Cure Terms Glossary

Dynamic Modulus

The modulus of a material is a measure of its stiffness, and the dynamic modulus is the stiffness under free or forced vibration. Modulus is the ratio of stress to strain, and for materials in oscillation, stress and strain are not necessarily in phase. Under sinusoidal oscillation, the measured modulus can be resolved into in-phase and (90°) out-of phase components as shown in the figure (for shear modulus) below.

  • The in-phase component (G') is the storage modulus and reflects the energy stored in the cycle.
  • The out-of-phase component (G"), the loss modulus, reflects the energy lost in the cycle.
  • The angle δ is the phase lag between stress and strain.

The terms storage and loss refer to the consevation or otherwise of mechanical energy - i.e. G' reflects the (perfectly elastic) component where mechanical energy is conserved and G" reflects the (purely viscous) component where mechanical energy is lost to friction heating. A material with elements of both viscous and elastic character is termed viscoelastic. The storage and loss moduli can also be represented in complex notation as G* = G' + iG".