Embedded optical fiber Bragg grating sensor in a nonuniform strain field: Measurements and simulations

This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due t o the nature of a stress concentration (i.e., the strong nonuniformity of t he strain field), the assumption that the grating spectrum in reflection re mains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical f iber Bragg gratings with different gage lengths are embedded near the notch tip. The form of the spectra in transmission varies between gages that are at different distances from the notch tip under given loading conditions. This variation is shown to be due to the difference in the distribution of strain along the gage length. By using the strain field measured using elec tronic speckle pattern interferometry on the specimen surface and a discret ized model of the grating, the spectra in transmission are then calculated analytically, For a known strain distribution, it is then shown that one ca n determine the magnitude of the applied force on the specimen. Thus, by co nsidering the nonuniformity of the strain field, the optical fiber Bragg ga ge functions well as an embedded strain gage near the stress concentration.