Development of structural health monitoring systems for composite bonded repairs on aircraft structures

The application of bonded composite patches to repair or reinforce defective metallic structures is becoming recognised as a very effective versatile repair procedure for many types of problems. Immediate applications of bonded patches are in the fields of repair of cracking, localised reinforcement after removal of corrosion damage and for reduction of fatigue strain. However, bonded repairs to critical components are generally limited due to certification concerns. For certification and management of repairs to critical structure, the "Smart Patch" approach may be an acceptable solution from the airworthiness prospective and be cost effective for the operator and may even allow some relaxation of the certification requirements. In the most basic form of the "Smart Patch" in-situ sensors can be used as the nerve system to monitor in service the structural condition (health or well-being) of the patch system and the status of the remaining damage in the parent structure. This application would also allow the operator to move away from current costly time-based maintenance procedures toward real-time health condition monitoring of the bonded repair and the repaired structure. To this end a 'stand-alone' data logger device, for the real-time health monitoring of bonded repaired systems, which is in close proximity to sensors on a repair is being developed. The instrumentation will measure, process and store sensor measurements during flight and then allow this data to be up-loaded, after the flight, onto a PC, via remote (wireless) data access. This paper describes two in-situ health monitoring systems which will be used on a composite bonded patch applied to an F/A-18. The two systems being developed consists of a piezoelectric (PVDF) film-based and a conventional electrical-resistance foil strain gauge-based sensing system. The latter system uses a primary cell (Lithium-based battery) as the power source, which should enable an operating life of 1-2 years. The patch health data is up-loaded by the operator using an IR link. The piezoelectric film-based sensing system is self-powered and has been designed to operate using the electrical power generated by an array of piezoelectric films, which convert structural dynamic strain to electrical energy. These transducers power the electronics which interrogate the piezoelectric film sensors, and process and store the patch health data on non-volatile memory. In this system the patch health data is up-loaded by the operator using a magnetic transceiver. This paper describes the development and evaluation of the two systems, including issues such as system design and patch health monitoring techniques.