Cure Behavior Study and Elastic Modulus Characterization of Resin System of a Quasi Poloidal Stellarator Modular Coil

Composites materials are increasingly becoming choice materials because of their tremendous strength-to-weight properties and impressive design flexibility. A more recent application of composites is in nuclear fusion reactors. One such reactor is the Quasi-Poloidal Stellarator (QPS) being developed by Oak Ridge National laboratory. QPS, with a non-axisymmetric, near-poloidally symmetric magnetic configuration, has stranded copper/epoxy composite coils, used for magnetic confinement of plasma. CTD- 404 and CTD-101K are the resins under consideration for the modular coils with copper fiber as reinforcement. Structural integrity of the modular coils over wide range of temperatures, including liquid nitrogen temperature, is of vital importance and appropriate resin with optimal cure cycle has to be used for this purpose.

In this regard, a study of the stresses induced on the fibers during cure of CTD-404 and CTD-101K was performed using the Cure Induced Stress Test (CIST) setup at UT composites laboratory. Carbon fiber was used for comparison purposes. It was observed that both CTD-404 and CTD-101K induced low cure stresses and high cool down stresses.

Later in this study a new method was developed to calculate the elastic modulus of a resin during cure. The knowledge of elastic modulus development of a resin during cure is vital in minimizing the residual stresses by appropriately changing the parameters of cure cycle. The method was developed based on difference in the displacements of the resin sample during cure, with fiber and without fiber. The method was developed for 3501-6 as the volume change data for CTD-101K and CTD-404 were not available. The volume change data for 3501-6, obtained by using volumetric dilatometer previously, was used and the load data of the reinforced fiber was obtained from cure-induced stress test. The curve for elastic modulus was developed for two isothermal cure cycles. Results obtained were compared with available experimental data and the data available in literature from three-point bend tests of cured samples at different cure times. The values of modulus obtained with this approach compared well with the available data.

Also, a study of the effect of liquid nitrogen temperature on the elastic modulus of the modular coil composite was performed. A fixture was designed to perform a cantilever bend test in liquid nitrogen on a MTS machine. It was observed that the liquid nitrogen temperature did not affect the modulus.