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Overload Failure of UHMWPE Seals

Background: A client contacted KnightHawk Materials Lab with multiple failures in polymer seals in a mobile, low temperature piping system. KML was hired to determine the cause of the failure within the polymer jackets of the seals in order to extend their service life.

Figure 1: Brittle Fracture Surface (Left) and Ductile Fracture with Feathering (Right) of Two Different UHMWPE Seals

Summary: The failures of the UHMWPE seals were caused by sudden overload events. There was no sign of fatigue damage or chemical/mechanical degradation of the polymer. The overload events led to the formation of two distinctly different types of fractures in seals that were installed in two different locations, with one seal failing in a brittle fashion (left image in Figure 1), while the other seal failed in a ductile fashion (right image in Figure 1). This difference in fracture mode was the result of differing levels of constraint on the seals. The seal that failed in a brittle fashion was more heavily constrained and thus did not have the ability, from a fracture mechanics perspective, to fail in a ductile fashion. When the system was less constrained, the natural ductility of the UHMWPE led to a ductile overload failure instead, as evidenced by the other seal.

Take Away: While ductile materials are generally expected to fail in a ductile fashion, it is possible for constraints on the system to force even a highly ductile material into a brittle fracture mode. Thus, understanding the system and the fracture mechanics that drive the failure can be critical in understanding how and why a fracture progresses the way it does.