A Novel Optical Fatigue Monitoring And Crack Sensing System
Aircraft airworthiness management has dramatically extended the lifespan of an increasing number of aging fleets. In many other aerospace communities, the design and development of an integrated vehicle health management system has been their major focus of research, particularly in the area associated with the monitoring of hidden cracks prior to structural failure. The ability to detect and monitor cracks provides the opportunity to take preventative action prior to impending failure. However, current structural monitoring technologies are limited by their weight, electromagnetic compatibility, power tolerance requirements, and cost. Due to weight, many of these sensors have limited value in critical areas where cracks form and residual strains need to be measured such as the fastener locations in aircraft structures. These challenges represent opportunities for optical based strain measuring sensors. This work is focused on the optimization and assessment of a novel optical metrology application. Optical metrology is the technology and science pertaining to measurements with light. These measurements can either target properties such as distance or of light itself. The notion of distance change of a small straight line segment is the fundamental metric of continuum mechanics and when coupled with light sensing has led to the important field of optical stress analysis. Optical sensors have been developed that measure force, torque, pressure and strain. This data is used to determine the effects of vibrations and/or loads on operating systems. Results are presented to demonstrate the optical metrology theory applicability to measure displacement fields as a bonded polyimide substrate is subjected to deformation. Using this method, one-dimensional strain data was measured as a metric pre-cursor to crack initiation. Quadratic approximations relating to nodal values of displacements were computed for different stages of crack growth. This technique was found to be comparable in sensitivity with that of conventional strain sensors under different loading scenarios. Although this technology still has the potential to be further refined, the digital strain encoder was found to be an exceptional method for small-scale measurements characterizing cracks and other flaws.
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