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Incomplete Weld in a HDPE Pipe Reducer

ENVIRONMENT:

Eastern U.S. soil.

EQUIPMENT:

Buried HDPE Reducer

MATERIAL:

<12 months

FAILURE:

Incomplete Weld

 

Description of the Material

A pipe subassembly approximately 4-ft long  was comprised of three parts; 1) a 42-inch diameter pipe section which made up about half of the entire length of the specimen, 2) a 30-inch diameter pipe section approximately 1-ft long, and 3) joining sections (1) to (2) was a series of three reducers, all of which were fused together to make one unit.

Background 

The reducer section had been installed 11 months prior to its failure as a replacement for cast iron pipe of similar diameter.  The pipe line was used to transport river water to a plant where it was used as cooling water for a manufacturing processes.  Although it is unknown when failure occurred, it is suspected that the fissure at the joint propagated to the point where water emerged from the ground.  At this stage, the pipe section was excavated and removed for analysis.  During our field investigation to the site, it was observed that a new HDPE section had been installed and would soon be buried in concrete. 

 
Findings
Visual and Macro Examination

There were a total of four joints examined.  Starting from the 30” diameter pipe, each of the first three fusions appeared to be fabricated under controlled conditions.  The interior surfaces of each of these joints showed signs of mechanical smoothing of the beads that are expected from fusion bonding of polymeric materials, such as this.  The integrity of the first three fusions has not been compromised and therefore will not be discussed any further.  The beads from the fourth fusion were still present on the specimen, which would indicate that this fusion was not performed under the same conditions as the previous three. 

The remainder of this investigation focused on the fourth fusion joint, between the largest reducer and the 42” diameter pipe, at which the failure occurred. 

Initially two dominant features were noted regarding this largest fusion on the reducer section.  They were 1) the ‘crack’ extends nearly semi-circumferentially, and 2) the mid-point of the crack accommodates a smooth 26” long bevel on the outer surface of the 42” pipe end, where the pipe would have joined the reducer. 

The double bead on the interior surface of the fourth joint implies that each mating surface melted during the attempted fusion, however the bevel found at the epicenter of the failure implies a lack of fusion.  The bevel appears to have been present prior to fusion, and was likely caused by heating due to the rounded corners of the cross section.  Had the corners of the bevel evidenced a mitered appearance, cleavage would have been suspected as the root cause. 

Upon dissection of the failed joint, an area of disbondment adjacent to the bevel was observed.  This second section shows a planar surface with good fusion around an area of unbonded polymer (Figure 1).  

 

 

Figure 1. Unbonded area, found adjacent to beveled portion.  Area surrounded by bonded material, which broke under stress.

 

Chemical Analysis

A portion of material was removed from the pipe section, the reducer section and the weld for chemical analysis.  All materials have been identified as polyethylene by FTIR spectrometry.  

 

Discussion

The largest fusion joint on the pipe subassembly failed by cracking due to improper technique during fusion of the two parts.  The contact between the surfaces was either incomplete, or the mating surface temperature did not promote good fusion.  In either case, the result was a poorly and incompletely fused joint, which led to crack propagation under stress and ultimate failure.

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