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Sensor Integration to Characterize Thick Laminates



Optical and piezoresistive sensors are well known and available. They can be structurally integrated between the plies of a laminate. In the case of thick laminates, these integrated sensors can help to characterize stress and temperature gradients along the laminate thickness. The mechanical characterization can be used in many aspects to improve the development and manufacturing of thick composite structures. In this work, we present efforts on how to use Fiber Bragg Grating (FBG) sensors, strain gauges with contact pins and Carbon Fibers Sensors (CFS) to characterize composite structures. The FBG sensors consist of a periodic modulation of the index of refraction along the optical glass fiber. Strain and/or temperature variations cause(s) to a shift of the Bragg-wavelength. The strain gauge can be electrically connected after embedding by means of the two vertically attached contact pins. The CFS consist of a standard carbon fiber roving with electrically connected endings. This carbon fiber is isolated by means of a thin glass layers to avoid circuit contacts. Based on the linear piezoresistive behavior of this carbon fiber, the CFS can be used for strain measurements, crack and delamination detection. The use of integrated sensors is presented for several applications in following examples: The strain gauge with contact pins is used to determine the stress distribution in lugs (laminate thickness > 10 mm). Furthermore, the strain gauge is also used to determine residual thermal stresses in adhesively bonded hybrid structures. The CFS is applied to characterize the crack density and the energy release rate in composite pressure vessels. Several CFS can be applied to form a sensor mesh with triangular or rectangular sensor elements. CFS-meshes can be applied for impact and delamination detection. In conclusion, the application of integrated sensors can improve the efficiency and safety of the design process for thick composite laminates.

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