Cure monitoring

Real-time computing of cure monitoring is an essential component for the control of the manufacturing process of composite materials. The rationale for cure monitoring relies on the various physical or chemical properties that can be used to follow the transformation of an initially liquid thermoset resin into its final rigid solid form (curing). The relationship between the monitoring output and the requirements for feedback-loop control is the subject of extended research activities including considerations of the modelling of the cure reaction.

Techniques

• Dielectrometry: The technique of conventional dielectrometry carried out typically in a parallel plate configuration of the dielectric sensor (Capacitance probe), has the capability of monitoring the resin cure throughout the entire cycle, from the liquid to the rubber to the solid state and remains the only technique capable of monitoring phase separation in complex resin blends curing within a fibrous perform. The same attributes belong to the more recent development of the dielectric technique, namely microdielectrometry. Several versions of dielectric sensors are available commercially. The most suitable format for use in cure monitoring applications are the flat interdigital capacitive structures bearing a sensing grid on their surface. Depending on their design (specifically those on durable substrates) they have some reusability, while flexible substrate sensors can be used also in the bulk of the resin systems as embedded sensors.

• Optical fiber cure monitoring is performed by measuring
  • changes in the concentration of specific reactive resin species using spectroscopy methods (FTIR & Raman)
  • changes in the refractive index or fluorescence of the resin (optical property)
  • changes in internal resin strain (mechanical property) with the use of Fiber Bragg Grating (FBG) sensors

• Ultrasonic cure monitoring methods based on relationships between changes in the characteristics of propagating ultrasound and the real-time mechanical properties of a component by measuring:
  • ultrasonic time of flight, both in through-transmission and pulse-echo modes
  • natural frequency using impact excitation and laser-induced surface acoustic wave velocity measurement.

• Thermal cure monitoring methods based on the amount of heat produced during the thermoset cure reaction by measuring:
  • the amount of heat flux per second through a given surface
  • the change in heat capacity for small resin quantities

References

• I.Partridge and G.Maistros, ‘Dielectric Cure Monitoring for Process Control’, Chapter 17, Vol. 5, Encyclopaedia of Composite Materials (2001), Elsevier Science, London, page 413
• P.Ciriscioli and G.Springer, ‘Smart Autoclave cure in Composites’, (1991), Technomic Publishing, Lancaster, PA.