Non linear behaviour generally occurs in many mechanical systems. At the present time, with the significant development of new materials such as polymeric rubbers and other materials, it is important to define and to characterise the behaviour of non linear materials.
While in linear systems it is normal to use modal analysis to obtain the parameters of the system, this does not happen in non-linear systems. One of the hypotheses assumes that modal analysis represents the system linearity and for that reason it is not applicable in the non linear case. Nevertheless, for small degrees of non-linearity, it is possible to consider this approach valid.
It is possible to differentiate between global non-linear behaviour like the one caused by the properties of the material, and local non-linear behaviour, that appears in localised elements of the mechanical system such as comforts or springs.
On the other hand, we can also distinguish:
No-linearity due to damping (hysteresis of the damping, friction of the Coulomb type).
No-linearity due to rigidity (soft non-linear, Young modulus non-linear).
No-linearity caused by the mass (change of inertia in the time as a result of the movement).