Cure Monitoring of Composite Carbon / Epoxy through Electrical Impedance Analysis
Composite materials are increasingly used in aeronautic; they offer many benefits such as mechanical strength, mass and consumption reduction. However, their process development needs to be known and controlled, in order to adjust the process parameters and optimize the characteristics of structures made from these materials. This paper is focused on impedance spectroscopy measurement and analysis technique to characterize material’s properties. In fact, the composites based on carbon fiber have electrical proprieties; therefore a three-dimensional modeling of the electrical conduction in the material is established by using a distributed allocation of an electrical resistance (RP) in parallel with a capacitance (CP). Then, thin electrodes (40 μm thick) are inserted inside the material and a specific impedance measurement bench is developed to perform real-time measurements of RP and CP on unidirectional (UD) mono-ply and multi-plies samples. During curing (in an oven) the change in values of both RP and CP in different stages of the curing cycle is showed. Then, problems that occur during the curing cycle (layup defect, loss of vacuum) were detected by a large gap of the measured electrical parameters in comparison with the ordinary case. Therefore, by this electrical measurement, we present a way to ensure an automated real-time monitoring of the composite curing
Insulation cure monitoring on global VPI large AC motors
Epoxy resins and micaceous insulation systems are used to insulate high voltage large rotating machine components. Composite insulation systems undergo marked changes in dielectric response as the cure of the system progresses. It is possible to exploit these changes to monitor and optimize the curing process. This paper presents the first of some work done on the global insulation cure monitoring of large AC motor stator windings processed through global vacuum pressure impregnation (VPI). Changes in the measured permittivity and dissipation factor in the range of 1 Hz to 10 kHz for laboratory samples, as well as capacitance test results on full size stators at 100 Hz, are reported. It is shown that permittivity (capacitance) appears to be ideal for cure monitoring purposes. It is also shown that the best frequencies for monitoring of this type are in the 100 Hz to 500 Hz range. Changes in the slope of the natural logarithm of permittivity (capacitance) with time (d[Ln(/spl epsi/')]/dt) are shown to be promising as an indicator of the level of cure. By terminating the curing when d[Ln(/spl epsi/')]/dt reaches a certain terminal value, it is believed that a high level of reproducibility in the level of cure of the resulting insulation system can be achieved. Discussion is given on analyses of d[Ln(/spl epsi/')]/dt, both in the laboratory and in the factory. The level of cure should also be correlated to other relevant physical properties such as the glass transition temperature or other global mechanical properties. It is shown how dielectric properties monitoring optimizes global VPI processing by producing a more reliable level of cure and reducing cure time.
Dielectric monitoring of epoxy cure
The cure process of mixtures of a DGEBA-type epoxy resin and ethylenediamine, in different molar ratios, was investigated by dielectrometry in a wide frequency range, 103-2·1010 Hz, at three temperatures, 25, 60, and 70°C. Two relaxation processes were found whose relevant parameters were obtained by a proper fit procedure. A deeper knowledge and a quantitative description of the observed effects was achieved by analyzing the change of dielectric data caused by the chemical reaction. The change of the dielectric strength of both processes, as well as of other dielectric parameters, reflect the chemical modifications of the system; the permittivity, both static and at microwave frequencies, was linearly related to the conversion. These results support the interpretative model based on the disappearance of dipoles, and confirm the permittivity as a valuable quantity to monitor the kinetics of the reaction. © 1996 John Wiley & Sons, Inc.