Stress relaxation testing of pre-compression ring mock-up for the ITER magnet system

The ITER pre-compression ring system is based on two sets of 3 rings made in fibreglass-epoxy composite and is located on top and bottom of the inner straight leg region. These rings will provide a radial force of about 70 MN/coil (35 MN at the top and 35 MN at the bottom) under cryogenic conditions, pulling the TF coils into contact, reducing the gap between the poloidal shear keys and their keyways during operation and also reducing the toroidal tension in the four Outer intercoil structures bands. The paper describes the stress relaxation test performed in ENEA Frascati laboratories on a reduced scale glass fibre-epoxy composite ring. The ring was manufactured with a diameter of 1 m (1/5 of the real scale one) by winding S2 glass fibres then impregnated with epoxy. The volumetric glass content was of 69% and no significant presence of defects was verified by an X-ray survey. The test has been carried out following as close as possible the ASTM standard E 328-86. The ring has been inserted into a dedicated hydraulic testing machine consisting of 18 radial pulling actuators and then loaded up to a ring membrane hoop stress of 800 MPa. The deformation has been maintained constant during the test by inserting a solid steel disk in the machine centre restraining the machine radial motion. Average hoop stress has been daily monitored measuring the force required to lift the ring just free of the constraints during the test period. The ring performed 125 testing days showing no significant stress relaxation and a small reduction of the ring stiffness that appeared mainly in the first days of the test. (C) 2009 Published by Elsevier B.V.