Papers by Keyword: Slow Crack Growth

Abstract: Outstanding durability of plastic pressure and non-pressure pipes can cause difficulties, when a reasonable lifetime estimation is needed. It is impossible to prove the lifetime by testing, but there is a method of calculation that can provide a certain idea about the expected lifetime. The lifetime estimation is based on the assumption that the failure occurs as a result of the slow crack growth mechanism and it is calculated using the linear elastic fracture mechanics approach. Numerical simulations of crack growth in the pipe are necessary for this calculation. These simulations must consider various effects that can play a role in the lifetime. This paper deals with the lifetime calculations of a pressure and a non-pressure corrugated pipe considering the soil loads acting on pipes when they are buried. In the simulation of the pressure pipe, a combination of loads is applied that consists of internal pressure, residual stress and the soil loads. The influence of the loads is discussed. The non-pressure corrugated pipe is loaded by the soil loads only.

Abstract: Accelerated tests of the polyethylene pipes are necessary for the pipe lifetime calculations. Accepted methodology for prediction of slow crack grow rate in these materials is fatigue testing of CRB (cracked round bar) specimens. This paper deals with the FEM modelling of the crack propagation during the CRB test under the influence of residual stresses. The crack growth is described based on the stress intensity factor and Paris-Erdogan law. The purpose of this research is to determine, if the residual stress influences crack behavior during CRB test.

Abstract: Fracture behaviour of a three-layer polymer pipe subjected to nonhomogenous distribution of external pressure induced by soil embedding is studied in this paper. Both long term and additional short term loading is considered. Such loading induces tensile stresses in the inner pipe wall which can lead to crack initiation and further slow crack propagation. The material interface between a protective layer and the base pipe can contribute to crack deceleration and can prolong the residual lifetime of the pipe. The paper presents three-dimensional numerical analysis of a commercial three-layer pipe containing an internal semi-elliptical crack. The effect of soil load on the fracture behaviour of the cracked pipe is quantified and discussed.

Abstract: This paper presents a methodology suitable for estimation of residual lifetime of polymer pipes. The linear and non-linear distribution of residual stresses in the pipe wall is studied using FEM analysis. The approximate relation for the stress intensity factor calculation is presented. It is shown that the presented relation gives a stress intensity factor similar to the FEM analysis for the linear or non-linear distribution of residual stress. The suggested procedure produces a slightly conservative lifetime estimation where the accuracy of the procedure increases with increasing ratio between applied internal pressure level to residual stress level. The accuracy of the residual stress estimation and corresponding stress intensity factor is discussed and a final recommendation for lifetime determination based on simplified methodology is given.

Abstract: In this paper a methodology for assessment of residual stress effects on crack behaviour in the polymer pipe is developed. For simplicitys sake, a linear distribution of residual stresses across the pipe wall is assumed. Linear elastic fracture mechanics is used for the fracture mechanics analysis of the cracked pipe. An approximate relation for the stress intensity factor estimation for a crack in a polymer pipe, with residual stress taken into account is suggested and discussed. The methodology presented can be helpful for a rapid lifetime estimation of polyolefin pipelines.

Abstract: Ba_0.5Sr_0.5Co_0.8Fe_0.2O_{3 – δ} is a mixed ion-electron conductor with high application potential as high-temperature gas separation membrane. However, in practical use the integrity of this brittle perovskite is challenged by the mechanical boundary conditions of transient temperature exposure. Moreover, long term failure mechanisms such as static fatigue at room temperature and creep rupture at operation temperature might occur. The relevance of both effects for BSCF has been investigated. The slow crack growth at room temperature has been determined using bi-axial bending under different loading rates. The creep rupture at elevated temperature has been analyzed from three-point bending tests. The results indicate favourable behaviour of BSCF in both cases. A low risk of failure due to slow crack growth exists and the strain to failure in combined tensile - compressive mode reaches up to 40 %.

Abstract: The multi-layered structure components of dissimilar materials are used in many engineering applications to protect the base structure from outer damage. Typical examples of coated structures are pipes with the dimensional addition of protective layers (multi-layer pipes). The purpose of this development is to protect the main (functional) part of the pipes from damages (e.g. surface scratches, internal crack propagation). In the contribution the attention is paid to cracks existing and propagating in inner protective layer. In many cases the cracks are stopped at the interface between protective and functional layers. The important task is to decide if they penetrate further through the interface in the bulk material and thus cause the failure of the system. The critical stress for the crack propagation through the interface depends on the relation between bulk and coating materials as well as on the crack geometry. All these important topics are taken into account and their effect on crack propagation is broadly discussed.

Abstract: In the contribution solution of special fracture mechanics problems connected with multi-layer plastic pipes was investigated. The assumptions of linear elastic fracture mechanics were accepted. A complex three-dimensional numerical model of multi-layer pipe system consisting of main (functional) pipe and protective layers has been suggested and numerically solved by finite element method. Two basic problems connected with lifetime expectation of multilayer pipe system have been considered and discussed, namely: question of fracture mechanics description of multilayered pipe system and corresponding measurements of the material properties. The suggested approach can help for more accurate estimation of the multilayer pipe damage.

Abstract: The relationship of KR versus crack length c (R curve) for Al2O3-30wt.% Ti(C,N).and for comparison alumina ceramics has been examined. The R-curve has been evaluated using pronounced long-crack formed during the three point bending (3PB) of the double edge notched beam. A combination of in situ microscopic crack growth observation and mechanical testing enabled measurement of crack growth resistance curves. The special device consisting of light microscope coupled with CCD camera, was fitted to Zwick 1446 testing machine. These observations reveal the existence of flat R-curve for Al2O3-30wt.% Ti(CN) and increasing R-curve for pure alumina. A study of slow-crack-growth (SCG) in tested materials was carried. The load-relaxation technique was used for observation at slow-crack-growth. The crack length was evaluated by linear-elastic analysis from the compliance of single-edge-notched specimen in three-point bending test. Parameters of stable crack growth n and logA, work-of fracture (WOF), stress intensity factor at the moment of crack initiation KI0 and maximum values of stress intensity factor KImax were determined. Mechanism of grain bridging responsible for occurrence of R-curve was observed by SEM and TEM.

Abstract: For explaining the SCG behavior of polyethylene, the crack layer theory is applied based on the description of two driving forces: crack and PZ. The relations between the speed of SCG, crack length and elapsed time are the most important characteristics of polyethylene resistance to crack propagation, or long-term brittle fracture. The crack layer model of slow crack growth in polyethylene is designed in such a way that it qualitatively reproduces the main features of the process indicated above and makes it possible to quantitatively match any pattern of step-wise crack growth. In this paper, the behavior of SCG of polyethylene is developed for numerical simulation based on the crack layer theory. Some parametric study and applications are addressed based on the developed simulation program.