Evaluation of the Viscoplastic Strain of High-Density Polyethylene/Multiwall Carbon Nanotube Composites Using the Reaction Rate Relation

A novel semiempirical method for separating the viscoplastic strains from the total creep strains is proposed and validated by examples of high-density polyethylene (HDPE)/multiwall carbon nanotube (MWCNT) nanocomposites. The method is based on Eyring’s reaction rate relation and an analysis of creep data in semilogarithmic strain rate–strain coordinates. The initial linear part of the relation corresponds to the reversible viscoelastic behavior, but the deviation from it is related to the accumulation of viscoplastic strains. The viscoplastic strains are determined by simple calculations using four approximation coefficients determined from two linear parts of the strain rate–strain relation. A common relationship between the viscoplastic and total creep strains is established from data of 57 creep and creep recovery tests for samples filled with various content of MWCNTs and performed under for different stresses and loading times. The validity of the method is proved by the existence of a reasonable correlation between the calculated viscoplastic strains and the residual strains measured experimentally at the creep recovery stage. The method proposed contributes to an effective assessment of viscoplastic strains found from creep tests with no need to study the creep recovery.