In this paper, the feasibility of real-time monitoring of the fatigue damage of composite laminates under fatigue loading using the embedded intensity-based optical fiber (IBOF) sensors was evaluated. The IBOF sensor system used in this study has a relatively simple sensing principle. Thus the system can be composed of inexpensive components compared with other optical fiber sensors such as Fabry-Perot and Bragg grating sensors. The performance of the fabricated IBOF sensors was evaluated by comparison with a surface-mounted extensometer. The process of conversion of the detected IBOF sensor signal into the stiffness was proposed. Experimental results showed that the sensing response of embedded IBOF sensors showed good agreement with that of the surface-mounted extensometer. The IBOF sensor system showed good potential as a health monitoring system which can give composite structures the capability of fatigue damage monitoring by detecting the stiffness change of the composite structures under fatigue loading. The IBOF sensors showed relatively good durability under fatigue loading compared with commercial electrical strain gages.

Whenever a fibre optic is used to convey a light signal through a radiation field, it is likely that an unwanted background signal will arise from Cerenkov or fluorescent light which will contaminate the signal. In luminescence dosimetry of high energy beams, when a fibre optic is used to convey the signal from the radiation field to the detector, Cerenkov light is the dominant contributor to the background signal and must be corrected for. In this work, a novel method is demonstrated to separate the signal from the unwanted background. A remotely operated shutter is used to block the signal, allowing the residual background in the fibre optic to be quantified. This background is subtracted from the total measurement acquired in a subsequent irradiation, enabling the luminescence signal to be extracted. Two types of shutter mechanism are considered: an electro-mechanical device to intercept the light path and an LCD device to block the light by cross-polarization. Both shutters were characterized and incorporated into a fibre optic dosimetry system used to measure the radiation dose produced by external beam radiation linear accelerators. The dosimeter using each of the shutters in turn was exposed to a 6 MV photon beam to determine their performance, including the measurement of field size dependent output factors. The mechanical shutter determined the output factors to within 0.29% of those measured with an ionization chamber, whereas the LCD shutter gave results that deviated by up to 2.4%. The switching precision of both shutters was good with standard deviations of less than 0.25% and both were able to completely block the light signal when closed. The use of shutters could therefore be applied to any fibre optic based system to quantify and remove a reproducible background arising from any source including ambient, fluorescent and Cerenkov light.