Defects classification and failure modes of electrofusion joint for connecting polyethylene pipes

Abstract

As electrofusion (EF) technology is widely used in connecting polyethylene (PE) pipes and other plastic pipes or composite pipes, research in safety assessment of EF joints has been of major concern. EF joints with defects are very common in practical applications. These defects may greatly reduce the mechanical performance of the EF joints and threat safety running of the pipeline system. To evaluate hazard of these defects and provide a basic understanding for the failure mechanism of EF joints, a comprehensive study on defects and failure modes is conducted in this work. The defects in EF joints are classified into four categories: poor fusion interface, over welding, voids, and structural deformity. The forming reasons of these defects are analyzed in detail. The mechanical properties of EF joint containing these defects are investigated by conducting peeling tests and sustained hydraulic pressure tests. Test results show that there are three main failure mode of EF joint under inner pressure, that is, cracking through the fusion interface, cracking through the fitting, and cracking through copper wire interface.

A review of nondestructive examination technology for polyethylene pipe in nuclear power plant

“…Defects in EF joints of PE pipes can be categorized to poor fusion interface, void, structural deformity (dislocation of heating wires), and cold welding [12]. Meanwhile, defects in butt fusion (BF) joints are classified as crack, poor fusion interface, void, and cold welding [13].…”

Low temperature tensile lap-shear testing of adhesively bonded polyethylene pipe

“…However, one key disadvantage of using PE in these applications is the difficulty in producing safe and reliable pressure tight joints when joining the material together to connect pipelines or services and fittings. This safety critical area is receiving increasing attention as any failure in these pipeline systems invariably leads to significant economic losses as well as potential loss of lives [8].…”

“…Fig 2 shows three failure modes that can occur as a result of defects in EF joints, the failure mode is known to depend on the length of the cold welding zone and the input energy, but it remains largely inexplicable in practical applications [8]. [8].…”

Mechanical behaviour of adhesively bonded polyethylene tapping tees

“…Polyethylene (PE) has been applied as an effective pipeline material since its 2 introduction in the 1960s [1] and has played an important role in the rebuilding and 3 modernisation of mains gas and water supply networks as well as sewage systems 4 [2]. PE pipes in the form of medium-density polyethylene (MDPE), which has a 5 minimum required strength of 8 MPa and is designated PE80 and high-pressure 6 polyethylene (HPPE) which has a minimum required strength of 10 MPa and is 7 designated PE100, both offer a low cost, fast way to repair or replace old networks 8 with many advantages over metal pipes, such as higher strength-to-weight ratio, 9 ease of jointing, higher impact strength, higher flexibility and higher chemical and 10 corrosion resistance [3][4][5][6][7].…”

“…Electrofusion, however, does not guarantee a perfect joint; 4 where mixing of the two molten parts contained within the fitting does not occur 5 properly, either by surface contamination, part mismatching or a short circuit, only a 6 partial bond will be achieved [14]. Where defects occur in the fusion zone, these can 7 act as stress initiation sites for slow crack growth (SCG) which will propagate 8 through the fused joint, eventually leading to failure [1,3] The possibility of using adhesives as an alternative solution to electrofusion and 1 mechanical fixing of tapping tees has received little consideration due to the difficult 2 nature of bonding PE. Previous attempts to bond PE to PE have resulted in poor 3 mechanical and physical adhesion properties due to its low surface energy [15] This current study will focus specifically on the mechanical behaviour of PE80 21 tapping tees bonded to PE80 pipe using various commercially available adhesives.…”