A Digital Mems-based Strain Gage for Structural Health Monitoring
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Conventional visual and nondestructive inspection methods must be scheduled in such a way as to ensure that any damage missed at one inspection will be discovered during a subsequent inspection before it poses a safety threat. The inaccessibility of many structural components, the dependence of results on inspector and inspection conditions, and the variability in service conditions experienced by nominally identical aircraft make this a challenging and costly job. In view of the above, there has been considerable interest in developing Health Usage and Monitoring Systems (HUMS) for application in old and new aerospace structures alike [4-5]. The goals of utilizing HUMS technologies include 1) reducing the frequency and cost of time-based maintenance and tear-down inspections by providing diagnostic data based on the flight loads history of specific individual aircraft, 2) reducing catastrophic structural failures during operation through early warning, 3) insuring combat readiness of operational aircraft, and 4) expanding the operational envelopes for aircraft based on airframe loads measured in-situ, commonly known as Condition Based Operation (CBO), and providing feedback to the pilot for situational awareness. A number of HUMS approaches can be employed to meet these goals. For instance, discrete strain measurements can be taken within critical aircraft structural assemblies to compare, at scheduled intervals over the life of an aircraft structure, strain (or vibration) "signatures" with a baseline signature identified early in the vehicle's life. Advanced "feature identification" algorithms may also be applied in real time for early warning of structural failure. Monitoring structural health can also be based on analysis of changes in strain (or spectral power density) distribution due to evolution of fatigue crack growth, impact related damage, or MSD. Finally, monitoring in-flight cumulative loading cycles can be used to provide a basis for remaining fatigue life predictions. HUMS for each of the above cases will require an array of distributed sensors [6-7]. Monitoring the stress distribution within an airframe is probably the most useful and can be traditionally approached by instrumenting critical load paths with strain gages. However, metal foil (or semi-conductor) strain gages have a number of disadvantages in the context of their use as HUMS sensors where large arrays of devices may be needed. For instance, strain gages require labor intensive installation, their analog outputs drift and require frequent calibration (making them unsuitable for monitoring long-term creep or plastic deformations), their use in large numbers results in excessive wiring and shielding requirements, and they are not reusable. Recent advances in the fabrication of MicroElectroMechanical s
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