Polyethylene (PE) pipes and fittings are used extensively in distributing natural gas and water safely, reliably and economically, and enjoy an excellent performance track record. PE offers the pipe industry:
* Economical, high volume manufacture--extrusion, injection moulding;
* Design flexibility--easily shaped;
* Integrated design--multifunction, ready assembled components--couplers and fittings;
* Low material cost;
* Light-weight design--ease of transport and handling;
* Flexibility--ease of transport and handling, use in conjunction with trenchless technologies and resistance to seismic activity;
* Relative ease of jointing (compared to metallic pipe systems);
* Corrosion and chemical resistance;
* Biologically inert capabilities;
* Toughness, impact resistance, abrasion resistance and long term durability--technical lifetime of 50 years;
* Low temperature performance;
* Leak-free fusion jointing--low maintenance costs;
* Low friction bore--no scale build-up and efficient flow of transfer medium; and
* Environmental benefits--recyclable.
Today, PE is highly engineered and provides a good balance of strength, stiffness, toughness and durability meeting the demands of the gas pipe industry.
The failure of PE pipe systems is typically due to older generation materials whose properties offer limited resistance against severe environmental and operating conditions. The other factor is human error, committed in the supply chain and site construction of installed pipe systems.
The general mode of field failure reported for PE pipe is brittle, slow crack growth (SCG) through the pipe wall. These cracks can initiate at microscopic stress-raising flaws. These brittle mechanical failures are typically slit-type fractures that lie parallel to the pipe's extrusion direction. Circumferential hoop stress in the pipe wall is the driving force for crack opening. Typical slit type fractures are shown in Figure 1.
Circumferential cracks can also be initiated on either the outside or inside surface of pipes due to secondary stresses such as bending or impingement on the material. The premature failure of melt-fused joints is also common failure where cracking initiates at stress concentrations created by poor installation practices.
[FIGURE 1 OMITTED]
Visually, brittle cracks are typically smooth, featureless and devoid of any yielding and deformation process, as shown in Figure 2.
[FIGURE 2 OMITTED]
There are three major failure modes for PE pipe, as shown in Figure 3.