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Influence of Residual Stress on Lifetime Prediction of Polymer Pipelines
Poduška, Jan ; Nezbedová, Eva (referee) ; Malíková, Lucie (referee) ; Hutař, Pavel (advisor)
The lifetime of plastic pipes for water supply and other applications is demanded to exceed at least 50 years. Such a long lifetime is difficult to prove by standard testing methods like the hydrostatic pres-sure test. However, it is possible to calculate an estimation of the lifetime, as the most frequently oc-curring mechanism of failure of plastic pipes is the creep crack propagation and subsequent failure. The method is based on describing the crack propagation by parameters of the linear-elastic fracture mechanics. An important part of this method is a finite element simulation of crack propagation in a pipe loaded by various types of loads. Residual stress, a side product of solidification after extrusion, is one of these loads. This thesis begins with an introductory part and literature review of the relevant topics – most of all the typical material properties of the pipe materials, mechanisms of failure, methods of residual stress determination suitable for plastic pipes and their results, methods of testing the materials and calculating lifetime. After the introduction, the problems to be solved are defined. The description of the residual stress state in the wall of various plastic pipes is the main topic. Both tangential (hoop) and axial residual stress in pipes of different dimensions and materials are determined using a combination of experiments and numerical simulations. Also, a simplified method of tangential residual stress is designed that can provide a sufficiently precise description of the tangential resid-ual stress state and is not difficult to carry out. A method to include the residual stress in the lifetime calculations and its influence on the lifetime is also dealt with. Apart from residual stress, the influence of soil loads in case of a buried pipe is studied. The residual stress can also influence the experimental determination of crack growth rate. If the CRB (cracked round bar) test is used to measure the crack growth rate, the crack can propagate asymmetrically due to the presence of residual stress in the specimens, which affects the results. Based on a finite element simulation of crack propagation in a CRB specimen, the severity of the influence is assessed.
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Influence of Residual Stress on Lifetime Prediction of Polymer Pipelines
Poduška, Jan ; Nezbedová, Eva (referee) ; Malíková, Lucie (referee) ; Hutař, Pavel (advisor)
The lifetime of plastic pipes for water supply and other applications is demanded to exceed at least 50 years. Such a long lifetime is difficult to prove by standard testing methods like the hydrostatic pres-sure test. However, it is possible to calculate an estimation of the lifetime, as the most frequently oc-curring mechanism of failure of plastic pipes is the creep crack propagation and subsequent failure. The method is based on describing the crack propagation by parameters of the linear-elastic fracture mechanics. An important part of this method is a finite element simulation of crack propagation in a pipe loaded by various types of loads. Residual stress, a side product of solidification after extrusion, is one of these loads. This thesis begins with an introductory part and literature review of the relevant topics – most of all the typical material properties of the pipe materials, mechanisms of failure, methods of residual stress determination suitable for plastic pipes and their results, methods of testing the materials and calculating lifetime. After the introduction, the problems to be solved are defined. The description of the residual stress state in the wall of various plastic pipes is the main topic. Both tangential (hoop) and axial residual stress in pipes of different dimensions and materials are determined using a combination of experiments and numerical simulations. Also, a simplified method of tangential residual stress is designed that can provide a sufficiently precise description of the tangential resid-ual stress state and is not difficult to carry out. A method to include the residual stress in the lifetime calculations and its influence on the lifetime is also dealt with. Apart from residual stress, the influence of soil loads in case of a buried pipe is studied. The residual stress can also influence the experimental determination of crack growth rate. If the CRB (cracked round bar) test is used to measure the crack growth rate, the crack can propagate asymmetrically due to the presence of residual stress in the specimens, which affects the results. Based on a finite element simulation of crack propagation in a CRB specimen, the severity of the influence is assessed.
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Simulation of the crack growth in the weld of the polyolefin pipes
Ševčík, Martin ; Hutař, Pavel ; Náhlík, Luboš
The aim of this work was to study the effect of the material properties change due to the butt welding on the stress intensity factor in polyolefin pipe. The gradient change of the Young’s modulus was assumed. The crack was located in the middle of the affected zone. The stress intensity factors estimated in both graded region and basic material (homogenous case) were compared. Strong influence of the graded region on the value of the stress intensity factor was observed. Therefore, for realistic estimation of the welded pipe lifetime the change of the material properties inside the polymer weld should be taken into account.
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