Jan Poduška

The Effect of Residual Stress on the Process of Crack Growth Rate Determination in Polymer Pipes

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.

Compression-Loaded Cracked Cylinder - Stress Intensity Factor Evaluation

As polymer parts are made by injection molding, most defects are usually caused by the shrinkage of material during solidification. These defects are then source of cracks in these parts. Present contribution is dealing with central cracked compression loaded polymer cylinder. In order to describe crack behavior under operational load, parametrical finite element model was developed. As results of this study stress intensity factors for central crack with different crack lengths are determined to describe crack behavior.

The Effect of Specimen Size on the Determination of Residual Stress in Polymer Pipe Wall

As a result of the production process, there are axial and tangential residual stresses present in pressure pipes made of polymer materials such as polyethylene or polypropylene. The residual stress magnitude and distribution have a significant influence on the pipe lifetime. In this contribution the results from experiments focused on determining the tangential residual stress distribution in the walls of polypropylene pipes of different dimensions are compared. The experimental method used involves measuring the deformation of ring shaped specimens that were slit in the axial direction. Measured deformation of the ring specimen is a result of the tangential and axial stress superposition. However, the effect of the axial residual stress depends on the specimen axial dimension and tangential residual stress estimated basing on experimental data should be corrected according to axial dimension of the specimen used. The correction suggested in this article is determined based on three-dimensional FEM simulations of the experiment.

Inaccuracy in Residual Stress Estimation and its Influence on the Residual Lifetime of Polymer Pipes

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.