Observations and analysis of fatigue crack growth in circular notched polyethylene specimens

Many fundamental as well as technical issues with PENT (ASTM F2136) are identified, for instance the inability of separating crack growth from crack initiation, too long duration of the test for new generation of PE, outdated test conditions unchanged since the original formulation designed for old PE materials, etc. Therefore, there is a quest for a new accelerated test method. Recently, a circular notched specimen (CNS) has been proposed as an accelerated fatigue testing. However, formation of an asymmetric crack in CNS and early transition to a ductile failure limit the applicability of CNS test for studies of SCG. There are several factors responsible for the asymmetry of SCG. Especially, geometric misalignments of CNS are the most common practical weakness of the test. The present paper reports the observation of asymmetric SCG in CNS under fatigue conditions and analysis by finite element method (FEM) of the CNS misalignments effect on the crack growth asymmetry. Root causes of the asymmetric crack growth and its effect on the time to failure (fatigue lifetime) are discussed.