A thermal gradient is inherent to the solidification of extruded profiles such as pipes. Depending on the magnitude of the gradient, varying levels of residual stresses are frozen-in during the pipe fabrication step. In this investigation, the residual stress state of a polyethylene (PE) pipe was altered by changing its wall thickness. Creep rupture testing of the subject pipes of varying wall thickness provided an insight on how residual stresses influences both ductile and brittle fracture processes.

BACKGROUND

Plastics have steadily replaced clay, copper, asbestos-cement, aluminum, iron and concrete pipes in various applications. Among the plastics employed in pipes, PVC accounts for about 75 % while polyethylene is employed in about 20 % of the plastic pipe applications. Polyethylene (PE) pipes are used extensively for the transportation and distribution of natural gas, accounting for about 80% of the new piping installations.

PE pipes used for gas transport are under pressure for the duration of their useful service. Therefore, it is important to establish the maximum load that such a pipe can withstand without deformation and damage over its expected lifetime (typically, many decades). The design stress and the useful service lifetime (durability) of PE pipes are typically estimated by performing creep rupture tests at multiple temperatures. In this test, the pipe of interest is subjected to a certain hydrostatic pressure (expressed as hoop stress) and the failure time is recorded; failure is defined as a continuous loss of pressure (leakage) from within the pipe.

In a previous report (1), it was suggested that residual stresses in a pipe can accelerate the fracture process in creep rupture testing. Residual stresses within a pipe (or any other extruded or molded part) are a consequence of a gradient in temperature during the fabrication process. In pipe extrusion, the outside surface of the extrudate is quenched (usually with a water spray) in the immediate vicinity of the die. The inner surface of the pipe wall, exposed to air, solidifies soon after the outer surface is set. This fixes the solid boundaries of the pipe. However, the core of the pipe wall solidifies several minutes after the pipe inner and outer surfaces have solidified. Therefore, as a consequence of the thermal gradient across the pipe wall, the crystallization (shrinkage) of the molecules within the core regions of the pipe wall produces residual stresses.

For extruded profiles such as pipes, the magnitude of residual stresses frozen-in during processing is generally expected to increase with increasing wall thickness assuming all other extrusion parameters are kept constant. In this investigation, two pipes of different wall thickness (SDR-11: Thick-Walled Pipe; SDR-17: Thin-Walled Pipe) were produced from the same PE resin by maintaining similar extrusion conditions. These two pipes were then subject to creep rupture testing at 23 °C and at 80 °C to assess the role of wall thickness and the consequent residual stresses on the performance of PE pipes meant for gas transport applications. The creep rupture testing conditions chosen encompassed both ductile and brittle failure events.

CONCLUSIONS

We have investigated the creep rupture performance of a given PE converted into an SDR-11 and an SDR-17 pipe. The semi-crystalline morphology of the two pipes, produced from the same resin at similar extrusion conditions, was verified to be very similar. However, thermal characterization of the two pipes confirmed a smaller thermal gradient during the processing of the thinner wall pipe. This suggests, as one would expect, lower levels of residual stresses frozen-in during the fabrication of the SDR-17 pipe. Creep rupture testing of the two pipes at 23 °C and at 80 °C indicates better performance for the SDR-17 pipe as long as the fracture is ductile. We suggest that the tensile component of the residual stresses that act along the inner surface of the pipe wall can augment the applied hydrostatic pressure and accelerate the ductile fracture process. Consequently, a lower level of residual stresses frozen-in during pipe fabrication is perhaps favorable for creep rupture tests that promote ductile fracture. This can have a direct bearing on the predicted long-term load-bearing capacity (or design stress) of a given PE pipe. One can lower the residual stress level in a pipe either by ex-situ annealing or by in-situ annealing during the pipe fabrication step.

The reported observations indicate that the pipe wall thickness does not influence either the time for brittle fracture (at a given stress) or the transition (“knee”) from ductile to brittle failure in creep rupture testing. It is proposed that the formation of a slit (craze-zone) during the initial stages of the SCG process allows at least a portion of the residual stresses to dissipate. This dissipation can occur continuously during the crack growth process in a manner such that the level of residual stresses present in the initial (as-made) pipe does not exert a substantial influence on the brittle fracture of pressurized PE pipes.

REFERENCES

(1) R. K. Krishnaswamy, Polymer, 46, 11664 (2005).