Cure monitoring of thermosetting polymers and associated composites by means of active thermography

The polymerisation of thermosetting polymers is an essential part in the production of fibre reinforced composites in autoclaves. An accurate measurement of the curing process enables the optimisation of autoclave processing cycles, hence is of particular interest with regards to resource- and cost-efficient parts manufacturing. The aim of this work was to evaluate whether methods of active thermography, which determine thermophysical properties, can be utilised in principle to monitor the curing process of thermosetting polymers. Virtual thermography measurements, carried out on generic one- and two-dimensional simulation models that describe the thermal behaviour of the polymer during cure, demonstrate the applicability in theory. This holds for both methods, those that determine the in-plane and those that determine the out-of-plane thermophysical properties. Real pulse thermography measurements with optical excitation in transmission configuration and lock-in thermography measurements were conducted on neat epoxy resin during cure. The lock-in thermography experiments included measurements in transmission configuration with contacted heating as well as measurements in reflection configuration with full surface and spot laser illumination. After vitrification of the polymer, the measured thermophysical properties in the in-plane direction followed the behaviour expected from simulations. Furthermore, it was shown that multilayered materials, comprising of an optical absorption layer of high conductivity and a resin layer with changing properties underneath, have a fundamentally different behaviour compared to a single layer material. Completely different heating frequencies must be considered for lock-in measurements to guarantee a sufficient change of the phase due to changing material properties. Based on the theory of effective material properties of fibre reinforced polymers, it could be shown that especially the relative changes of the thermophysical properties in the in-plane direction resembles quantitatively and qualitatively those of neat thermosetting polymers. This work qualifies active thermography for the measurement of thermophysical properties of neat thermosetting polymers and associated fibre composites during cure. The reconstruction of cure related properties from the measured thermophysical properties and the low sensitivity are identified as challenges and call for further research. A combined approach of thermal and spectral measurements is proposed.