Cure Terms Glossary

Modulus

The modulus is the ratio of stress to strain, and is a measure of a material's resistance to deformation - i.e. its stiffness. The actual modulus is specific to the strain geometry and a number of different modulus types are recognised in simple short-term testing. Examples include:

  • Tensile modulusa (typically designated E) = (tensile stress) / (tensile strain)
  • Compression modulusb (typically designated Ec) = (compressive stress) / (compressive strain)
  • Flexural modulusc (typically designated Ef) = (flexural stress) / (flexural strain)
  • Shear modulus (typically designated G) = (shear stress) / (shear strain)

a) also known as Young's modulus
b) also known as bending modulus (Eb)
c) also known as bulk modulus (B, K) or support factor (flexible foams)

A modulus has units of stress (i.e. force per unit area), and the SI unit is the Pascal, where 1 Pa = 1 N/m2. Typical values for polymers cover a wide range of values, typically measured in kPa for flexible foams, MPa for solid rubbers and GPa for plastics.

At the molecular level, modulus is a function of the stiffness of the molecule itself. Thus anything which stiffens the molecule (backbone architecture, intermolecular associations, etc.) will increase the modulus. For a hypothetical freely-jointed chain (without entanglements), the modulus is directly proportional to crosslink density and this concept has been used in the development of theories of rubber elasticity.

In oscillatory measurements, the ratio of stress to strain gives a dynamic modulus.