Is a split in a buried plastic pipe evidence of a pressure surge?
Not by itself. When a buried plastic pipe splits, the first explanation offered is usually a pressure event — a surge, a hammer, a regulator that let the line run high — and sometimes that explanation is right.
Often, though, the split in a buried plastic pipe is the end of a crack that grew for years at a stress far below anything that would burst the pipe in a test, with ordinary pressure in the line the whole time. That mechanism, slow crack growth, leaves a distinctive fracture, and mistaking a slow crack growth fracture for a pressure event sends the investigation toward the wrong records.
Why does the stress-rupture curve for polyethylene pipe bend instead of running as one straight line?
The stress-rupture curve for polyethylene pipe bends where a first failure zone, a gradual straight-line decline in strength accompanied mainly by ductile fractures, gradually gives way to a second zone of more rapid decline in strength accompanied by brittle-like fractures only. Stress-rupture testing plots hoop stress against time to failure on log-log coordinates, and for polyethylene that plot is not a single straight line.
The National Transportation Safety Board’s special investigation report Brittle-Like Cracking in Plastic Pipe for Gas Service (NTSB/SIR-98/01, adopted April 23, 1998) recorded what elevated-temperature testing had established: “two distinct failure zones were indicated for polyethylene piping in stress rupture testing. … The first zone is characterized by the gradual straight-line decline in strength accompanied primarily by ductile fractures. The first zone gradually transitions to the second zone, which is characterized by a more rapid decline in strength accompanied by brittle-like fractures only.”
The time and magnitude of that more rapid decline, the National Transportation Safety Board report noted, “varies by type and brand of polyethylene.”
How does a brittle-like slit in polyethylene pipe differ from a ductile burst?
A ductile stress-rupture failure in polyethylene pipe balloons, while a brittle-like failure is a narrow slit that frequently shows no visible deformation at all. The National Transportation Safety Board’s caption to its own test figure, in the special investigation report Brittle-Like Cracking in Plastic Pipe for Gas Service (NTSB/SIR-98/01, adopted April 23, 1998), describes the ductile failure as showing “substantial deformation (ballooning) at the failure.”
The same National Transportation Safety Board report records in a footnote that slit fractures “are also referred to as brittle-like fractures” because of “the frequent lack of visible deformation associated with them.” The report also notes that as hoop stress on the test piping is decreased, the time to failure increases and the amount of deformation apparent in the failure decreases.
A narrow crack through a polyethylene pipe wall of unchanged thickness is not evidence of a violent event; it is closer to the opposite.
What does ASTM D2837 assume when it sets a design basis for polyethylene pipe?
ASTM D2837 assumes that a pipe material’s stress-rupture data follow a straight line on log-log coordinates through at least 100,000 hours, because the method extrapolates that line to the 100,000-hour intercept — about 11 years, as the National Transportation Safety Board put it. ASTM D2837, Standard Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials or Pressure Design Basis for Thermoplastic Pipe Products, analyzes stress-rupture data covering a testing period of not less than 10,000 hours by linear regression, yielding a best-fit log-stress versus log time-to-fail straight line.
For polyethylene, ASTM D2837 “includes a supplemental requirement for the ‘validating’ of this assumption.” Data that do not approximate a straight line on log-log coordinates cannot be assigned a design basis under ASTM D2837 at all, and material whose data show “excessive scatter or a pronounced ‘knee’” is classified as unsuitable for analysis.
The National Transportation Safety Board recorded that this downturn restriction entered ASTM D2837 in 1988, and that the new requirements had no effect on pipe installed before their effective date.
What is stress intensification in plastic pipe, and what causes it?
Stress intensification in plastic pipe is stress that is higher in one area of the pipe than in the areas next to it, and it can be generated by external forces or by a change in the pipe’s geometry. The National Transportation Safety Board defined stress intensification, the companion condition to slow crack growth, plainly: “Stress intensification occurs when stress is higher in one area of a pipe than in those areas adjacent to it. Stress intensification can be generated by external forces or a change in the geometry of the pipe (such as at a connection to a fitting).”
The National Transportation Safety Board’s accident work identified rock impingement, inadequate support and the resulting differential settlement, bends sharper than installation recommendations allow, and damage from squeezing the pipe to control gas flow among the sources of stress intensification. The National Transportation Safety Board concluded that much of the plastic pipe manufactured and used for gas service from the 1960s through the early 1980s “may be susceptible to premature brittle-like failures when subjected to stress intensification.”
Slow crack growth in plastic pipe is a resin question and an installation question at once.
What happens inside the material during slow crack growth?
During slow crack growth, stress concentrations at notches, scratches or material imperfections nucleate microscopic voids in the amorphous phase of the polymer, those voids grow into crazes of highly oriented fibrils, and the fibrils fail by breakage or disentanglement of polymer chains as local stress rises. That sequence is described by Thuy, Pedragosa-Rincón, Niebergall, Oehler, Alig and Böhning, writing in Polymers 14(12):2415 (2022).
Thuy, Pedragosa-Rincón, Niebergall, Oehler, Alig and Böhning are explicit that the slow crack growth mechanism runs without a liquid environment and may merely be accelerated where one is present. That is what separates slow crack growth from environmental stress cracking.
Which tests measure resistance to slow crack growth, and can they date a crack?
ASTM F1473-24a, the notch tensile test known as PENT, and ISO 13479:2022 both measure resistance to slow crack growth, but neither ASTM F1473-24a nor ISO 13479:2022 dates a crack found in the ground.
ASTM F1473-24a runs at 80 °C and 2.4 MPa, or at other conditions chosen to be low enough “to preclude ductile failure and thereby eventually induce brittle type of failure.” ISO 13479:2022, third edition, takes a length of pipe with four machined longitudinal external notches and subjects it to a hydrostatic pressure test while immersed in a water tank at 80 °C, recording the time to failure.
Does passing a burst test show that a plastic pipe resists slow crack growth?
No. A certificate showing that a plastic pipe met a minimum burst requirement says nothing about the pipe’s resistance to slow crack growth, although burst results and slow crack growth resistance are routinely offered as though they were the same evidence. A burst test answers a burst question.
ASTM D1599-18(2024), Standard Test Method for Resistance to Short-Time Hydraulic Pressure of Plastic Pipe, Tubing, and Fittings, raises pressure to failure within a window the method sets at 60 to 70 seconds, and ASTM D1599 itself cautions that such results are generally not indicative of long-term strength.
What can an examination of a split polyethylene pipe establish?
An examination of a split polyethylene pipe can classify the fracture as brittle-like or ductile from the deformation and the wall geometry, and can locate the initiation site, including whether it sits at a fitting, a squeeze location, a scratch or an impingement mark.
An examination of a split polyethylene pipe can also measure residual slow crack growth resistance on unfailed pipe from the same run, and can identify the geometric features that concentrated the stress.
What can’t an examination of a split polyethylene pipe establish?
An examination of a split polyethylene pipe cannot date the fracture: no examination reads a date off a slit fracture, and no laboratory time to failure at 80 °C converts into service years at ground temperature. The pressure in the line at the moment of failure is a record question, not a fractographic one.
Where several stress sources are candidates for a split polyethylene pipe, the buried geometry that would separate them is destroyed by the excavation, which is why the position of the pipe, the bedding, the rock and the bend have to be documented before anything is lifted.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.