Modulus of High Elasticity

Modulus of High Elasticity 

a measure of the deformation resistance of rubbers and other rubberlike materials; it is the ratio of the stress σ to the reversible strain e. When e is small, the value of σ is proportional to ∊ (the linear region of the mechanical behavior of a material), and consequently the modulus of high elasticity by definition is analogous to the ordinary modulus of extension (Young’s modulus) or the shear modulus, depending on the type of stressed state that is being measured. When ∊ is large (region of high-elasticity ∊), the proportionality between ∊ and e is violated, and in this case the modulus of high elasticity is understood to mean an equivalent quantity that is a function of ∊ and, as before, is defined as the ratio ∊/σ;/. It usually ranges from fractions of a meganewton per sq m (MN/m²) to several meganewtons per sq m (from fractions to several tens of kilograms-force per sq cm [kgf/cm²]), whereas for metals and plastics, Young’s modulus attains values of the order of 10⁵ or 10³ MN/m² (106 or 104 kgf/cm2), respectively. Theoretically, the modulus of high elasticity should increase linearly with temperature, but in practice the temperature dependence can be neglected. For the high-elastic state there is typically no change in volume upon extension, and as a result, the modulus of high elasticity measured for shear is one-third of the modulus of high elasticity measured for uniaxial extension.

The pronounced difference between the values of the modulus of high elasticity for rubberlike substances and Young’s modulus for crystalline solids and glasses is associated with the different nature of the strains in the two cases. A decisive factor in the case of high-elasticity deformation is the flexibility of the polymer chain: deformation of a body as a whole occurs mainly by means of a change in the conformation of the macromolecules. However, ordinary elastic deformation results from a change in the interatomic distances and valence angles. The elastic forces that impede such changes are substantially greater than the forces required to prevent the elastic recovery of a rubberlike body. The absolute value of the modulus of high elasticity increases with the strengthening of the intermolecular coupling between the polymer chains and the increase in the thickness of the three-dimensional network of chemical bonds.

A. IA. MALKIN