Time-resolved small-angle X-ray scattering study of void fraction evolution in high-density polyethylene during stress unloading and strain recovery

Abstract

By combining time-resolved small-angle X-ray scattering and quantitative microscopy, it is shown that void fraction in high-density polyethylene has permanent and non-permanent components during tensile testing. By means of time-resolved small-angle X-ray scattering, we developed an analysis methodology to assess the void volume fraction ϕ{symbol}v in high-density polyethylene (HDPE) during tensile testing. The specimens were first drawn up to different imposed strains, and subsequently were subjected to stress unloading and strain recovery stages. During the loading stage, ϕ{symbol}v progressively increased with the strain level, starting from a well-defined onset strain prior to the yield point. In particular, ϕ{symbol}v reached a maximum of 8.75vol% for a strain of 12.5% in the case of a HDPE grade with a molecular weight of 105 000gmol-1. Stress unloading and strain recovery caused a decrease in ϕ{symbol}v attained at the end of the loading stage. For a HDPE grade with a molecular weight of 55 000gmol-1, ϕ{symbol}v was more important during the loading stage and the decrease in ϕ{symbol}v was less marked during the stress unloading stage when compared to the HDPE with molecular weight of 105 000gmol-1. The residual and reversible components of void volume fraction were revealed.