Failure Assessments for MQXF Magnet Support Structure with a Graded Approach

The High-Luminosity Large Hadron Collider (HLLHC) upgrade requires new quadrupoles, MQXF, to replace the present LHC inner triplets. The MQXFA magnet is the first prototype that has a 150 mm aperture and uses Nb3Sn superconducting technology in a 4.2 m magnetic length structure. The support structure design of the MQXFA magnet is based on the bladder-and-key technology, where a relatively low pre-stress at room temperature is increased to the final preload targets during the cool-down by the differential thermal contraction of the various components. The magnet support structure components experience different load levels from pre-load to cool-down and excitation. Consequently, a few parts experience high stresses that may cause localized plastic deformations or internal fracture development. The concept presented in this paper for the failure assessment of support structures integrates nonlinear finite element analysis with detailed sub-models and fracture mechanics into an advanced engineering tool. The nonlinear FE solutions enable estimations of the structural response to the given loads, and the advanced fracture analysis with failure assessment diagram (FAD) assesses the structure safety index of results obtained from the FE model. The paper describes how the MQXFA shell end segments are being optimized based on the failure analyses.