Plastic Pipes Conference Association # 2004 Milan
Janson, Bergstroem, Baeckman, Blomster
When performing strength quality control of PE pressure pipes made of new grades it is important to take the creep vs. time into consideration and not only the common stress vs.
time behaviour. Thus, it is just the relation between linear strain and logarithmic time, which will within a reasonable testing time help us to reveal important information regarding the long-term functionality of the pipe. Due to a physical ageing process it has been found that a rectilinear strain vs. time relation will show up after a certain time for a high quality pipe material subjected to a feasible hoop stress. Janson (1).
Since the introduction of PE 100 grade pipes and based on the above mentioned observation the Swedish KP-Council has required a special verification in this respect as a basis for approval and KP-marking of such pipes. The actual problem was connected to the fact that the long-term strength of the PE 100 material had at that time only been studied according to the traditional relation between hoop tensile stress and time to burst. As the stresses referred to, seem to give rise primarily to the ductile type of failure, or failures which will be preceded by large relative strains, it is obvious that also the relation between strain and loading time has to be recognised. Hence it was found necessary to introduce a maximum design strain after long time in addition to the conventional maximum design stress requirement. An application of the verification principle is illustrated in Figure 1, from Janson (1).
The modern “Low Sag” resins makes it possible to produce pipes with larger diameters and thicker walls than previously. Thus, a pipe with a diameter of 1 400 mm and a wall thickness of 100 mm was recently produced by Pipelife Norge. AS. The pipe material was a Borealis PE 100 compound classified after ISO/TR 9080. The long-term strength tests were performed by Bodycote Polymers, Sweden using ordinary small pipes with dimensions 32 × 3 mm. Without questioning the fulfillment of this extensive test it must be realised that the small 32 × 3 mm pipes not in all respect can be representative for a 1 400 × 100 mm pipe. –Therefore it was recommended to make a verification of pipe properties by the method illustrated in Figure 1. As a complement also the density and MFR variation through the pipe wall was measured and the thermal stability of the material in the internal surface of the pipe was checked. Using a separate pipe sample it was also possible to determine the level of frozen in stresses in the pipe.
Having the capability of performing pressure tests of up to 1 600 mm pipes the SP, Swedish National Testing and Research Institute in Gothenburg, Sweden, was given the task to perform the tests. SWECO was commissioned by Borealis to define the test programme, follow the tests, evaluate the measurements and to summarise the results in a final report.
This work should also be made in co-operation with Gunnar Bergström, SP
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