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

Abstract

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.

Modelling of Environmental Ageing of Polymers and Polymer Composites—Durability Prediction Methods

“…Primary creep is the viscoelastic region, where strain rate decreases with time and strain. During secondary creep, the strain rate reaches a constant steady plastic flow rate, which gradually accelerates (tertiary creep), eventually leading to strain localisation and failure [ 76 , 112 ].…”

“…Strain rate minima are determined from the strain rate vs. strain dependences called Sherby–Dorn plots [ 69 , 70 , 112 ]. As examples, Sherby–Dorn plots for glass-fibre reinforced isostatic polypropylene (iPP) [ 72 ] and carbon nanotube (CNT) reinforced polycarbonate [ 69 ] composites tested in uniaxial creep at 23 °C under various stresses are shown in Figure 8 .…”

“…Creep lifetime is strongly related to the accumulation of irreversible strains; thus, models for predicting their evolution are of great interest. The viscoplastic strain is expressed as nonlinear functions of stress, time, temperature, etc., e.g., Zapas–Crisman model discussed elsewhere [ 65 , 112 , 116 , 117 ]. Identification of multiple model parameters requires an extensive testing campaign that is often not justified in terms of costs.…”

“…Simple, cost-effective methods with a minimum required amount of a priori known material parameters are advantageous for practical applications. In a recent study by Starkova et al [ 80 ], a simple power-law relationship between the residual (viscoplastic) and the total creep strain was established ( Figure 10 ): where is the accumulated viscoplastic strain measured as the residual strain in creep-recovery tests, and n are material constants; n takes values in the range 0.7–2 and is equal to unity in the case of linear dependence between the residual and total creep strain [ 112 , 118 ]. The data were generalised for many polymers and composites reinforced with different types and amounts of fillers and tested under a wide range of stresses and temperatures.…”

Recursive multiscale homogenization of multiphysics behavior of fuzzy fiber composites reinforced by hollow carbon nanotubes

“…Modern engineering applications require the development of composite materials with advanced mechanical, thermal, electric, etc., properties that provide high performances when employed for structural and/or functional components. Carbon nanotubes (CNTs) have shown excellent characteristics when introduced into composite structures (Aniskevich and Starkova, 2021; Ay and Tanoğlu, 2020; De Luca et al, 2020; Karger-Kocsis et al, 2020; Qian et al, 2010; Song et al, 2012). They are probably the strongest and stiffest materials that have been discovered with regard to the tensile strength and modulus.…”