Plastic Pipes Conference Association # 1985 York
Wolters
The modified Robertson test is a test method by which ina rather simple and quick way crack arrest and (limited) crack propagation are distinguished, provided that crack speed over the crack length is measured.
Rather large variations in rapid crack propagation (RCP) behaviour of different PE types have been found. At lower temperatures the crack speed is higher, and hence, the possibility of RCP increases. Annealing of PE pipes at 80 ~C strongly improves the resistance to RCP.
Polyethylene pipes are used to an increasing extent for gas distribution pipeline systems at the higher pressure levels (up to 4 bar). The allowable internal pressure, however, is limited by a.o. the possibility of rapid crack propagation.
At higher pressures this possibility increases.
Rapid crack propagation (RCP) is a phenomenon in which a crack runs at a very high speed (in the order of some hundreds of metres a second) in axial direction over great distances along the pipe. Particularly from the view-point of safety this is an undesirable situation.
A distinction should be made between crack initiation and (rapid) crack propagation. The initiation of a crack is possible by for instance impact blows (third party damage). By proper material selection crack initiation can be largely suppressed, but not completely avoided. The initiation of such a crack in plastics pipelines under operating pressure and temperature may lead to RCP. The conditions at which RCP in PE pipelines may occur, therefore, should be established and tests and requirements regarding RCP should be developed to guarantee a safe and reliable use of the PE gas pipeline system.
RCP in PE pipes has been investigated intensively during the last few years by Wolters and Ketel (1, 2), Vancrombrugge (3), Greig and Ewing (4), Prabhat et al.
(5), Gaube and Mueller (6) and Bragaw (7). Various test methods have been used in these investigations, like the so-called modified Robertson test, developed by Vancrombrugge (3).
This paper reviews investigations performed at VEG-GASINSTITUUT into RCP in PE gas pipes, in which use was made of this modified Robertson test. The significance of this test will be shortly discussed and the effects of temperature, pressure, PE type, and residual stresses, will be given.
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