Solids, resisting deformation shearing force

In our attempt to differentiate between fluids and solids, we can now say that solids are substances which can permanently resist very large shear forces. When subjected to a shear force solids move a short distance (elastic deformation), thereby setting up internal shear stresses which resist the external force, and then they stop moving. Materials that obviously are fluids cannot permanently resist a shear force, no matter how small. When subjected to a shear force, fluids start to move and keep on moving as long as the force is applied. 

Fluids are substances that move continually when subjected to a shear force as long as the force is applied. Solids deform slightly when subjected to a shear force and then stop moving and permanently resist the force. There are, however, intermediate types of substance the distinction between solid and liquid is one of degree rather than of kind. ... 

The polymer s wear and impact resistance is due to the extremely long molecular chains of the material and its semi-crystalline solid state conformation. The surface and subsurface structure of the polymer is highly ordered and the crystal sites offer resistance to deformation and conformance to a wear mating surface and to sections being tom loose. (3). UHMWPE does not easily flow when high shear forces are applied due to the excessive entanglements of the long polyethylene chains. 

Viscoelastic materials are those which exhibit both viscous and elastic characterists. Viscoelasticity is also known as anelasticity, which is present in systems when undergoing deformation. Viscous materials, like honey, polymer melt etc, resist shear flow (shear flow is in a solid body, the gradient of a shear stress force through the body) and strain, i.e. the deformation of materials caused by stress, is linearly with time when a stress is applied [1-4]. Shear stress is a stress state where the stress is parallel or tangencial to a face of the material, as opposed to normal stress when the stress is perpendicular to the face. The variable used to denote shear stress is r which is defined as ... 

In response to external forces, particulate solids have characteristics of solid- and liquidlike behaviour, capable of resisting flow until a limiting shear stress is exceeded, after which infinite flow will occur. Unlike liquids, however, shear stress is proportional to the normal load applied rather than the rate of deformation and, unlike solids, the magnitude of shear stress is generally indeterminate, depending on the inter-particle static coefficient... 

When the applied shear stress system is removed from an elastically deformed solid, the solid regains its original shape completely and the work of deformation is also recovered. No such recovery occurs in a liquid the work done in producing a given rate of shear is completely dissipated against the liquid friction forces. The resistance to flow offered by a liquid when it is subjected to a shear stress is called a viscous force and the liquid is said to possess viscosity. 

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