Long term characterization of unidirectional fiberglass for ITER pre-compression rings

In the frame of the development of pre-compression rings for ITER magnet system a unidirectional fiberglass composite has been developed having a ultimate tensile strength (UTS) as high as 2200 MPa at room temperature. During the development of the material a particular grip system has required to be developed, in order to limit the bearing stress on the sample and to obtain a sufficient load to test the sample. To understand the long term creep behaviour room temperature tests have been performed in ENEA Faenza facilities starting in 2007. The long term tests, performed at a constant stress level as high as 80% UTS, showed a behaviour of the material during the test similar to the one of creep tests, although different from the one of metallic materials. In these tests a very low creep strain and very low creep rates, in comparison with the ones of metallic materials, have been recorded, showing a marked dominance of the glass behaviour and a limited influence of the resin on the global behaviour of the composite. From the long term tests, performed at stress levels varying from 63 to 80% of UTS, very different failure times have been recorded for samples loaded at stresses of 75% of UTS and lower and samples loaded at greater stresses. The high stressed samples showed failure times of the order of tenths or hundreds of hours, while the low stresses are all in the order of thousands and more of hours. For the engineering work on the rings the results indicate that at the stress levels considered in the ITER pre-compression rings (50% or less of UTS) the failure time and the deformation of the rings during the ITER operating life are of limited concern. Future activities on this material foresee a set of tests at decreasing stress level (up to 40%), these tests presumably will not lead to an evaluation of the failure time of the material at such a low stress level, however they will give relevant results in order to understand the creep rate of the material at low stress level. (C) 2010 Elsevier B.V. All rights reserved.