Glossary term
Loss Factor
A dimensionless measure of energy dissipated relative to energy stored during cyclic material or structural deformation.
Definition
metricLoss factor is a dimensionless damping metric that relates energy dissipated per cycle to energy stored in a material or structure.
In dynamic materials and vibration analysis, loss factor measures internal damping. It is often denoted eta and is related to the phase lag between stress and strain in viscoelastic behaviour. A high loss factor means the material or system dissipates more vibrational energy as heat, reducing resonance peaks and transmitted vibration. It is used for polymers, rubbers, constrained-layer damping, composites, acoustic treatments, mounts, and structural damping design.
Loss factor measures damping through energy dissipation. In a cyclic loading process, a perfectly elastic material stores and returns energy without loss. A real material or structure dissipates some energy as heat, friction, microstructural motion, or hysteresis. The loss factor expresses that dissipation relative to stored energy.
For many vibration problems, loss factor is related to energy per cycle:
where \Delta E is energy dissipated per cycle and E_s is peak stored energy. In viscoelastic material testing, the loss factor is often expressed as:
where E' is storage modulus and E'' is loss modulus.
Engineering use
Loss factor is important in vibration control, acoustics, damping treatments, isolators, mounts, panels, tires, adhesives, polymers, rubbers, composites, and constrained-layer damping. A higher loss factor can reduce resonance amplitude and transmitted vibration. It can also reduce noise by dissipating vibrational energy before it radiates as sound.
However, damping performance is frequency and temperature dependent. A polymer may have high loss factor near its glass transition but much lower damping outside that range. Strain amplitude, preload, ageing, humidity, and manufacturing process can also change measured damping.
Relation to damping ratio
For lightly damped single-degree-of-freedom systems, loss factor and damping ratio are approximately related by:
This approximation is useful but should not be applied blindly to complex, nonlinear, or heavily damped systems.
Common mistakes
A common mistake is quoting one loss factor as if it applies to all frequencies and temperatures. Another is confusing material loss factor with system damping. A structure made from a high-loss material can still have low effective damping if strain energy is not concentrated in that material. Good damping specifications state frequency, temperature, strain amplitude, test method, material condition, and whether the value is a material property or assembled-system result.