The Dutch gas distribution network consists of about 20% (22,500 km) of unplasticised poly(vinyl chloride) (uPVC) pipes, most of which have been installed from the mid-sixties up to the mid-seventies of the previous century and have been in service ever since. Replacing the uPVC gas pipes exactly after the specified service lifetime of 50 years will lead to a costly and extremely labour intensive project in the next decade. Postponing the replacement is only an option when it can be done without compromising the integrity of the network. It is therefore of great value for the network operators to have full knowledge on the condition of the pipes in their network. In this thesis the framework for a method that can determine the condition, and therewith the residual lifetime, of uPVC gas pipes is developed. Recent failure data shows that themajority of the failures in uPVC gas pipes is caused by excavation activities (third-party damage). The risk of life threatening situations after such a failure is considerably higher for a brittle fracture than for ductile failure behaviour of the pipe. Brittle uPVC gas pipes should therefore be replaced, which makes the impact behaviour the limiting factor for the service lifetime of these pipes. A review of the degradation mechanisms occurring during the lifetime of uPVC pipes shows that physical ageing is expected to be the most important mechanism that causes embrittlement. During physical ageing the polymer chains move towards their thermodynamically favoured positions, causing an increase in resistance against plastic deformation. Moreover the deformation behaviour localises, causing embrittlement on a macroscopic scale. The focus of this thesis is therefore on the influence of physical ageing on the mechanical behaviour of uPVC gas pipes. The procedure of determining the residual lifetime is based on these findings and is split into four aspects/chapters: the choice for the yield stress as a measure for the condition of the pipe material, characterisation of the change of the yield stress time (its ageing kinetics), determining the critical condition and development of a method of measuring the current condition.
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