Digital Repository, CONVEGNO IGF XXII ROMA 2013

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FEM-DBEM coupled procedure for assessment of cracks in lateral supports of the magnet system of Wendelstein 7-X
R. Citarella, M. Lepore, Joris Fellinger, Victor Bykov, Felix Schauer

Last modified: 2013-06-27


ABSTRACT. The superconducting coils of the magnet system of Wendelstein 7-X (W7-X) are bolted onto a
central support ring and interconnected with five so-called lateral support elements (LSEs) per half module.
After welding of the LSE hollow boxes to the coil cases, cracks were found in the vicinity of the welds that
could potentially limit the allowed number N of electromagnetic (EM) load cycles of the machine. In response
to the appearance of first cracks during assembly, the stress intensity factors (SIFs) were calculated and
corresponding crack growth rates of theoretical semi-circular cracks of measured sizes in potentially critical
position and orientation were predicted using Paris’ law, whose parameters were calibrated in fatigue tests at
cryogenic temperature. The Miner's rule was adopted to allow for different load combinations. The predefined
semi-circular initial crack shape and crack growth limit, were set in such a way to avoid multiple crack
coalescence even if such restrictions could have a significant effect on N. These results have been published in a
first paper, whereas, in the current paper, this work has been extended with analyses of propagation of cracks
with different shapes and affected by nearby cracks. For this purpose, the Dual Boundary Element Method
(DBEM) was applied in a coupled FEM-DBEM approach: the crack path is assessed with the Minimum Strain
Energy density criterion and the SIFs are calculated by the J-integral approach. The Finite element method
(FEM) was adopted to model the overall component whereas the DBEM was adopted for the submodel
analysis in the volume surrounding the cracked area. With this approach, the effects of the crack shape and the
presence of nearby cracks on the allowed number of EM load cycles of the machine were demonstrated.

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