Plastic Pipes Conference Association # 1995 Edinburgh
Marshall, Ingham, Brogden
When clients purchase plastics pressure pipes, they have frequently made that decision because they know that plastics do not corrode and they assume that if they do not exceed the pressure rating then a long lifetime should be assured. (Low cost has also been known to be a factor).
It is thus of concern for the plastics pipe industry to have test methods which can reproduce the same loading conditions which are obtained in service and knowledge of materials behaviour which allows for the extrapolation of short term data to predict performance long into the future. In designing tests, it is most important to learn from historical service failures of pipe systems and to model/simulate the worst case conditions which are likely to occur. It is also important to recognise that the client may frequently have a less than perfect knowledge of the loadings which will be imposed and is unlikely to employ contractors who will install the system precisely to instructions. This is particularly true in the UK where contractors seldom read detailed specifications and are driven by economic considerations to install pipe as quickly as possible with labour forces who are often poorly trained.
In such circumstances, materials and products are required which have large reserves of toughness to withstand at least three factors being contrary to expectations. Usually: 1) pipes will be damaged and defects will be introduced during installation, 2) the service loading will seldom be static pressure alone and 3) backfill, bedding and surround materials may differ from those specified and will often be compacted inefficiently.
4) With fusion welded PE systems there will be added problems where contractors will disobey welding instructions.
There are materials which will withstand all such conditions. The problem is to determine with assurance that they are chosen and that the process of converting the raw polymer into pipe does not alter properties.
History has shown that a Fracture Mechanics approach forms the best framework for considering both the design and assessment of products, since it is implicitly assumed that the worst case of brittle failure caused by the propagation of sharp cracks 1s always a possibility. The objective of all FM testing and appraisal is to assume that materials are imperfect and analyses exist which allow ductile/brittle transitions to be predicted.
It is the purpose here to review how the Fracture Mechanics approach pioneered for plastics pipes in the UK by both academics [1] and industry [2] has helped to improve the understanding of pipe behaviour and develop new methods of assessment, which address the conditions actually obtained in service rather than the ideals so often assumed.
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