ABSTRACT
Liquids are effectively incompressible when subjected to direct stress (hydraulic power systems depend on this property), which implies that the elementary particles (atoms or molecules) are in direct contact. However, liquids obviously flow under the action of the shear stress, which shows that the particles are able to move relative to each other; that is, there are no primary bonds between them. In many cases, it is not possible to draw a sharp dividing line between the mechanical behaviour of liquids and solids; there is a large group of materials, known as viscoelastics, whose behaviour is part liquid and part solid. There are two important consequences of viscoelasticity. The first is that the stress–strain relationship is non-linear. The second consequence is known as Boltzmann's superposition theory. This states that each increment of load makes an independent and additive contribution to the total deformation.
