Dynamic modulus (sometimes complex modulus) is the ratio of stress to strain under vibratory conditions (calculated from data obtained from either free or forced vibration tests, in shear, compression, or elongation). It is a property of viscoelastic materials.
Viscoelastic stress–strain phase-lag
- In purely elastic materials the stress and strain occur in phase, so that the response of one occurs simultaneously with the other.
- In purely viscous materials, there is a phase difference between stress and strain, where strain lags stress by a 90 degree ( radian) phase lag.
- Viscoelastic materials exhibit behavior somewhere in between that of purely viscous and purely elastic materials, exhibiting some phase lag in strain.
Stress and strain in a viscoelastic material can be represented using the following expressions:
- Stress: 
- where is frequency of strain oscillation,
- is time,
- is phase lag between stress and strain.
Storage and loss modulus
The storage and loss modulus in viscoelastic materials measure the stored energy, representing the elastic portion, and the energy dissipated as heat, representing the viscous portion. The tensile storage and loss moduli are defined as follows:
- Loss: 
Similarly we also define shear storage and shear loss moduli, and .
Complex variables can be used to express the moduli and as follows:
where is the imaginary unit.
Ratio between loss and storage modulus
The ratio between the loss and storage modulus in a viscoelastic material is defined as the , which provides a measure of damping in the material. can also be visualized in vector space as the tangent of the phase angle () between the storage and loss modulus.
For example, a material with a greater than one will exhibit more damping than a material with a less than one. The reason that a material with a greater than one shows more damping is because the loss modulus of the material is greater than the storage modulus, which means the energy dissipating, viscous component of the complex modulus prevails as the defining factor of the material's characteristics.
- The Open University (UK), 2000. T838 Design and Manufacture with Polymers: Solid properties and design, page 30. Milton Keynes: The Open University.
- PerkinElmer "Mechanical Properties of Films and Coatings"
- Meyers and Chawla (1999): "Mechanical Behavior of Materials," 98-103.