The susceptibility of a high density polyethylene geomembrane to long-term stress cracking is investigated under field conditions simulating municipal solid waste landfills. Experiments are con-ducted in the large-scale geosynthetic landfill liner longevity simulators (GLLS) in which the 0.6 m diam-eter pre-aged geomembrane sample is underlying a gravel drainage layer and a geotextile protection layer and overlying a geosynthetic clay liner GCL layer. The composite liner configuration is tested under 250 kPa vertical pressure and at a range of temperatures between 40 and 85oC. The results reported by Ab-delaal et al. (2014) showed that, at temperatures between 55 and 85oC, the geomembrane failure times (due to brittle stress cracking) followed an Arrhenius relation. However, at 40oC, the geomembrane sam-ple had remained intact for 20 months. The objective of this paper is to present an update to the failure times of the geomembrane liner after four years of testing in the GLLSs. The current results show that the GMB at 40oC is still intact after further 28 months of testing under simulated field conditions. The current duration of the 40oC test substantially exceeds the failure time extrapolated from the Arrhenius relation established based on temperatures between 55 and 85oC suggesting a change in the behaviour of the GMB at lower temperatures below 55oC.