Determination of dynamic elastic modulus of polymeric materials using vertical split Hopkinson pressure bar

• The stress equilibrium influence is evaluated quantitatively and its effect is little on the determination accuracy of elastic modulus.
• The indentation affects the accuracy of the dynamic elastic modulus and this can be remedied by suitable selections of the specific diameter and modulus of bar to specimen.
• The tilt between specimen and bars results in large errors in measured strains and elastic modulus for traditional SHPB, but this can be rectified by the proposed vertical SHPB apparatus.

This paper is focused on the determination of dynamic elastic modulus of polymer materials under high strain rate loading using the split Hopkinson pressure bar (SHPB) technique. Experiments conducted on the epoxy specimen by the traditional SHPB and the proposed vertical SHPB equipment demonstrates that the vertical SHPB can give more accurate measurements. The related factors, namely, the effect of stress inequilibrium in specimen, indentation in bars due to specimen and the tilt between specimen and bars, are extensively studied. It is concluded through theoretical analysis and numerical calculations that the influence of stress inequilibrium becomes negligible after two characteristic times. The numerical study on the indentation effect shows that the bar to specimen specific elastic modulus ratio and specific diameter ratio are critical to the level of influence on indentation. However, polymers with low elastic modulus values can still be accurately measured regardless of the indentation displayed. The numerical investigation on tilt effect indicates that the imperfect contact condition severely affects the accuracy of measured elastic modulus. This issue can be rectified by the newly proposed vertical SHPB. It can improve the contact conditions between bars and specimen significantly and offer acceptable accurate measurements for the dynamic elastic modulus of polymeric materials.