Abstract
Structural materials of the new millennium will be hybrid materials, composed of polymeric, metallic and ceramic components. A common characteristic of the hybrid materials is their time-dependent behavior, which arises from the time-dependency of polymeric constituents. The intensity and the rate of time-dependency depends on boundary conditions, i.e., temperature, pressure, moisture, and mechanical loading, to which material has been exposed during the processing (i.e. extrusion, injection molding, blowing, …) and later in the exploitation. It is important to note that the theory of elasto-plasticity for these materials is usually not applicable. It is also necessary to state that, at present, there is no generally accepted theory (constitutive model), which would enable reliable predictions of the stress-strain state of structural materials made of these materials. For that very reason the automotive industry still uses steel for the brake pedals, while the clutch and gas pedals are commonly made of polymeric materials. With increase use of polymers in structural applications the research activities in this field become more and more intensive.
Our research group, in collaboration with the colleagues from the California Institute of Technology have developed the nonlinear viscoelastic model - »Knauss-Emri« (the authors received for this achievements in 1997 the prestigious »Kapitsa« medal; the same year they were elected as Editors-In-Chiefs of the international journal Mechanics of Time Dependent materials, published by Kluwer.). This model enables predictions of the stress-strain state of the viscoelastic materials simultaneously exposed to temperature, pressure, moisture and mechanical loading. The model is currently one of the four leading models in the field [1].
The model is very reliable in predicting the volumetric changes in the vicinity of the glass transition temperature (physical aging), resulting from the pressure and temperature variation, e.g., in the phase of polymer processing.
At present this is still the only model enabling good predictions of the flow of the material exposed to the simultaneous influences of temperature, moisture and mechanical loading [2], e.g., in the melt spinning process of the PA and PE fibers, which are drawn with the acceleration of 6000 g and higher.
The model is less appropriate for predictions of the material behavior under the shear loading, with the simultaneous influence of temperature, moisture and hydrostatic pressure. The research program Mechanics of Time Dependent Materials is orientated towards systematic analysis of the combined influence of temperature, moisture, hydrostatic pressure and shear loading variations on the behavior of time dependent materials. The ultimate goal of this research program is to upgrade the »Knauss-Emri« model into the general nonlinear theory of thermo-elasto-visco-plasticity.
[1] Landel, R.F., Peng, S.T.J., Equation of State and Constitutive Equations. J. of Rheology, Vol. 30. Iss. 4, pp. 741-765, (1986).
[2] Chan, R.W., Haasen, P., Kramer, E.J., Materials Science and Technolgy, Materials, Vol. 12. Structure and Properties of polymer, VCH Publishers, Weinheim, (1993)
