In this study, frequency response functions (FRF) measured by piezoelectric Macro-Fiber Composites (MFC) are used to detect invisible delamination in a composite plate. The plate is impacted to seed damage in the form of fiber delamination. Then, the active-sensing system exerts an excitation into the plate, and measures the subsequent responses. FRF and damage indicator features are derived from the measured signals and used to assess the condition of the plate. Validation of the delamination is completed using an ultrasonic scan method. The effective area of observed damage is well correlated to the damage indicator feature.Copyright © 2004 by ASME