Real and Imaginary or Magnitude and Phase, Finding a Robust Damage Index for the Impedance Method Structural Health Monitoring

Structural vibration behavior is an essential signature of the integrity of structures and hence has been used for damage detection. Structural vibration impedance by way of piezoceramic patch excitation and sensoring offers a local damage detection technique. The commonly used damage index in this method is the rate of change of the real part of the measured electromechanical impedance. This paper studies several damage indices constructed by the real and imaginary parts or magnitude and phase of the measured impedance and attempts to answer the question of which part of the complex valued electromechanical impedance is better for damage assessment. It theoretically deducts and concludes that the damage index in terms of the change in real part is in fact not a properly defined index; on the other hand, the indices in terms of the change in magnitude and phase are shown as clear and properly defined in physical concept. It is further demonstrated by experiment that the damage index in the change of real part is not robust and is prone to measurement noise, whereas the whole complex valued impedance offers optimized and robust damage index