit) is presented. The data obtained by tracer studies are compared to those got by other methods of evaluating openness of natural fractures, such as a Warren-Root formula applied to process pressure build up curves in reservoirs with natural fracturing, and the relation discovered by V.D. Viktorin for carbonate reservoirs of Perm krai. An error of the results received by different methods does not exceed 5 %, which means fidelity and overall accuracy of the techniques applied and their feasibility for investigation of fracturing in carbonate reservoirs. It is established that pressure conductivity in carbonate reservoirs varies from the highest value corresponding to pressure conductivity in fractured-porous formation to the lowest value corresponding to pressure conductivity in the matrix pores. This allows us to conclude that a reservoir features natural fracturing. One of the most serious issues in 3D-simulation, especially of carbonate reserves, is accurate determination of permeability, openness and directional attitude of natural fractures. The studies with tracers coupled with hydrodynamic research (pressure interference test) in the process of carbonate reserves development permit to improve the quality of input data and fidelity of prediction calculations

On the residual opening of hydraulic fractures

Hydraulic stimulation technologies are widely applied across resource and power generation industries to increase the productivity of oil/gas or hot water reservoirs. These technologies utilise pressurised water, which is applied inside the well to initiate and drive fractures as well as to open a network of existing natural fractures. To prevent the opened fractures from complete closure during production stage, small particles (proppants) are normally injected with the pressurised fluid. These particles are subjected to confining stresses when the fluid pressure is removed, which leads to a partial closure of the stimulated fractures. The residual fracture openings are the main outcome of such hydraulic stimulations as these openings significantly affect the permeability of the reservoirs and, subsequently, the well productivity. Past research was largely focused on the assessment of conditions and characteristics of fluid driven fractures as well as proppant placement techniques. Surprisingly, not much work was devoted to the assessment of the residual fracture profiles. In this work we develop a simplified non-linear mathematical model of residual closure of a plane crack filled with deformable particles and subjected to a remote compressive stress. It is demonstrated that the closure profile is significantly influenced by the distribution and compressibility of the particles, which are often ignored in the current evaluations of well productivity. © 2013 Springer Science+Business Media Dordrecht.Luiz Bortolan Neto, Andrei Kotouso

Low cycle fatigue behavior of elbows with local wall thinning

There have been a number of studies concerning the integrity of high-strength carbon steel pipe elbows weakened by local pipe wall thinning, the latter can be typically caused by flow accelerated erosion/corrosion. In particular, the focus of several recent studies was on low cycle fatigue behavior of damaged elbows, mainly, in relation to strength and integrity of piping systems of nuclear power plants subjected to extreme loading conditions, such as earthquake or shutdown. The current paper largely adopts the existing methodology, which was previously developed, and extends it to copper-nickel elbows, which are widely utilized in civil infrastructure in seismically active regions. FE (finite element) studies along with a full-scale testing program were conducted and the outcomes are summarized in this article. The overall conclusion is that the tested elbows with various severity of local wall thinning, which were artificially introduced at different locations, demonstrate a strong resistance against low cycle fatigue loading. In addition, elbows with wall thinning defects possess a significant safety margin against seismic loading. These research outcomes will contribute to the development of strength evaluation procedures and will help to develop more effective maintenance procedures for piping equipment utilized in civil infrastructure

Stress singularities resulting from various boundary conditions in angular corners of plates of arbitrary thickness in extension

Copyright © 2005 Elsevier Ltd All rights reserved.The stress singularities in angular corners of plates of arbitrary thickness with various boundary conditions subjected to in-plane loading are studied within the first-order plate theory. By adapting an eigenfunction expansion approach a set of characteristic equations for determining the structure and orders of singularities of the stress resultants in the vicinity of the vertex is developed. The characteristic equations derived in this paper incorporate that obtained within the classical plane theory of elasticity (M.L. Williams’ solution) and also describe the possible singular behaviour of the out-of-plane shear stress resultants induced by various boundary conditions.Andrei Kotousov and Yaw Tong Lewhttp://www.elsevier.com/wps/find/journaldescription.cws_home/297/description#descriptio

Comments on paper by Yang and Guo 'On using the Kane-Mindlin theory in the analysis of crack problems'

The original publication is available at www.springerlink.comAndrei Kotouso

Special Issue on “Mechanical Behaviour of Aluminium Alloys”

Aluminium alloys are the most common type of non-ferrous material utilised for a wide range of engineering applications, namely in the automotive, aerospace, and structural industries, among others. [...