Vibro-acoustic modulation baseline-free non-destructive testing

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 amplitude, B: MI ~ B^β. Thus, quadratic nonlinearity yields linear dependence, β =1, and Hertzian nonlinearity results in β<1. Other nonlinear mechanisms yield different power laws. Therefore, measuring power damage coefficient β instead of MI may offer testing without established reference value. It also offer some insights into the nonlinear mechanisms transformation during damage evolution. This approach was experimentally investigated and validated.