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Embedded sensing micro-components for fibre reinforced composite material systhesis and monitoring
1 : Laboratoire de Tribologie et Dynamique des Systèmes
(LTDS)
-
Website
Ecole Centrale de Lyon
36 Avenue Guy de Collongue, 69134 Ecully Cedex -
France
The failure of materials involved in large infrastructures, such as bridges or roads, may cause a huge loss of life, economy or a loss of services An attractive solution to upgrade existing infrastructures and increase their life-span is to bond fibre reinforced thermosetting composite patches to strengthen the damaged structure and to prevent failure growth. And in doing so, understanding the properties as well as failure mechanism of the bonded thermosetting composites is essential. Initiations and growths of defects occur during almost all life of the composite up to final failure which occurs by coalescence of previous damages (cumulative damage). Hence, how to detect the latent cracks in materials and how to fix the damaged materials with more cost-effectively ways are research emphasizes.As a non-destructive inspection method, in-situ Structural Health Monitoring (SHM) implies the use of sensors, such as piezoelectric ceramics (PZT), which are embedded within a structural material and provide real time performance feedback based on the measurement, such as electrical impedance. These sensors can be used to monitor the health state of thermosetting composites, from their curing process to the propagation of microcracks then to the end of their life cycle, which is useful for analyzing the fabrication or failure procedure of materials. By monitoring the change in the impedance spectrum which is linked to the changes of matrix viscoelastic properties as cure progressed, the different steps of the epoxy cure regarding molecular motion, viscosity, crosslinking density and their consequences on the mechanical properties of the material can be understood.In this work, the resin for fabricating the patch composite is designed for ambient temperature cure. Its prepolymer is a low molecular weight “green” epoxy resin that made from bio-based epichlorhydrine. The hardener used is Cardolite NX5619, a solvent-free, low viscosity phenalkamine curing agent made through the Mannich reaction of cardanol from cashewnuts, formaldehyde, and amines. The natural fiber used is flax, and a quasi-unidirectional fabric made of untwisted rovings was used. The weft and warp ratio is 9/91 and the areal density is 200 g/m2. The manufacturing of the natural fibers reinforced epoxy composite was accomplished by the use of wet hand lay-up process (2 plies) at room temperature.For monitoring purpose (curing and damage), small PZT ceramic disks (0.2 mm thick disk with a diameter of 7 mm) were embedded in the composite materials during manufacturing at room temperature. The change in the impedance spectrum which is linked to the changes of matrix viscoelastic properties as cure progressed was used to follow the different steps of the epoxy matrix cure.The results showed that the piezoelectric transducers are well suited to in-situ monitor the reaction progress during isothermal curing of a flax reinforced epoxy materials. After curing, the sensor was used as damage detector. In order to assess the efficiency of such a system for health monitoring, tests were performed based on tensile measurements using digital image correlation (DIC), classical acoustic emission and scanning electron microscope (SEM).
