Composite Damage Detection in Aircraft Structures Using Vibration Measurements of Modulation Under Operational Loading

It is desirable in composite helicopter blades, airframes, and fuselage to detect incipient material damage and track the growth of structural damage as the aircraft undergoes rotordynamic and structural loading. In this paper, damage in composites is identified using vibration measurements to detect the presence of modulation between the low-frequency harmonic vibration response that is excited by the rotordynamics and the high-frequency acoustic response that is excited by a ceramic actuator. The hypothesis is that modulation between these two forced response components occurs if the composite material is damaged because the damage introduces nonlinear stiffness or damping restoring forces. For example, a delamination will produce clapping, buckling, or rubbing of the composite material when it is loaded depending on the direction and frequency range of the static and dynamic loads. One merit of this proposed approach for detecting incipient damage is that the low-frequency vibration is of sufficient energy to "pump" the composite structure to stress the damaged region whereas the high-frequency acoustic waves are of small enough wavelength to "probe" the structure to detect the damage. In addition, this approach to composite damage detection would ideally not be affected by the complexity of the composite structure (geometry, material configuration - sandwich, stiffened laminate, etc.) or variability in the operating regime of the aircraft. In order to determine the feasibility of using this damage detection method on military aircraft, experimental testing was conducted on a similar carbon fiber composite material to what is used on modern military helicopter structural components.