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Description of
the Material
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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.
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| Background |
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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.
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| Findings |
| Visual and Macro Examination |
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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.
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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.
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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.
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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.
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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).
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Figure
1. Unbonded area, found adjacent to beveled portion. Area surrounded
by bonded material, which broke under stress. |
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Chemical Analysis
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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.
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Discussion
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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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