A designer who has trained in traditional engineering materials and who is now designing in plastic should have a grasp of the general concept of viscoelastic response in order to understand the behaviour of thermoplastics. When discussing viscoelasticity, the following definitions are relevant.
Viscoelastic Material
A material whose response to a deforming load combines both viscous and elastic qualities. The common name for such a material is 'plastic'.
Stress
The force per unit area, which is acting on a material and tending to change its dimensions. It is the ratio of the amount of force divided by the cross-sectional area of the body resisting that force. In engineering formulae, stress is represented by s. Among other types, stress can be tensile - as when the body is subject to a tension load; compressive - when the body is subjected to compression loading; or shear - when the body is subject to shearing load.
Strain
The percentage deformation of a body when subjected to a load. Tensile strain occurs when there is an increase in the original dimension, and is numerically expressed as the change in length per unit length of the specimen under load. It is represented in formulae by e. There can also be compressive, shear and volumetric strains.
Young's Modulus
This is the ratio between stress and strain, i.e. stress divided by strain, and is denoted by E.
Elastic Material
A material that deforms under stress, but regains its original shape and size when the load is removed. A practical example of elastic material is any spring working within its limits. For completely elastic materials, stress is directly proportional to strain. However, when material has viscous as well as elastic properties (as in 'plastic'), deviation from this linear relationship occurs.
Proportional Limit
This is the point on the stress-strain curve where deviation from the linear relationship starts to occur. The point at which the proportional relationship starts to deviate is often expressed in term of stress (s), and also the deformation is often expressed in terms of the strain produced.
Elastic Limit
The point on the stress-strain curve which marks the maximum stress a material can absorb and still recover to its original dimensions with no permanent deformation. The recovery may not be immediate, and the elastic limit may be at stress levels higher than the proportional limit.
Viscous Material
A material which after being subjected to a deforming load, does not recover its original shape and size when the load is removed. An example is a piston in a dashpot containing a viscous fluid. If a load is applied to move the piston in the dashpot, the piston will not return to its original position after the displacing load is removed, unless a returning load is applied - opposite to the original load.
Rheology
The science of the deformation and flow of materials under load.