A Novel Fatigue Damage Sensor for Stress/Strain-Life Based Prediction of Remaining Fatigue Lifetime of Large and Complex Structures: Aircrafts

Abstract

In this paper, a new novel smart fatigue damage sensor (US Patent 8,746,077 B2) for continuous monitoring of fatigue health state of structural members of aircrafts is presented. The sensor has multiple parallel beams, each sensitive to different levels of fatigue lifetime. These beams are designed to fail prematurely but progressively as the sensor goes through the same fatigue cycles as the structural member it is attached to. Whenever fatigue level on an individual beam of the sensor exceeds the number of engineered fatigue cycles, that particular beam fails and sensor electronics can detect that failure and transmit this information wirelessly. Just like mileage signs on the road informing you about the distance left to your destination as you drive, multiple beams of the sensor serve similar purpose informing the user about the distance to failure progressively. Just as mileage signs can be placed at desired intervals, multiple beams can be engineered to give indication at desired fatigue milestones. This gives ability to monitor aging status of the structure and also help schedule predictive maintenance accordingly. The beams inside the sensor are designed to work based on different stress concentration factors (Notch Factors)/geometry to measure the level of structural fatigue health. The sensor needs to be mounted on the surface of structural member at fatigue critical locations just like strain gauges. Unlike strain gauges, a unique feature of the new sensor is its ability to operate without power source. This way it can serve for a long time without maintenance. Since sensor does not need power to operate, it can be embedded or mounted on critical components including composite structures or rotating helicopter shafts, gears, etc. After being attached to critical location of the real structure, the smart fatigue damage sensor goes through the same fatigue life experience of critical structural elements or mechanical components from the beginning of service life to the end. The fatigue sensing beams with different stress-strain and fatigue lifetime levels are designed to estimate the fatigue damage accumulation and remaining fatigue life of unidirectional and multidirectional structural or mechanical elements including composite structures.

Since distributed fatigue sensor network system monitors the fatigue health conditions of structures periodically or on demand, the collected data can be used not only for condition-based fatigue life prediction but also for sensor based predictive fatigue maintenance and development. This new approach could also pave way to new fatigue design tools for fatigue sensitive complex, large and expensive engineering structures or mechanical systems of aircraft structures. Full paper will be concentrating design principles of the sensor based on Stress/Strain-Life Based Prediction principles.