Page 25 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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History and Overview 5

polyethylene (PE) is currently the material of choice in natural gas transportation systems in the
U.S. One advantage of PE piping is that maintenance can be performed on a small branch of the
line without shutting down the entire system; a local area is shut down by applying a clamping
tool to the PE pipe and stopping the flow of gas. The practice of pinch clamping has undoubtedly
saved vast sums of money, but has also led to an unexpected problem.
In 1983, a section of a 4-in. diameter PE pipe developed a major leak. The gas collected beneath
a residence where it ignited, resulting in severe damage to the house. Maintenance records and a
visual inspection of the pipe indicated that it had been pinch clamped 6 years earlier in the region
where the leak developed. A failure investigation [3] concluded that the pinch clamping operation
was responsible for the failure. Microscopic examination of the pipe revealed that a small flaw
apparently initiated on the inner surface of the pipe and grew through the wall. Figure 1.2 shows
a low-magnification photograph of the fracture surface. Laboratory tests simulated the pinch
clamping operation on sections of PE pipes; small thumbnail-shaped flaws (Figure 1.3) formed on
the inner wall of the pipes, as a result of the severe strains that were applied. Fracture mechanics
tests and analyses [3, 4] indicated that stresses in the pressurized pipe were sufficient to cause the
observed time-dependent crack growth, i.e., growth from a small thumbnail flaw to a through-
thickness crack over a period of 6 years.
The introduction of flaws in PE pipe by pinch clamping represents a Type 2 failure. The pinch
clamping process was presumably tested thoroughly before it was applied in service, but no one
anticipated that the procedure would introduce damage in the material that could lead to failure
after several years in service. Although specific data are not available, pinch clamping has undoubt-
edly led to a significant number of gas leaks. The practice of pinch clamping is still widespread in
the natural gas industry, but many companies and some states now require that a sleeve be fitted
to the affected region in order to relieve the stresses locally. In addition, newer grades of PE pipe
material have lower density and are less susceptible to damage by pinch clamping.
Some catastrophic events include elements both of Type 1 and Type 2 failures. On January 28,
1986, the Challenger Space Shuttle exploded because an O-ring seal in one of the main boosters
did not respond well to cold weather. The shuttle represents relatively new technology, where

FIGURE 1.2 Fracture surface of a PE pipe that sustained time-dependent crack growth as a result of pinch
clamping. (Taken from Jones, R.E. and Bradley, W.L., Forensic Engineering, Vol. I, 1987.) (Photograph
provided by R.E. Jones, Jr.)