Overview of the testing activities on ITER sub-scale pre-compression rings

Highlights: ► ENEA developed a high strength glass fiber-epoxy composite for ITER pre-compression rings. ► High UTS values were obtained at RT on linear specimens (2200 MPa) and on scaled ring mock-ups (1550 MPa). ► Creep tests showed very low creep strain and creep rates. ► Long term tests showed no significant stress relaxation on the ring mock-ups. - Abstract: After a first R and D and testing activity to develop and characterize by tensile and creep tests a high strength glass fiber-epoxy composite as reference material for the manufacture of ITER pre-compression rings, ENEA designed and manufactured a dedicated testing facility and different sub-scale composite ring mock-ups in order to characterize their mechanical properties. The paper reports the results of the overall testing activities performed during the last years on a total number of eleven sub-scale pre-compression ring mock-ups manufactured by winding S2 glass fibers on a diameter of 1 m (1/5 of the full scale) both by vacuum pressure epoxy impregnation (VPI) and filament wet winding techniques (WW). The first three rings were manufactured by ENEA Frascati thanks to a particular VPI technique; one of them was used as base composite material to manufacture different sets of specimens for shear, compression and non destructive tests (NDT). Then, five other mock-ups were manufactured following ENEA VPI process and three using WW technique by two different industrial companies. The rings were tested in ENEA Frascati in a dedicated hydraulic testing machine consisting of 18 radial actuators working in position control with a total load capability of 1000 tons. The complete testing campaign consisted of six ultimate tensile strength (UTS) tests and four stress relaxation (SR) tests. The tests demonstrated that the composite (S2 glass-epoxy) is a valid and viable solution for the ITER pre-compression rings (non conductive, non magnetic, capable to work at high stress level without relaxation). UTS tests showed an overall average ultimate strength of 1550 MPa (mean hoop stress in the cross section) and a constant hoop modulus to failure of all the mock-ups. Stress relaxation tests did not show any significant relaxation at different stress levels and durations.