Thermoplastics time-dependent behavior

A number of more advanced and general models attempting to predict the yielding, viscoplastic flow, time-dependence, and large strain behavior of fluoropolymers and other thermoplastics have recently been developed.in this section, we discuss the Dual Network Fluoropolymer (DNF) model. 

Figure 3-35 shows the curves of a family of thermoplastics describe a failure process that is fairly typical of the behavior of TPs. The time-dependent strains resulting from several levels of sustained or creep stress are shown, together with the development of... 

A particular thermoplastic melt behaves as a viscous liquid or an elastic solid during processing operations depending on the relationship between the time scale of deformation to which it is subjected and the time required for the time-dependent mechanism to respond. The ratio of characteristic time to the scale of deformation is defined as the Deborah number De = by Reiner [1,2], where X. is the characteristic time and X, is the time scale of deformation. The characteristic time, X for any material can always be defined as the time required for the material to reach 63.2% or 1 — (1/e) of its ultimate retarded elastic response to a step change. If De > 1.0, elastic effects are dominant, whereas if De < 0.5, viscous effects prevail. For any values of Deborah numbers other than these two extremes, the materials depict viscoelastic behavior. 

The mechanical properties of thermoplastics are time dependent with respect to both temperature and strain [1, 2]. Many approaches for modelling the time-, stress-and temperature dependent deformation behavior are based on a temperature dependent elastic deformation [3,4,5]. It is still not clear how superimposed elastic and viscoelastic parts of the deformation can be considered separately. 

When an engineering plastic is used with the structural foam process, the material produced exhibits behavior that is easily predictable over a large range of temperatures. Its stress-strain curve shows a significantly linearly elastic region like other Hookean materials, up to its proportional limit. However, since thermoplastics are viscoelastic in nature, their properties are dependent on time, temperature, and the strain rate. The ratio of stress and strain is linear at low strain levels of 1 to 2%, and standard elastic design... 

The behavior of plastic structures under compression plays a critical role in numerous applications. It has been recognized that the buckling of metals under elevated temperatures presents important distinctions from the classical Eulerian case, [11]. During an experimental study, [12], buckling times were registered for a range of compressive loads applied to the top of compression molded and annealed thermoplastic samples (see Fig. 2). A typical time - load dependence is shown in Fig. 3. 

The behavior of the material under high stresses and strains in the microregion at the crack tip should also reflect specific features determined in macroscopic experiments. In thermoplastic materials the dependence on strain and time is of prime importance. These questions will be addressed in the following section. 

Also, the AUC (area under curve) of the different materials represents their resilience. Cast iron and ceramics are very brittle steel, copper, and aluminum, as well as the thermoplastics PA and PP, are highly deformable and can therefore absorb large amounts of energy, for example from (impact) load application. It must be remembered here that the deformation behavior of plastics is highly dependent on time and temperature factors (see Fig. 15). Simplified explanations of deformation terminology follow. 

Generally, two different types of measurement are applied to determine the linear viscoelastic behavior, namely static (or equilibrium) and dynamic mechanical measurements. Static tests involve the imposition of a step change in stress and the observation of any subsequent development in time of the strain, whereas dynamic tests involve the application of a harmonically varying strain. In ordinary thermoplastic polymer systems, test conditions such as strain or frequency must be in the linear range otherwise, the results will be dependent on the experimental details rather than on the material under test. 

Compared to rubber, thermoplastic-modified systems show a much more complicated phase separation and structure evolution, and consequently very few studies were initially undertaken related to phase separation and rheological behavior. Examples of these studies included Pethrick et cd. [47,48], who applied a curome-ter to monitor the pot life and gel time of thermoplastic-modified epoxy systems. In 1999, Bormet and Pascault et al. [49] were the first to conduct a study of the relationship between rheological behavior and thermoplastic concentration in pol-yetherimide-modified epoxy-amine systems. By using an isothermal steady shear test with parallel plates at low deformation (1%), these authors showed that the rheological behavior at phase separation was greatly dependent on the initial concentration of thermoplastics. 

---