Application of fiber optic sensors for elevated temperature testing of polymer matrix composite materials

Advanced polymer matrix composite (PMC) materials have been more frequently employed for aerospace applications due to their light weight and high strength. Fiber-reinforced PMC materials are also being considered as potential candidates for elevated temperature applications such as supersonic vehicle airframes and propulsion system components. A new generation of high glass-transition temperature polymers has enabled this development to materialize. Clearly, there is a requirement to better understand the mechanical behaviour of this class of composite materials. In this study, polyimide-coated fiber optic sensors are employed to continuously monitor strain in a woven carbon fiber bismaleimide (BMI) matrix laminate subjected to tensile static and fatigue loading at elevated temperatures. A unique experimental test protocol is utilized to investigate the capability of the optical sensors to monitor strain and track stiffness degradation of the composite material. An advanced interrogation system and an optical spectrum analyzer are utilized to track the variation in the optical fiber wavelength and the wavelength spectrum for correlation with strain gage measurements. Isothermal tensile static and fatigue tests at room temperature, 105°C, 160°C and 205°C suggest that these optical sensors are capable of continuously monitoring strain and tracking the stiffness loss of a highly compliant PMC specimen during cyclic loading. The results illustrate that employing optical sensors for elevated temperature applications has significant advantages when compared to conventional strain gages.