ABSTRACT4This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e., the strong nonuniformity of the strain field), the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical fiber 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 electronic speckle pattern interferometry on the specimen surface and a discretized model of the grating, the spectra in transmission are then calculated analytically. For a known strain distribution, it is then shown that one can determine the magnitude of the applied force on the specimen. Thus, by considering the nonuniformity of the strain field, the optical fiber Bragg gage functions well as an embedded strain gage near the stress concentration.KEY WORDS--Optical fiber sensor, Bragg grating, embedded sensor, strain distribution, nondestructive evaluationThe use of optical fiber gages to measure temperature, strain, or even detect fracture in a material has great potential due to their relatively small size, sensitivity and immunity to electrical fietds.l Surface-mounted optical fibers, for example, have been used in a variety of strain gage applications. 2 Furthermore, due to their dimensions, optical fiber gages can be embedded unobtrusively into materials, particularly composites already containing fiber reinforcements. However, once an optical fiber is embedded, the interpretation of the gage response becomes more complex due to the effects of May 20, 1999. Final manuscript received: October 10, 2000 interface between the fiber and the material, as well as the multiple components of strain applied to the fiber. 3-6 The problem is further complicated when the strain field surrounding the gage is not sufficiently uniform with respect to the scale of the gage length.As an example, the optical fiber Bragg grating (OFBG) sensor permits the localized measurement of axial strain in an optical fiber. In comparison with a simple optical fiber displacement gage, the response of the OFBG sensor is only affected by the strain (or temperature) field at the location of the grating and not along other portions of the opticai fiber. This property makes the OFBG sensor especially useful for measuring localized phenomena such as the strain conditions near a region of fracture. 7The conventional treatment of the OFBG as a strain gage assumes that the reflected spectrum is a single distinct peak whose shift is linearly proportional to the applied strain. Naturally, this assumption is only valid if the gage le...
