Reanalysis of the Creep Test Data and Failure Behavior of Polyethylene and Its Copolymers

Creep behaviors of four polyethylenes (PE) were investigated at temperatures from 296 to 358 K. Two PEs showed only ductile failure (DF) at all temperatures, the third one changed from DF to brittle failure at high temperatures (HTBF) and the fourth one showed three failure modes which are low-temperature brittle failure (LTBF), DF and HTBF with the increase of temperature. The study found out that activation energy for deformation at the secondary creep stage, based on the Eyring’s law using two parallel processes, can provide a clear indication on the possibility of HTBF, as the two PEs that showed HTBF have much lower activation energy for the low-stress process, named process one here, than the corresponding activation energy for the other two PEs. However, another creep model, commonly known as the Norton power law (NPL), does not provide such a clear distinction. The study also found that the Monkman–Grant relationship is consistent among data for DF and LTBF, but not for HTBF. Therefore, activation energy based on the Eyring’s model can be used to quantify the likelihood of HTBF which has long been linked to an unusual ductile-brittle transition for PE pipe in a long service.