Prediction of the creep behaviour of polyethylene and molybdenum from stress relaxation experiments

Abstract

In many applications it is useful to be able to convert observed creep data of a material to corresponding stress relaxation data or vice versa. If the material exhibits non-linear viscoelasticity such a conversion can be rather difficult. In this paper two semi-empirical flow equations, the power law and the exponential law, are used to convert stress relaxation data into corresponding creep behaviour data. These two flow equations are often used to describe non-linear viscoelastic behaviour. The procedure adopted here is based on the assumption that the creep data during the experiment decrease due to an increase in the internal stress level, thus decreasing the effective stress for flow. The conversion method is applied to high density polyethylene and polycrystalline molybdenum at room temperature. In general predictions using the power law are in better agreement with the experimental results than predictions using the exponential formula. The concepts of secondary and ceasing creep are discussed in terms of build-up of internal stress during the creep process.

The effect of a coupling agent on the viscoelastic properties and internal stresses in high density polyethylene filled with glass spheres

“…It has earlier been shown [14] that for HDPE this power law provides a reasonable description of the relaxation behaviour at room temperature. The internal stress level is determined by the method proposed by Li [12].…”

“…The intercept with the stress axis is then identified with the internal stress ai. The internal stress o-i increases with increasing initial strain of the relaxation test for HDPE [14]. The counterpart of eq.…”

“…Since ai increases with the strain, the strain rate will decrease until a steady state is obtained or the creep process ceases [14].…”

Correlation between interfacial interactions and internal stresses in filled high density polyethylene

“…It has been shown earlier [21] that the power-law eq. (2) provides a reasonable description of the relaxation behaviour of HDPE at room temperature.…”

“…It is possible to predict the creep behaviour from stress relaxation data using the internal stress concept. The influence of the internal stress on primary and secondary creep and the interconversion from stress relaxation to creep is discussed in more detail in [21].…”