Page 1 of 6 The 7 th International Conference on FRP Composites in Civil Engineering International Institute for FRP in Construction PULTRUDED FRP GIRDER WITH EMBEDDED OPTICAL SENSOR NETWORK Markus GABLER Dr.-Ing., Buckland & Taylor Ltd., Canada magb@b-t.com Jan KNIPPERS Prof. Dr.-Ing., Universitaet Stuttgart, Germany j.knippers@itke.uni-stuttgart.de ABSTRACT: The presented work deals with the integration of fibre optic sensors into pultruded fibre composite sections. Opposed to previous work done is this field, the girders have multi-walled cross sections and the sensors are being positioned at any desirable place and aligned freely towards the pultrusion axis. To achieve this, the optical sensor fibre is being placed and fixed onto a fabric before further processing. This ‘sensor fabric’ is then being embedded during the fully automatic production. In the optical fibre, the wavelength of transmitted light is being diffracted depending on the 3D strain state and body temperature within the fibre core (4-dimensional dependency). To allow for conclusion on the internal forces of the composite girder, the dependency between ambient composite strain and the strain in the fibre core had to be derived. Therefore, a spatial FEA analysis and various laboratory tests have been performed in order to determine the respective dependencies. The interaction proved to be rather sensitive to irregularities in the sensor fibre placement as well as flaws in the laminate. The new development has been implemented and tested with the 'smart bridge', which will be described in this contribution. 1. Introduction The failure mode of a single FRP layer is rather brittle. Even though a multilayered laminate might allow for a pseudo-ductile moment-phi-curve, FRP lacks in redundancy compared to other construction materials. Thus, composite structures have to be designed with high safety factors ('safe life' concept), which yields to less economic use. An alternate approach is to guarantee the bearing capacity through continuous monitoring ('fail safe' concept). This allows for recognition of first-ply or local failure and provides the opportunity of taking countermeasures before complete failure. Therefore, it may be advantageous regarding economic design to continuously monitor FRP structures. However, the installation of externally bonded sensors is very expensive and the sensors may be short lived since they are exposed to weathering or mechanical damage. In contrast, embedded sensors are better protected and can be installed during production, which is reducing additional cost to a minimum and improves reliability. Fiber optic sensors are very well suited for implementation in FRP since the continuous fiber is reducing matrix disturbance to a minimum and is more corrosion resistant than metallic strain gauges. For the presented research project Bragg grating sensors have been utilized. This sensor consists out of gratings which are inscribed via laser along a continuous glass fibre. The glass fiber is usually protected by a polymer coating. When light is guided through the fiber, a portion of it is reflected at each grating. The reflected light has a specific wavelength, which is dependent on several grating parameters. If strain or temperature shift is applied on the fiber at the location of a grating, the wavelength is shifted based on the quantity of disturbance. Thus, the strain and temperature shift can be measured indirectly by monitoring the reflected wavelength. The dependency is described in (1). 2. Developed manufacturing process 2.1. Embedding of sensors An economic ways of FRP production is the continuous pultrusion process. The reinforcing fibers are processed as either fiber bundles (rovings) or fabric. The fibers are spun from a coil, wetted with liquid