Rheology and mechanical properties

Rheological knowledge combined with laboratory data can be used to predict stresses developed in plastics undergoing strains at different rates and at different temperature; rheology is the science of the deformation and flow of matter under force. The procedure of using laboratory experimental data for the prediction of mechanical behavior under a prescribed use condition involves two rheological principles. There is the Boltzmann's superposition principle that enables one to utilize basic experimental data such as a stress relaxation modulus in predicting stresses under any strain history. The second is the principle of reduced variables, which by a temperature-log time shift allows the time scale of such a prediction to be extended substantially beyond the limits of the time scale of the original experiment.

Regarded as one of the cornerstones of physical science, is the Boltzmann's Law and Principle that developed the ldnetic theory of gases and rules governing their viscosity and diffusion. This important work in chemistry is very important in plastics (Ludwig Boltzmann born in Vienna, Austria, 1844-1906). It relates to the mechanical properties of plastics that are time-dependent.

The rheology of solid plastics within a range of small strains and within a range of linear viscoelasticity, has shown that mechanical behavior has often been successfully related to molecular structure. It shows the mechanical characterization of a plastic in order to predict its behavior in practical applications and how such behavior is affected by temperature. It also provides rheological experimentation as a means for obtaining a greater structural understanding of the material that has provided knowledge about the effect of molecular structure on the properties of plastics, particularly in the case of amorphous plastics in a rubbery state as well as extending knowledge concerning the complex behavior of crystalline plastics. Studies illustrate how experimental data can be applied to a practical example of the long-time mechanical stability.

As reviewed, a plastic when subjected to an external force part of the work done is elastically stored and the rest is irreversibly dissipated. Result is a viscoelastic material. The relative magnitudes of such elastic and viscous responses depend, among other things, on how fast the material is being deformed. It can be seen from tensile stress-strain (S-S) curves that the faster the material is deformed, the greater will be the stress developed since less of the work done can be dissipated in the shorter time.

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