Pre-compression Rings Research Articles

Mechanical Tests on Reduced Scale Mock-Ups of ITER Pre-compression Rings

Mechanical Tests on Reduced Scale Mock-Ups of ITER Pre-compression Rings

Acceptance Test of Pre-Compression Rings for ITER Magnet System

Acceptance Test of Pre-Compression Rings for ITER Magnet System

F4E Procurement of the Pre-Compression Rings Made of Pultruded Composite Material

The highly innovative process, proposed by CNIM and chosen by F4E, is based on a pultrusion technique that involves the manufacture of profiles of epoxy S2-glass. Each pre-compression ring (PCR) is manufactured by winding the flat pultruded profile (2 mm thick and about 2800 m long) and utilizing an adhesive tape (0.12 mm thick) between layers to freeze the geometry. After the curing cycle, the PCR is machined to reach the required geometry tolerances. Each PCR will have a diameter of approximately 5 m, a cross-section of 332 mm × 288 mm and will weigh approximately 3 tons. The PCRs will finally be proof tested at 600 MPa in hoop stress that corresponds to 1.5 time the operational hoop stress. An extensive qualification phase has been released to prove that both materials and manufacturing technology can procure PCRs according to requirements. This paper describes the manufacturing processes, the results of the qualification phase and the status of the production.

Multi scale analysis of ITER pre-compression rings

The Pre-compression Rings of ITER (PCRs) represent one of the largest and most highly stressed composite structures ever designed for long term operation at 4K. Six rings, each 5m in diameter and 337mm×288mm in cross-section, will be manufactured from S2 fiber-glass/epoxy composite and installed three at the top and three at the bottom of the eighteen D shaped toroidal field (TF) coils to apply a total centripetal pre-load of 70 MN per TF coil. The composite rings will be fabricated with a high content (65% by volume) of S2 fiber-glass in an epoxy resin matrix. During the manufacture emphasis will be placed on obtaining a structure with a very low void content and minimal presence of critical defects, such as delaminations. This paper presents a unified framework for the multi-scale analysis of the composite structure of the PCRs. A multi-scale analysis approach employing various scales of ABAQUS FEM models and other analysis tools have been used to calculate the response and performance of the rings over the design life of the structure. We have studied the effects of various defects on the performance of the rings under the operating temperatures and loading that will be applied to the PCRs. The results are presented here.

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

Abstract After a first RD 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.

Ultimate tensile strength testing campaign on ITER pre-compression ring mock-ups

ENEA has developed and characterized a high strength glass fibre-epoxy composite as reference material for the manufacture of the two sets of 3 pre-compression rings located at top and bottom of the inner straight leg region of the ITER Toroidal Field (TF) coils. These rings will provide a radial force of about 70 MN/coil at cryogenic temperature pulling the TF coils into contact and reducing toroidal tension in the four outer intercoil structures. The paper describes the ultimate tensile strength (UTS) testing campaign carried out at ENEA Frascati laboratories on six different rings manufactured winding S2 glass fibers on a diameter of 1 m (1/5 of the full scale) by both vacuum pressure epoxy impregnation and filament wet winding techniques. The volumetric glass content was around 70%. The rings were expanded with radial steps of 0.1 mm into a dedicated hydraulic testing machine consisting of 18 radial actuators working in position control with a total capability of 1000 tons. All the mock-ups showed very high tensile strength (1550 MPa is the average of the mean hoop stresses at failure) and a practically constant tensile modulus. The test results are reported and discussed.

Mechanical characterization of glass fibre–epoxy composite material for ITER pre-compression rings

Pre-compressions rings have been designed to improve the ITER magnets structural support. They are made of high strength unidirectional S-2 glass fibres wound in an epoxy matrix. In order to obtain very high strength properties, ENEA developed and characterized this composite with a high volumetric glass content (∼68%). Benefits of this solution include high strength, no interference with magnet fields and long expected service life. This work illustrates the mechanical characterization of the material used to manufacture the rings. At first, linear specimens were produced to perform tensile and creep tests. Then, reduced scale ring mock-ups have been fabricated and tested in ENEA Fusion-Laboratories. One ring was used for the machining of several specimens for compression and shear tests. Standard and non standard specimens were machined from different geometric directions (longitudinal, radial and transversal with reference to the fibre–glass direction) and tested at room temperature and at liquid nitrogen (77 K) temperature to complete characterization with compression and shear tests. The experimental campaign has been carried out following as close as possible the related ASTM standards in order to evaluate material strength, Young and shear moduli. Test results showed high mechanical strength of the composite in fibre–glass longitudinal direction but lower values in transversal direction.

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.

Short-term tests on unidirectional fiberglass for ITER pre-compression rings

The unidirectional fiberglass unidirectional composite has been proposed as one of the candidate materials for ITER pre-compression rings. Material samples have been produced and tested for ITER pre-compression ring manufacturing and testing. In 2003, an initial batch was produced, giving interesting results, but presenting a lower limit for the mechanical characteristics of the material. Therefore, a grip system has been developed and tested, which requires longer samples (650 mm) for obtaining reproducible results which meet the standard. This development led to a grip system using 45° fiberglass for grips and INCONEL compression rings for keeping the grips in place. In 2006, in short-term test, the fourth batch resulted in an ultimate tensile strength of about 2200 MPa (room temperature) and a very limited dispersion of results. Tests at a temperature of 77 K resulted in a mean value greater than 2750 MPa with a similar dispersion. From the above results, an allowable stress value of 1000 MPa can be envisaged at the operating temperature. Stress relaxation tests are presently in progress.

Fibreglass unidirectional composite to be used for ITER pre-compression rings

The ITER magnet system will be kept in place by a system of pre-compression rings, which have stringent material requirements. The best solution, at the present level of knowledge, is to use a composite material consisting of unidirectional glass fibres in a resin matrix. To study the characteristics of the composite, ENEA performed a series of impregnations using S-2 glass and an epoxy resin matrix. Standard and non-standard specimens were tested both at room and at liquid nitrogen (77 K) temperature. The mechanical strength values were as high as 2200 MPa and the behaviour remained linear practically until failure. A set of specimens will be tested at room temperature with 70–80% of the collapse load in order to verify the creep behaviour of the material. A facility is under construction to test the rings on a scale of 1/5 of the final ITER rings in operating conditions and up to stresses leading to rupture. The test results will supply the information on the behaviour of massive structures in unidirectional fibreglass composite required for the design of the ITER rings.

Key features of the ITER-FEAT magnet system

The design of the ITER magnet system is being finalized. The reference design of the winding pack of the TF coil is based on the use of circular conductors supported by radial plates. This design has been chosen for its high insulation reliability during operation. The overall TF coil structure includes pre-compression rings made of unidirectional fiber glass, which reduce the stress level in the outer intercoil structures and the coil case. The design of the central solenoid, including pre-load structure, has been developed. Two conductor jacket options are still under investigation for the CS and the final choice will be based on the results of on-going R&D.