  Vibro-Acoustic Modulation method for detection and characterization of
various structural and material flaws has been actively researched for the last
two decades. Most of the studies focused on detection and monitoring of
macro-cracks requiring well established baseline (no-damage) value of the
modulation index. The baseline value is specific for a particular structure,
measuring setup, and other factors and can't be established in many practical
situations without a long term monitoring looking for a relative change in the
Modulation Index. In this work, we propose and investigate a baseline-free
Vibro-Acoustic Modulation method, which does not require monitoring of relative
Modulation Index change, unlike conventional approach. It was hypothesized that
the nonlinear mechanisms (and respective nonlinear response) of a structure are
different for undamaged and damaged material. For example: material without
damage or at early stages of fatigue have classic elastic or
hysteretic/dissipative nonlinearity while damaged (cracked) material may
exhibit contact bi-linear or Hertzian nonlinear mechanisms. These mechanisms
yield different power law dependencies of Modulation Index (MI) as function of
applied vibration frequency input amplitude, B: MI ~ B^b Thus, quadratic
nonlinearity yields linear dependence, b =1, and Hertzian nonlinearity results
in b<1. Other nonlinear mechanisms yield different power laws. Therefore,
measuring power damage coefficient b instead of MI may offer testing without
established reference value. It is also offer some insights into the nonlinear
mechanisms transformation during damage evolution. This approach was
experimentally investigated and validated.
