Bending measurement using embedded long-period fiber gratings

The use of LPG embedded in carbon-fiber composite laminates (ELPG), in a 4-3 configuration, for bending measurement has been demonstrated. With increased bending curvatures on the 4 layers side, the coupling strength of the cladding mode decreases while the resonance wavelength remains relatively constant. A reduction in coupling strength leads to a reduction of the resonance amplitude depth. From the bending test covering the range of curvatures from 0m(-1) to 2m(-1), the ELPG yields a sensitivity of 5.065dB/m(-1) and a repeatability of 98.1%. In another investigation, the ELPG ability to determine direction of bend has also been demonstrated by applying bending at the 3 layers side of the laminate. Despite having a short curvature range between 0m(-1) and similar to0.626m(-1), the test demonstrates an increase of the cladding mode coupling strength with an increase in bending curvature, thus showing the ELPG ability to differentiate bending directions. By exploiting the unique characteristics of ELPG, two ELPGs can be exploited for 2-axis measurement of structures. Hence the overall cost and complexity of the bending sensor system can be greatly reduced.