Internal friction is understood to be a set of physical phenomena, responsible for irreversibility, in terms of mechanical energy, of strain processes in solids. A part of the energy that a body receives when under strain is dissipated, that is, spread within the environment surrounding the body. Hondo and Jokibe also concluded that the decrement is directly proportional to vibration frequency. Glikman and Hejna observed that for moderate frequencies the damping decrement gets smaller with growth of frequency. The hysteresis loop shown in the figure corresponds to mild steel for different maximum values of stress. The presence of internal friction decreases resonant amplitudes in the case of forced vibration and generates hysteresis loops. Kelvin-Voigt's model is a very often used model of a linear viscoelastic solid. The resistive force opposing external loading is posed by the sum of the spring and the damper resistive forces.