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Monitoring the continued health of aircraft subsystems and identifying problems before they affect airworthiness has
been a long-term goal of the aviation industry. Because in-service conditions and failure modes experienced by
structures are generally complex and unknown, conservative
calendar-based or usage-based scheduled maintenance
practices are overly time-consuming, labor-intensive and expensive. Metal structures such as helicopters and other
transportation systems are likely to develop fatigue cracks under cyclic loads and corrosive service environments. Early
detection of cracks is a key element to prevent catastrophic failure and prolong structural life.
Furthermore, as structures age, maintenance service frequency and costs increase while performance and availability
decrease. Current non-destructive inspection (NDI) techniques that can potentially be used for this purpose typically
involve complex, time-intensive procedures, which are labor-intensive and expensive. Most techniques require access to
the damaged area on at least one side, and sometimes on both sides. This can be very difficult for monitoring of certain
inaccessible regions. In those cases, inspection may require removal of access panels or even structural disassembly.
Once access has been obtained, automated inspection techniques likely will not be practical due to the bulk of the
required equipment. Results obtained from these techniques may also be sensitive to the sweep speed, tool orientation,
and downward pressure. This can be especially problematic for
hand-held inspection tools where none of these
parameters is mechanically controlled. As a result, data can vary drastically from one inspection to the next, from one
technician to the next, and even from one sweep to the next.
Structural health monitoring (SHM) offers the promise of a paradigm shift from schedule-driven maintenance to
condition-based maintenance (CBM) of assets. Sensors embedded permanently in aircraft safety critical structures that
can monitor damage can provide for improved reliability and streamlining of aircraft maintenance. Early detection of
damage such as fatigue crack initiation can improve personnel safety and prolong service life.
This paper presents the testing of an acousto-ultrasonic piezoelectric sensor based structural health monitoring system for
real-time monitoring of fatigue cracks and disbonds in bonded repairs. The system utilizes a network of distributed
miniature piezoelectric sensors/actuators embedded on a thin dielectric carrier film, to query, monitor and evaluate the
condition of a structure. The sensor layers are extremely flexible and can be integrated with any type of metal or
composite structure. Diagnostic signals obtained from a structure during structural monitoring are processed by a
portable diagnostic unit. With appropriate diagnostic software, the signals can be analyzed to ascertain the integrity of
the structure being monitored. Details on the system, its integration and examples of detection of fatigue crack and
disbond growth and quantification for bonded repairs will be presented here.
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