Rheological Behavior and Material Classification

All real materials fall Theologically between two extremes the perfectly elastic Hookean solid, for which stress is directly proportional to strain, and the Newtonian liquid, for which (shear) stress is directly proportional to (shear) strain rate. Strain can be defined as deformation relative to a reference length, area or volume (Barnes et al., 1989) it is dimensionless. Strain 

Real materials are neither truly Hookean nor truly Newtonian, though some exhibit Hookean or Newtonian behavior under certain conditions (Barnes et al., 1989). Real materials may exhibit nonlinearity, which is a lack of direct proportionality between stress and strain, or between stress and strain rate. Real materials may exhibit either predominantly elastic behavior or predominantly viscous behavior, or a measurable combination of the two, depending on the stress or strain and the duration of its application (Barnes et al., 1989). Such materials are termed viscoelastic. Barnes et al. (1989) pointed out that it is better to classify rheological behavior than to classify materials, a given material can then be included in more than one rheological class depending on experimental conditions. 

However, for practical purposes, it is useful to distinguish between solids and liquids according to the following definitions (Barnes et al., 1989)  

A given material may be a solid under some conditions and a liquid under others for example a plastic material is a solid at stresses lower than its yield stress but a liquid at higher stresses. The term semi-solid is a convenient, though imprecise, description of materials that when unsupported by a container, change shape (deform) under the influence of gravity under ambient conditions. Many fat-based foods are semi-solid. 

Viscolelastic materials can be divided into viscoelastic solids and viscoelastic, or simply elastic, liquids. All viscoelastic liquids are non-Newtonian, but not all non-Newtonian liquids are viscoelastic. Non-Newtonian liquids show nonlinear rheological behavior, and this may be time dependent (Barnes et al., 1989). 

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