Monitoring the integrity of composite patch structural repair via piezoelectric actuators/sensors

Abstract

Critical to the success of composite repair of metallic aircraft structures is the integrity of the bond between the base aluminum panel and the reinforcing high-strength composite patch. Monitoring of the repair is equally important to insure the composite patch integrity throughout the service life of the structure. Described in this paper are the test methods and results of a vibration signature-based technique used to qualitatively identify a de-bond on several different composite repair coupons. The high-frequency domain vibration signature from the test coupon is obtained using a single patch of piezoelectric material (Lead Zirconate Titanate or PZT), functioning both as an actuator and sensor. The vibration signature is obtained as a variation in electrical impedance of the piezoelectric patch, while driven by a fixed alternating electric field over a frequency range. The current drawn by the actuator is modulated due to the structure's inherent dynamic characteristics. The modulated electrical impedance which is analogous to the frequency response function, but much more easily obtainable, is an indication of vital dynamic structural behavior and is used to identify damage. Damage is simulated by either growing an existing pre-crack under the composite patch through cyclic loading or by creating a de-bond close to the edge of the repair patch. High frequency excitation, which is greatly facilitated by the electrically driven low-power compact PZT patch, is critical to the success of this technique because it assures a clearly visible change in the impedancelvibration signature even for very minor damage/changes. The technique has met with great success in the first stage of this development effort. Even a very minor de-bond or crack growth, has been clearly detected.