Loss factor is defined as the ratio of the energy dissipated by the structural system to the total energy of the system, per cycle of vibration

Pd = dissipated power
ω = angular frequency
η = loss factor
Ed = dissipated energy
ETot = Total mechanical energy
ETot = EK + ES
Total mechanical energy can be represented by the summation of dissipated kinetic energy, (Ek) and strain energy, (Es).