Use of Leave‐in‐Place Sensors and SHM Methods to Improve Assessments of Aging Structures

The number of commercial aircraft operating at or beyond their initial design lives continues to grow. Multisite fatigue damage, hidden cracks in hard-to-reach locations, disbonded joints, erosion, impact, and corrosion are among the major flaws encountered in today's extensive fleet of aging aircraft and space vehicles. Furthermore, the extreme damage tolerance and high strength-to-weight ratio of composites have motivated designers to expand the role of advanced materials in aircraft structures. These developments, coupled with new and unexpected phenomena, have placed greater demands on the application of advanced nondestructive inspection (NDI) and health monitoring techniques. In addition, innovative deployment methods must be employed to overcome a myriad of inspection impediments stemming from accessibility limitations, complex geometries, and the location and depth of hidden damage. The use of in situ sensors for real-time health monitoring of aircraft structures appears to be a viable option to address these concerns. Recent requests for real-time monitoring of structures have produced a niche for active sensor systems using remote frequency eddy currents, fiber-optics, piezoelectric materials, and comparative vacuum monitoring. Reliable, structural health monitoring systems can automatically process data, assess structural condition, and signal the need for human intervention. Prevention of unexpected flaw growth and structural failure could be improved if onboard health monitoring systems are used to continuously assess structural integrity. Such systems would be able to detect incipient damage before catastrophic failure occurs. Condition-based maintenance practices could be substituted for the current time-based maintenance approach. Other advantages of onboard distributed sensor systems are that they can eliminate costly, and potentially damaging, disassembly, improve sensitivity by producing optimum placement of sensors with minimized human factor concerns in deployment, and decrease maintenance costs by eliminating more time-consuming manual inspections. In addition to mature MEMS (microelectromechanical systems) devices such as acoustic emission sensors, accelerometers, strain gauges, and pressure sensors, recent advances in microsensors have produced miniature eddy-current, ultrasonic, piezoelectric, fiber-optic, and other devices that lend themselves more directly to damage detection. Technology exists to co-locate the processing electronics with in situ sensor networks to produce real-time transmission of data and real-time diagnostics of structural health. When combined in a systems approach that includes sensors to monitor electronics, hydraulics, and avionics, it is possible to produce an aircraft prognostic health management architecture that can assist in maintenance scheduling and tracking. This article focuses on developments and testing of mountable sensors and how they can be integrated into such a health management system. Specific example applications are discussed along with issues that must be addressed to realistically deploy leave-in-place sensors. Successful field testing is presented to quantify the performance of real-time, health monitoring systems and to highlight their use in guiding condition-based maintenance activities. Keywords: nondestructive testing; in situ sensors; real-time health monitoring; eddy current; ultrasonic; piezoelectric; fiber-optic; comparative vacuum monitoring; condition-based maintenance; SHM validation