Structural health monitoring relies on routine inspection and maintenance to ensure safe operation and optimized service life of critical structures. Surface strain measurements can provide an important indicator of structural condition. Established strain sensor technologies, such as resistance strain gages and fiber Bragg grating (FBG) sensors, as well as some newer ones, [1-3] are all point-wise, unidirectional, and contact sensing methods that require physical connections in order to obtain strain readings. Some existing full-field non-contact optical methods have also been developed, such as interferometric techniques, [4] non-interferometric techniques, [5-7] and Raman spectroscopy. [8] The interferometric techniques usually require a model of actual structure, plus tedious calculations to separate the values of principal stresses, and expensive equipment. The limitation of non-interferometric techniques lies in the requirements for a random gray intensity distribution or speckle pattern distribution and heavy dependence on the quality of the imaging system. Over the past twenty years, Raman spectroscopy has also been investigated for strain sensing applications by many researchers. [8-12] However, Raman-based strain sensing methods are hampered by the low intensity of Raman scattering signals. A more promising approach for non-contact strain sensing technology uses carbon nanotube fluorescence.
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