The fellowship of the composite rings

The pre-compression rings will be one of the most challenging composite structures ever manufactured. Weighing more than 3 tonnes each, they will tightly hold the ITER toroidal field coils from the top and bottom with a radial load of 7,000 tonnes per coil and withstanding hoop stress of 350 MPa per ring.

Ten years of successful R&D performed by the Italian laboratory ENEA near Frascati, under Task Agreements with the European Fusion Development Agency (EFDA), the European Domestic Agency, and direct contracts with the ITER Organization, have recently been brought to a close. The work performed at ENEA by the team of Paolo Rossi identified two suitable fabrication processes for the rings, and developed applicable non-destructive examination methods by x-ray and ultrasound. The Italian team further completed the full mechanical characterization of the glass-fibre/epoxy composite at room and operating temperatures, allowing a final optimization of the ring design, and determined the ultimate tensile stress (UTS) of six mock-up rings (in average over 1500 MPa) in a purpose-designed machine that, with 18 independent hydraulic pullers, simulates the configuration of the 18 toroidal field coils (see related article).

The last challenge was to achieve the prediction of the long-term performance of the mock-up rings: this challenge has also been met. Tests on specimens at different constant loads during long periods had allowed the definition of the creep behaviour, however the correlation with long-term tests on the mock-up rings was pending. Early last year a ring was loaded at 950 MPa (65 percent of UTS) with no measurable degradation rate after 210 days maintained at such stress. A new ring was loaded last month at 1100 MPa (77 percent of UTS) and maintained under constant load until rupturing after 140 hours, right between the breaking times of two specimens loaded equivalently.

This accomplishment occurred concurrently with the end of the European Domestic Agency call for tender for the pre-compression rings. It enables our Division to wrap up the technical understanding of the rings' performance before industry enters in the game. The expertise and the purpose-design equipment available at ENEA will continue to play an essential role for industry in the years to come.