Use of Fabry Perot fiber optic sensors to monitor residual strains during pultrusion of FRP composites

This paper describes the use of an embedded optical sensor to measure the process induced strain in pultruded carbon fiber reinforced composite rods. The survivability of Fabry Perot optical sensors in the pultrusion process, real time strain monitoring during the pultrusion process and residual strain measurements within the composite rods, were the three objectives of the research. Strain profiles were recorded for individual experiments as the sensors traveled through the pultrusion die, and for the cool down period after the sensor had exited the die. For the total pultrusion process, the residual strain after cooling was found to present somewhat of a problem. For several experiments, the residual strain after exiting the pultrusion die was in the range of +200 to 400 microstrain, after which the sensors ceased to function. Calculations indicated that the radial shrinkage of the carbon fiber rods was sufficient to cause failure of the glass capillary portion of the Fabry Perot sensors. A novel method of reinforcing the Fabry Perot sensors before being pultruded was successful in allowing the sensors to survive the total process with only a slightly negative residual strain.