Interferometric Techniques Using Embedded Optical Fibers for the Quantitative NDE of Composites

Optical interferometric techniques using single mode optical fiber waveguide embedded in composites and other layered materials have been used to measure one- and two-dimensional stress distributions and acoustic emission caused by applied point source loads. By interferometrically comparing the phases of coherent optical signals propagated through an embedded sample fiber and a bypass reference fiber, a signal proportional to the instantaneous strain integrated along the embedded length of the sample fiber has been detected. System calibration has been obtained by applying a one-dimensional dc strain field to a cantilever beam containing the fiber. Using this calibrated system, an array of fibers attached to a 15cm × 15cm x 0.3cm plate simply supported at the corners and subjected to point loading on the surface has been used to quantitatively determine the two-dimensional dc stress field in the plate. Finally, the calibrated ac response of the interferometer to acoustic emission events in a composite panel has been demonstrated. Potential applications are discussed

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Ultrasonic detection of surface acoustic waves has been shown to have important applications in determining microstructural properties, inspection of materials for quality control and for detection of flaws in materials. Several laser generated optical techniques have been shown to be highly sensitive for the detection of ultrasound [1]. However, light transmission through air has several problems and fiber optic sensors are rapidly gaining popularity because of their high sensitivity, small size and low transmission losses