Investigations of Polyethylene Materials by Means of X-ray Diffraction
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Date
2011-01-21
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Abstract
This work deals with the investigation of polyethylene gas pipes by means of
x-ray diffraction, which have been in use for 35 years for the supply of gas. A
lifetime of 50 years was predicted for these polyethylene (PE) pipes of first
generation. A remaining life time of 15 years for all gas pipes installed in the
seventies has thus to be expected. In order to prevent expensive and complex
replacement of pipes after failure, parts of the pipes are removed from the
gas grid. Afterwards, so-called creeping tests are conducted, which serve as
a simulation method for the ageing behaviour and for the estimation of the
remaining life time.
Ageing processes are atomic modifications, for which creeping tests are in-
sensitive. In order to gain a more detailed insight into these ageing processes
and to identify indicators for ageing, pipe parts have been exposed to high
temperatures and pressures. This procedure leads to an artificial ageing of
the material additionally to the natural ageing due to the 35 years period
of use. Sample slices have been prepared out of these artificially aged pipe
parts and out of the corresponding natural aged pipe, in order to analyse
the material's structure depending on the position within the pipe wall and
to compare the results. X-ray diffraction, small angle x-ray scattering and x-
ray tomography experiments have been conducted at di erent experimental
stations to investigate the materials structure on nanometre and micrometre
length scales.
The sample's crystallinity of the semicrystalline material turned out to be
the searched indicator for ageing. With increasing age, the material becomes
more crystalline and thus more brittle. Furthermore, this change was the most
pronounced at the inner surface of the pipe wall. Texture analysis showed
moreover a preferred orientation of unit cells parallel to the long axis of the
pipe, which is also stronger pronounced at the inner surface than for the rest
of the pipe wall. These are thus most vulnerable for material failure.
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Keywords
Polyethylen, Alterung, Kristallinität, Röntgenkleinwinkelsteuerung, Pulverdiffraktion, Zeitstandsversuche, Restlebensdauer, Ageing, Crystallinity, X-ray diffraction, Powder diffraction, Small angle x-ray scattering, Texture, Brittleness, Creeping tests, Remaining lifetime