Simultaneous strain and temperature monitoring of the composite cure with a Brillouin-scattering-based distributed sensor

A Brillouin-scattering-based distributed sensing system has been used for in-situ strain measurement during the curing of AS4-3501 composite panels. It has a pulse length of 1.5 Ins, which is equivalent to spatial resolution of 15 cm and read-out resolution of 5 cm. Distributed Brillouin scattering sensors have been used to monitor either temperature or strain for concrete structures, dams, and fiber cables by NTT Transmission System Labs (Japan) and Ecole Polytechnique Federale de Lausanne (EPFL) groups with spatial resolutions >1 m. We report the first application for a Brillouin scattering sensor in a composite curing process. The challenges for this application are the requirements of centimeter spatial resolution due to the small size of the composite material; simultaneous temperature and strain measurement during the fast temperature changing process of the curing, and short signal processing times. We present experimental results on strain measurements from optical fiber embedded in the eight-ply panels during the heat-up, isotherm, and cool-down stages of the cure process. The materials studied are 177degreesC cure thermo-set materials used extensively in the manufacture of composite parts for the aerospace industry. The distributed sensor can detect the reaction advancement by measuring the cure shrinkage at the gelation and vitrification stages. Shrinkage is then correlated with the degree of cure data from a differential scanning calorimeter (DSC). The thermal response of the solidified composite during cooling is profiled. Details of the data processing of the Brillouin-scattering-based distributed sensor to obtain the strain variations as a function of cure temperature, time, and location are explained. (C) 2002 Society of Photo-Optical Instrumentation Engineers.