Monitoring of advanced composite weave structures using multi-axis fiber grating strain sensors

Abstract

The feasibility of using multi-axis fiber grating strain sensors to monitor transverse strain and transverse strain gradients to complex, woven composite structures has been evaluated. This paper overviews the multi-axis fiber optic grating strain sensors and how they can be applied to measuring multidimensional strain fields interior to composite parts with complex composite weave structures. Experimental results are given for the case of a bi-axially woven composite coupon as well as for an E-glass/epoxy composite sample . Multi Axis Fiber Grating Strain SensorsIn order to measure transverse strain and transverse strain gradients in textile composite materials multiaxis fiber grating strain sensors were used. A multi-axis fiber grating strain sensor is formed by writing a fiber grating onto birefringent polarization maintaining optical fiber. For this type of fiber grating strain sensor, a single fiber grating results in two distinct spectral peaks. These peaks correspond to each of the polarization axes of the polarization preserving fiber, which differ slightly in index of refraction. When the fiber is loaded transversely the relative index of refraction of the polarization axes of the fiber change and the net result is that the difference in wavelength between the spectral peaks changes as well. When the fiber is strained axially, the fiber elongates or compresses changing the fiber grating spectral period and the output spectrum goes to longer or shorter wavelengths respectively. Figure 1a illustrates a multi axis grating written onto polarization preserving fiber, which is subject to uniform transverse loading. In this case the two spectral reflection peaks corresponding to the effective fiber gratings along each birefringent (polarization) axis will move apart or together uniformly providing a means to measure transverse strain 1,2,3,4 . In the case where load along the transverse axis is not uniform, as shown in Figure 1b, the peak associated with the nonuniform transverse load will split 1 . The transverse strain gradient can be measured quantitatively by the spectral separation between the peaks. The response of the fiber is approximately 1/3 of that of axial strain along the length of the fiber. As a specific example at 1300 nm a spectral shift of 0.01 nm along the fiber axis corresponds to 10 microstrain. A peak to peak separation of 0.01 nm due to nonuniform transverse strain corresponds to approximately 30 microstrain for 125 micron diameter bow tie polarization preserving fiber.

Failure mechanisms of fiber optic sensors placed in composite materials

“…The response of the fiber optic sensor to strain in the composite part depends upon its placement and orientation [7][8][9][10][11].…”