The influence of interfacial friction on the deformation of high density polyethylene in a split hopkinson pressure bar

Abstract

The results of an experimental study of the influence of interface friction on the yield behaviour of a high density polyethylene when it is compressively deformed to a large strain (25%) at high rates of strain ([if5 × 10³ s⁻¹) in a split Hopkinson pressure bar (SHPB) are described in this paper. The potential sources of error and uncertainty intrinsic in this method are described and evaluated. It is shown that for discs of polymer with diameterto-thickness ratios of about 10, which are required for these experiments, interface friction can produce a very significant increase in the apparent flow stress. Three model lubricants have been used, and it is found that even a modest friction coefficient of 0.05 produces an enhancement in the flow stress of about 25%. A simple rigid-plastic analysis based on the von Mises yield criterion is found to serve as a useful empirical correction. Results from previous studies of polymer deformation in the SHPB configuration have been evaluated using this analysis, and these results confirm the generality and importance of interface frictional constraint in these experiments. The most suitable lubricant for high speed deformation studies is found to be a petroleum gel (10 μm thick) which shear softens and produces a nearly ideal lubricating layer.