Influence of Residual Stresses on Lifetime Prediction for Plastic Pipes

Session 8B
9:50 am

Jurgen Heinemann, Techische Universitat Darmstadt

The principal suitability of the procedure for the lifetime analysis of plastic pipes described in ISO 9080 is substantiated by long-lasting test and practical experience for almost 60 years now. Also, no cases are known of systematic damage due to the failure of plastic pipes because of an insufficient lifetime analysis. Tests conducted on pipes in operation for over 40 years do not provide any contrary evidence.

Nevertheless, the production of plastic pipes is usually done via extrusion through a form-giving profile tool, causing a different orientation of the polymer chains. Additionally, the developing temperature gradient in the pipe wall during the cooling process leads to the formation of a distinct morphology profile across the pipe wall. Furthermore, the solidification process which progresses from the outside inwards causes an uneven volumetric contraction.

The effects of residual stresses resulting from the aforementioned processes and their later relaxation have not been yet a subject to a systematic investigation. With the aim to establish this relationship a collaborative research project is conducted, which will enable an even more accurate lifetime analysis for plastic pipes compared to the standard lifetime extrapolation.

Based on internal pressure creep rupture tests at four different temperatures conducted with plastic pipes made of three different materials (PE 100, α-nucleated PP-H and β-nucleated PP-H), with two different dimensions and three different residual stress levels, a lifetime analysis using the standard extrapolation method is performed.

Subsequently, an evaluation of the results as a function of the residual stresses is carried out with respect to an accompanying material characterisation and chemical analysis. This includes the determination of the distribution of the residual stresses, the degree of crystallinity and the orientation of the polymer chains over the pipe wall as well as the analysis of the thermal behaviour and different mechanical characteristics.

Dipl.-Wirtsch.-Ing. Jürgen Heinemann (1), Dipl.-Ing. Hansgeorg Haupt (1), Dipl.-Ing. Alexander Bockenheimer (1), Prof. Dr.-Ing. Christina Berger (1), M.Sc. Raquel Maria2, Dr. Robert Brüll (2)
1) Technische Universität Darmstadt, Zentrum für Konstruktionswerkstoffe, Staatliche Materialprüfungsanstalt / Fachgebiet und Institut für Werkstoffkunde, 64283 Darmstadt, Germany,
2) Deutsches Kunststoff-Institut, 64289 Darmstadt, Germany