Vibro-acoustic amplitude and frequency modulations during fatigue damage evolution
Vibro-Acoustic Modulation method (VAM) utilizes effect of the nonlinear interaction between higher frequency ultrasonic wave (carrier signal) and much lower frequency structural vibration (modulating signal). This interaction is taken place at the nonlinear interfaces (cracks, bolted connections, delaminations, etc.) manifesting itself in the spectrum as side-band components around the carrier. There are numerous studies applying VAM for nondestructive testing and structural health monitoring. Most of them utilize resonance structural bending vibrations as the modulating signal and measure a ratio of sideband to carrier spectral components defined as Modulation Index (MI). The present VAM study utilizes in-plane non-resonance very low frequency (10 Hz) tensile oscillations for monitoring fatigue and stress-corrosion damage evolution in steel. Experiments consistently demonstrated significant increase in MI during 70% - 80% of the fatigue life. Additionally, newly developed algorithm separates Amplitude and Frequency Modulations during the damage evolution demonstrating FM dominance at initial micro-crack growth stages and transition to AM dominance during macro-crack formation.