Field failure mechanisms in HDPE potable water pipe.
As early as the mid-1980s, hydrostatic pressure testing of HDPE pipe had demonstrated three generic stages of pipe failure . These are identified on the schematic creep rupture curve shown in Figure 1. Stage I failure involves a purely mechanical failure mechanism due to ductile overload of the material. Stage I failures in pipe testing manifest as ductile bursting of the pipe with yielding of the material. Stage II failure also involves a mechanical failure mechanism but manifests itself as non-ductile slit or pinhole cracks in the pipe wall, permitting leakage from the pipe. Stage III failure also manifests itself as leakage from non-ductile cracking of the pipe wall, but it is not purely mechanical. Stage III failure occurs at lower stresses that Stage II failure and requires some minimum level of oxidative degradation of the HDPE pipe material. Degradation of the material throughout the full pipe wall is not necessary for pipe performance to be adversely affected. Degradation of a layer 50 to 60 microns deep at one of the pipe surfaces is all that is required to yield Stage III failure. This "surface embrittlement" phenomenon has been thoroughly studied and confirmed by a number of authors [2-6].
There have been occasional reports of significant performance problems with the use of HDPE pipe in potable-water service since at least the late 1970s. Some of these early issues were evaluated in a study funded by the Plastics Pipe Institute and carried out by the consulting firm Simpson, Gumpertz and Heger, Inc. (SGH) . The principal investigator of this study was Richard E. Chambers, and the report produced has come to be known as the Chambers Report. The largest number of these early incidents were connected to pipe made from one pipe resin and extruded by one pipe manufacturer. In service the pipe became extremely brittle in only a few years of use. The Chambers Report noted that this embrittlement was caused by extensive in-service oxidative degradation of the polyethylene resin used to manufacture the pipe.
We performed failure analyses on this product used in several different locales in the U.S. In many cases the oxidation was so severe that the HDPE pipe could be snapped like a dry twig. These field failures were clearly of the Stage III type. The Chambers Report opined that this condition was caused by having either insufficient or no antioxidant in the pipe resin, exacerbated by extreme extrusion temperatures and/or shear rates. Documents disclosed in ensuing litigation validated this opinion.
The Chambers Report identified other performance issues in the use of HDPE pipe in potable-water service that were Stage I and Stage II type failures. Some were determined to be due to excessive bending of the very flexible pipe, especially at fitting connections at the water main and at the customer's water meter. Others were attributed to rock impingement, in which a large rock or other hard object in the backfill around the pipe pushed into the pipe wall, creating highly localized excessive stresses that resulted in premature Stage II failure. There were also failures attributable to the design of the mechanical fittings used to connect the HDPE service lines to water mains and meters. The insert stiffeners employed in these fittings were singled out as a particular problem, due to bending of the pipe at fitting connections.
The conclusions presented in the Chambers Report did result in some changes to HDPE water pipe, associated fittings, and installation practices.
Forensic Analysis of Recent Failures
In a 2009 study , more than 50 service-aged HDPE pipe samples were acquired from water utilities across the United States. These included over 20 pipe specimens that had leaked in service. Fracture surfaces were examined to determine fracture initiation locations, crack propagation directions, and other crack growth characteristics. The samples were subjected to a variety of analytical techniques commonly used to assess oxidation in polyolefin piping (HDPE, polybutylene, and polypropylene). The procedures employed are described below.
Bend Back Tests 
This test involves an optical examination of the inner surface of a ring of pipe that has been slit and bent open to place the inner pipe surface in tension. The visual examination looks for evidence of surface crazing or cracking that would be a sign of embrittlement of the inner pipe surface. The results of this test are purely qualitative. They demonstrate the presence of inner surface embrittlement, but not the extent.
Fourier Transform Infrared Spectroscopy (FTIR)
Infrared spectroscopy has been utilized for decades as a means of identifying and quantifying oxidative degradation in polyethylene. Oxidized polyethylene absorbs radiation in an area of the infrared spectrum (the carbonyl region) where unoxidized polyethylene does not. A "carbonyl index" can be calculated by taking the ratio of the absorbance of radiation in the carbonyl region to that in a region where infrared absorption is due only to the basic elements of the polyethylene molecule. The application of this procedure to quantifying oxidation in HDPE pipes has been described in detail by numerous authors [3, 5, 6, 10, 11]. When performed with an FTIR instrument equipped with a special microscope, the method can provide a profile of the extent of oxidation at different locations within the pipe wall.
Two slightly different FTIR techniques (ATR and micro-transmission) were utilized to measure the degree of oxidation on pipe samples, both in the material in the first few microns (1 micron = 0.00004 inch) into the pipe wall from the inner surface and in the layer extending to about 50-60 microns (approximately 0.002 inch) in from that surface. Carbonyl index calculations were performed to assess the extent of polyethylene oxidation at various depths from the inner surface. Specimens from 30 pipe samples were tested in this way.
The carbonyl index is determined by taking the ratio of two absorption peaks in the infrared spectrum. One peak is from absorbance by ketone carbonyl groups that are created in the polyethylene molecules by oxidation. The other peak is from absorbance by -C[H.sub.2]- groups (methylene groups) that are the basic building blocks of polyethylene molecules. Historic research into surface embrittlement of polyethylene pipe [3, 5, 12] has shown that a carbonyl index of > 0.1 for a depth into the pipe wall from the inside surface of 50-60 microns is sufficient to induce Stage III failure. Sixty percent of the pipes tested by FTIR, failures and non-failures alike, exhibited carbonyl index values in the first 0.002 inch into the pipe wall from the inner surface that were at or greater than that level.
Oxidative Induction Time (OIT)
Samples taken from the failed pipes were subjected to OIT measurements per ASTM D3895  to determine the relative amount of antioxidant remaining in different locations within the wall of these pipe samples after potable water service. Measurements were made on specimens taken from both the middle of the pipe walls and the inner surfaces. The tests were performed at 200[degrees]C. While the results of OIT testing cannot be used to compare different stabilizer packages and also cannot be used to predict long-term oxidation resistance, they can be used to assess the extent of antioxidant depletion for a given stabilization system. These results show less antioxidant at a pipe's inside surface than in the mid-wall. This demonstrates that the antioxidants in the pipe compounds were being depleted while the pipes were in service.
Pang Tensile Tests
A modified ring tensile test, based on ASTM D2290  and described by Rozental-Evesque , was carried out on several samples. Rings cut from pipe have a reduced cross-section area created at locations 180[degrees] around the ring circumference from each other. Elongation to break was measured to determine decreases that would indicate embrittled surfaces or degraded mechanical properties. Ring specimens from seven samples were subjected to this test. Several of these exhibited a reduction in elongation and a large scatter in the individual elongation values, both of which can be associated with the onset of surface embrittlement. Ring tensile tests show reduced elongations of between 32% and 91% of the values obtained on unused pipe specimens.
Visual and Microscopic Examination
Pipe specimens and fracture surfaces were examined visually and with a Leica Model MZ 16 stereo-optical microscope equipped with optics enabling it to operate at magnifications ranging from 3.5X to 110X. Digital photographs were taken of samples under the microscope using a Nikon Model DXM 1200F digital camera and Nikon ACT-1 imaging software.
Twenty-three of the pipe samples included the failure location. Following are the results obtained on four of the failed specimens, which were typical of the entire population of failed pipes evaluated in this study.
Location A: Desert Southwest U.S. Environment
This sample exhibited severe degradation of the inner surface with oxidation penetrating approximately 5 thousandths of an inch (0.005 inch) into the pipe. The pipe, which had been in service for 25.5 years, exhibited very brittle behavior that was clearly shown in the bend back test and the fracture surfaces. The pipe had a longitudinal slit visible on the outer surface, approximately 0.5 inch long. Figure 2 shows the inside surface cracks at the leak location.
The pipe was apparently squeezed off near the failure site. This may well have occurred after the failure, during repair of the leak. The pipe had cracks at the leak site and opposite the leak, indicating that the pipe was squeezed and that the squeeze-off process led to some of the cracking on the opposite side. The inner surface of the pipe exhibited "mud cracking," which is typical of severely oxidized material (Figure 3). In this case, material properties have been degraded to such an extent that stresses in all directions cause the material to crack. The depth of this degraded layer is approximately 125 microns thick, leading to multiple crack initiation sites and subsequent slow crack growth as seen in Figure 4.
This sample has effectively reached the end of its life because of oxidative degradation. At this level of degradation, normal service stresses would lead to fracture, as happened in this case. The level of oxidation has been documented by FTIR, with carbonyl index values exceeding 0.1 for a depth of 125 microns into the pipe wall, OIT values near zero at the inner surface, and substantial reduction of antioxidant in the core.
Location B: Central California (U.S.) Environment
Several samples were provided from a distribution system in Central California that included the failed area in the sample. These ranged in time in service from 5 to over 30 years. Two of the samples appeared to have failed because of rock impingement and one exhibited significant erosion that partially destroyed the fracture surface. This is a common phenomenon in water piping failures. The water flows out of the leak at a high enough velocity to churn up the soil around the leak. This creates an abrasive mix that wears away the surfaces. Figures 5 and 6 show one of these samples. The carbonyl indices of the material in the first 50 microns from inside surfaces of these pipes were less than 0.10. Another sample from this location, which failed because of oxidative degradation and embrittlement of the inside surface of the pipe, had been in service for over 32 years. The carbonyl index in the first 50 microns of material inward from the inner pipe surface of that sample was 0.15.
Location C: Midwest (U.S.) Location With Chlorine Dioxide Disinfectant
Samples were received from a distribution company that all exhibited intense oxidation at the inner surface. These samples had failed after service times of from 1 to 12 years. This oxidation was limited to the first 50 microns, but did lead to crack initiation and failure in many instances. This location was the only one in this study where chlorine dioxide was used as the secondary water disinfectant.
Other samples from this location appear to have failed because of rock impingement and exhibited eroded surfaces where the fracture surface features were destroyed.
Location D: Southeast U.S. Location--Pipe With Manufacturing Defect
The pipe sample in Figures 8 and 9 is a 6-inch DIOD (Ductile Iron Outside Diameter) HDPE water main, supposedly manufactured to the requirements of AWWA C906. This pipe failed where the pipe wall thickness was less than the allowable minimum value. Figure 8 shows multiple fracture initiations all located along this thin longitudinal strip of the pipe wall. Figure 9 shows just a portion of the fracture surface with multiple initiations. While multiple initiations are often associated with surface embrittlement controlled failure, in this case the inner surface oxidation in this pipe was relatively slight. The carbonyl index values measured on the material in the first 50 microns inward from the inside pipe surface were 0.02 or less.
Discussion and Conclusions
The results of this study demonstrate that many of the same failure mechanisms noted in the Chambers Report over 25 years ago are still being seen today. Mechanical Stage I and Stage II failures are still observed, driven by excessive stresses from rock impingement and excessive bending at the rigid fittings used to connect HDPE service lines to mains and meters. Although very rare, failures due to manufacturing defects are sometimes still found, as in the thin-walled pipe from Location D. Also, Stage III failures attributable to oxidative degradation and embrittlement of the HDPE material are still observed, both in older installations and with relatively new pipe products. The Stage III failures observed today, however, are not due to wholesale degradation of the entire pipe wall, as was the case with the pipe in the Chambers Report. Contemporary embrittlement failures involve degradation of only a 50- to 60-micron-thick surface layer to a carbonyl index of 0.1. Lesser levels of degradation, either with less depth into the pipe wall or to a carbonyl index of less than 0.1, have not to this point in time been found as the root cause of Stage III HDPE pipe failure.
The fact that premature oxidative aging of pressurized HDPE pipe, in the presence of the water disinfectants employed in potable water treatment, is still a cause of field failures (Stage III failures) needs to be addressed. The stabilizer systems employed in HDPE pipe compounds since 1980 have eliminated the short-time, wholesale degradation of pipe material discussed in the Chambers Report. However, premature failures due to oxidative degradation and surface embrittlement are still found today in certain circumstances, e.g., in hot climates (Location A), long-term installations in excess of 20 years but still less than the advertised 50- to 100-year life for HDPE pipe (Location B), and systems that utilize very aggressive disinfectants like chlorine dioxide (Location C).
Currently; no industry guidance is available for designers and owners who wish to incorporate design factors to account for varying service conditions such as service temperature, disinfectant type, disinfectant concentration, pressure, or resin grade on the Stage III failure mode. While the science of polyethylene oxidation is well understood and HDPE oxidation is observed in the field, more work is required to further the water industry's understanding of how service conditions affect HDPE pipe service lifetimes in order for designers and owners to accurately forecast service life and set design factors for their specific service conditions. Recent research has shown that the specific composition of the stabilizer package in an HDPE pipe compound is the controlling factor in the resistance of such pipe to Stage III failure . Consideration should be given to qualifying HDPE pipe compositions with specific stabilizer packages by ASTM F2263  in the same manner that long-term strength of HDPE pipe compounds in the absence of oxidation is done by ASTM D2837 .
The authors wish to thank Underground Solutions, Inc., which funded this study. They also wish to thank Mr. Joe Grzetic and Mr. Tom Jeka of ESI for sample preparation and mechanical property testing and Materials Engineering Inc. for performance of the FTIR and OIT tests.
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Donald E. Duvall, Ph.D., P.E., and Dale B. Edwards, P.E. ESI, Aurora, Illinois, USA
In 2011, this paper was awarded the Dr. Myer Ezrin Best ANTEC Paper Award by the Failure Analysis & Prevention Special Interest Group (FAPSIG) of SPE. FAPSIG has annually presented a Best Paper Award to a paper presented at one of its ANTEC sessions. In 2011 the award was named to honor Dr. Myer Ezrin, an SPE Fellow and one of the founders of FAPSIG, for his contributions both to the group and to the advancement of the practice of failure analysis of polymeric materials.
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|Author:||Duvall, Donald E.; Edwards, Dale B.|
|Date:||Mar 1, 2012|
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