G. Tsamasphyros, K. Kalkanis, G. Kanderakis, N. Pantelelis, M. Tur, Y. Botsev, N. Gorbatov, G. Maistros, M. Kazilas, S. Ronen, U. Ben-Simon, I. Kressel, A. Green, R. Chemama, E. Inglisian, F. Voelkermeyer, F. Fischer
The combined use of Bragg grating and dielectric sensors has been studied for the monitoring of bonded composite
repairs to metallic parts, using standard composites field repair equipment and specially built "sensing mats"
Measurements retrieved via a variety of sensors were correlated, to analyze the interactions of the chemical and physical
changes occurring during the bonding process. It was concluded that the combined use of Bragg gratings and dielectric
sensors can provide important information for the cure monitoring of bonded composite repairs.
Conventional aircraft repair techniques employ bolted or riveted metallic reinforcements, which frequently introduce additional stress concentrations leading to further cracking and creating areas difficult or impossible to inspect. Bonded composite repairs (“patches”) result in the elimination of stress concentrations caused by additional fastener holes, improved strength to weight ratio and present a sealed interface. This reduces even further the danger of corrosion and fretting under the repair, gives greater flexibility in design and lessens application time while lengthening fatigue life.
Embedding optical fibres and sensors into the patch, and combining this with advanced data collection and processing systems, creating a so-called “smart patch”, will enable the real-time assessment of aircraft structural integrity resulting in reliable prediction of maintenance requirements for repaired structures. This paper describes the current state of the art in smart patch technology, and includes a detailed description of the measurement problem and of the work being undertaken to solve it, at both the component and system level. An analysis of typical crack behaviour, based on FE modelling is presented and this demonstrates the need for optical strain sensors having a very short gauge length. The paper discusses the advantages and limitations of very short Fibre Bragg Gratings (FBGs) in this context and also provides early experimental data from 1mm and 2mm gratings which have been fabricated for this purpose. The paper also describes the impact of the measurement and environmental constraints on the design of the FBG interrogation system and presents the results of initial trials. The work is being undertaken in the framework of a collaborative project (ACIDS) which is co-funded by the European Commission.
A classical cracked metallic structure repaired with a "smart" bonded composite patch has been studied, using finite element analysis, in order to determine the debonding detection capabilities of the optical network and to select the appropriate optical fibers paths and Bragg Grating sensors locations. The patch is bonded over a cracked aluminum plate, by means of a thin adhesive layer, while the primary loading axis of the metal is assumed to be parallel to the direction of the optical fibers. Different optical fiber paths and sensor positions were considered and their ability to measure the variation of the developed strain field due to the patch debonding propagation around the crack tip was studied. It was concluded that a fiber optics network is capable of evaluating the increasing debonding area around the crack tip and can provide adequate information concerning the critical parameters required for the monitoring of the structural integrity of composite patch reinforced structures (i.e. strains developed at the patch debonding boundary and position of the crack tip). At least two Bragg Grating sensors should be used at each side of the crack per optical fiber, to enable adequate monitoring of the adhesive debonding and crack propagation.
A three-dimensional model of a structurally integrated optical fiber sensor into a composite patch bonded over a cracked metallic structure is built using ANSYS finite elements analysis program. Composite patch is modeled assuming three- dimensional epoxy matrix with embedded cylindrical carbon fibers and optical fiber sensors. The latter is composed from two different materials to represent the core/cladding and the protective coating. The whole patch is perfectly bonded over a cracked aluminum sheet. External loads are applied only on the metal structure, as in a real repair case. The primary loading axis of the metal is parallel to the direction of the optical and the carbon fibers. However, because of the different nature of the materials that form the composite patch, complex mechanical interactions between the fibers and the surrounding material occur, resulting in a complicated strain field along the optical fiber sensor, which causes significant sensing variations. Different crack lengths are assumed in order to study their effect on the strain field of the optical fibers at different points across its radius, as well as the variation of the stress field along the optical sensor and the corresponding plots were produced.
The eddy current method is used to trace crack propagation under a composite patch repair of a cracked metallic structure, after mechanical testing in fatigue. The capability and the reliability of the eddy-current method to detect cracks under a composite obstacle of significant thickness are checked for several patch thicknesses. Notched specimens 6 mm thick were fabricated using 2024-T3 Aluminum. Boron Epoxy patches bonded with film adhesive were applied to the one side of the metallic specimens. Initial notches were 10 mm long, while the thickness of the reinforcement was varying from 2 layers (0.25 mm) to 7 layers (0.875 mm) in order to represent actual structural composite patch repairs. Crack propagation from the tip of the notches was achieved by fatigue loads. The estimation of required loads to cause fatigue crack propagation was done by means of three-dimensional finite elements analysis. The eddy current method was then applied to trace the crack tip under the patch after their mechanical testing. Accuracy of the eddy-current method was verified by measuring the crack lengths on both sides of the specimen and comparing the results. The eddy-current method was found to be fully capable of tracing the crack propagation under the composite patch, requiring only proper calibration for the generator. Small differences in the crack lengths between the patched and the unpatched side of the specimen were explained by their non-symmetric configuration, which induced different stress intensity factors at the patched and the unpatched sides, as finite elements analysis has clearly shown.
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