Investigating the Time Dependent Behavior of Thermoplastic Polymers under Tensile Load

Abstract

The relationship between the results of the tensile and the stress relaxation tests of polypropylene specimens were analyzed and an attempt was made to find a way to estimate the former from the latter based on the measurements and the theory of linear viscoelasticity. The mechanical response of real polymers are basically of nonlinear character, therefore their behavior patterns do not meet the idealized (linear) ones. Experiments were performed on poly(propylene) (PP) as a test material and the stress relaxation behavior, as well as the linear elastic and linear viscoelastic approximation of the tensile load‐time curve were analyzed. To demonstrate the applicability of our idea and to perform the numerical calculations we have chosen a flexible function with three parameters to realize the nonlinear behavior.

Estimating the creep strain to failure of PP at different load levels based on short term tests and Weibull characterization

“…Considering mechanical tests a polymer specimen behaves as a system characterized by operator S that gives a relationship between the time-dependent stimulus as input, X(t), and the response, Y(t), as output (Equation (1)) [19,20]:…”

“…In case of LVE behavior the LVE estimation of the creep curve is the derivative of the tensile test one (Equation (4)) [19,20]:…”

“…In the reality the slope should be finite therefore the real creep stimulus is given by Equation (5): (5) where t o is the uploading time needed for setting the creep load level (F 0 = F · 0 t 0 ) (Figure 1a). Nagy and Vas showed that in the case of real creep stimulus where the uploading, that is the setting the creep load, is performed with constant load rate and assuming LVE behavior the proper form of Equation (4) is as follows (Equation (6)) [19,20]:…”

“…Real polymers exhibit LVE behavior only in a relatively small load range and beyond it they behave in a non-linear manner. It was also proven by Nagy and Vas [19,20] that at a given creep load the real creep curve can be estimated by a non-linear variable transformation (T) of the LVE estimation that has got a kind of analogy with the load level -time superposition principle described by Urzumtsev and Maksimov [5] similar to that used in the case of the time-temperature superposition principle (Equation (7)):…”

“…In general these methods can be used just at or below a given load level and in most cases they can-not estimate the expected lifetime and/or the creep strain-to-failure. Nagy and Vas [19,20] have developed a combined method for estimating the long term behavior from short term measurements based on the linear viscoelastic theory and non-linear variable transformations and they employed that to stress relaxation. In the present paper this method is applied to estimate the creep strain to failure of PP at arbitrary load levels by making use of the information provided by tensile tests and short term creep measurements performed at room temperature as well as a Weibull distribution based stochastic model.…”