Table 1.
Glossary of Terms Relating to Viscoelasticity.
| Term | Definition/explanation |
|---|---|
| Elastic/elasticity | Tendency of a solid material to return to its original shape after being deformed. A material’s elasticity is described by a stress–strain curve |
| Elastic modulus (λ) | Defined as stress divided by strain—number that describes the resistance of a material to being deformed elastically. There are several types of elastic modulus, relating to the application of different forces (eg, shear modulus [G or µ], Young modulus [E]). Young modulus, which relates to tensile or compressive stress (ie, opposing forces along one axis), is sometimes referred to as “elastic modulus.” |
| Hysteresis | Difference between stress–strain curves as a material is being unloaded versus loaded. Consider an elastic band being first loaded then unloaded: during unloading, a given force produces a slightly longer length compared to that observed while the elastic band was being loaded. The effect becomes more pronounced if loading and unloading are done rapidly |
| Linear viscoelasticity | Rate of change in strain (strain rate) increases linearly with stress |
| Phase lag (δ) | Extent to which strain lags behind stress when the stress is oscillatory. Viscoelastic materials have a phase lag between 0° (value for a purely elastic material; stress and strain in phase) and 90° (purely viscous material). |
| Shear | Application of a force with direction perpendicular to the cross section of a material (eg, material with square cross section: bottom held in place, force applied to the top from left to right) |
| Shear complex dynamic modulus (G*) | Complex dynamic modulus represents the ratio of stress to strain under vibratory conditions. It is calculated from the storage modulus and the loss modulus as follows: G* = G’ + iG’’, where G’ is the shear storage modulus, G’’ is the shear loss modulus and i is the imaginary unit (square root of −1). As well as shear, it can be measured for tension (tensile complex dynamic modulus) or compression. |
| Shear loss modulus (G’’) | A measure of the deformation energy used up by the sample during the shear process. This parameter represents the viscous behavior of the material. As well as shear, it can be measured for tension (tensile loss modulus) or compression |
| Shear modulus/modulus of rigidity (G or µ; denoted by Hartert as ∊) | Defined as shear stress divided by shear strain. It is a type of elastic modulus, specifically for shear stress |
| Shear storage modulus (G’) | A measure of the deformation energy stored by a material during the shear process. This parameter represents the elastic behaviour of the material. As well as shear, it can be measured for tension (tensile storage modulus) or compression |
| Strain (∊) | A measure of the extent to which a material deforms when under stress. Measured as a ratio of the measurement under stress to the measurement at baseline. |
| Stress (σ) | Force per unit area—to study the properties of a material, a force is applied and the extent to which the material deforms is measured |
| Stress–strain curve | Graph showing relationship between stress and strain |
| Tensile | Relating to tension or the application of opposing forces along an axis. The act of stretching an elastic band in a straight line would involve the application of tensile stress |
| Viscoelasticity | Term used to describe materials possessing both elasticity and viscosity. When a stress is applied to a viscoelastic material, molecular rearrangement known as creep occurs. However, when the stress is removed “back stresses” within the material cause it to return to its original form |
| Viscous/viscosity | A measure of resistance to gradual deformation by shear stress or tensile stress. Corresponds to the “thickness” of a fluid—water has low viscosity and honey has higher viscosity |
| Young’s modulus (E) | Defined as tensile stress divided by tensile strain—similar to elastic modulus but specifically for tensile stress |