Figure 1.

Fundamental quantities involved in mechanical characterization of a material. (a) Stress and strain defined as force per unit area and deformation per unit length respectively, are basic quantities that allow characterization of the mechanical response of materials. Materials deform differently under compressive, tensile, and shear forces. (b) The relationship between the stress and strain defines the material static mechanical properties. For simple elastic and purely viscous materials a simple linear relationship between the stress and strain/strain rate governs the mechanical properties. The elastic and shear moduli are measures of material rigidity and describe the tendency of a material to deform under normal and shear forces respectively. The viscosity is a measure of material resistance to flow under applied force and defined as the ratio of shear stress to shear strain rate. (c) For soft materials including cells, typically the stress is proportional to the strain under small deformations. However under larger deformations the stress–strain relationship is non-linear and the stress increases more rapidly under application of large strains.