PSI - Issue 24

Andrea Chiappa et al. / Procedia Structural Integrity 24 (2019) 898–905 Andrea Chiappa et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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In order to reduce the computational burden, the FEM representation of the WP deploys a continuous medium, with smeared orthotropic properties. The adopted homogenization procedure extracts equivalent stiffness properties from an array built by the repetition of the same representative volume element (RVE). In this case, a proper RVE includes a cable with its steel jacket and both inter-turn and double pancake insulations. Details on the procedure and examples of its application can be found in Giorgetti (2018). The mechanical properties for the steel casing and the homogenized WP are reported in Table 2 and Table 3, respectively.

Table 2. Mechanical properties for SS316LN. Mechanical property

Value

E (Young modulus) [GPa]

205 0.29

ν ( Poisson’s ratio )

α (thermal contraction) [1/K]

10.38∙10 -6

Table 3. Mechanical properties for the homogenized WP referred to the local coordinate system of Fig. 1. Mechanical property Value T = 4.2 K Value T = 293 K E x [GPa] 20.3 19.9 E y [GPa] 25.3 24.7 E z [GPa] 51.4 49.5 ν xy 0.355 0.348 ν yz 0.146 0.148 ν xz 0.113 0.115 G xy [GPa] 1.15 1.13 G yz [GPa] 12.4 12.0 G xz [GPa] 9.9 9.6 α x [1/K] 12.5 ∙10 -6 12.5 ∙10 -6 α y [1/K] 12.6∙10 -6 12.6∙10 -6 α z [1/K] 10.5∙10 -6 10.5∙10 -6

A condition of frictionless contact is set between the filler wrapping the WP and the steel walls of the casing. Preliminary analyses showed that the presence of friction at this interface has a little influence on the overall stress assessment and neglecting its action is a slightly conservative assumption. Results refer to the end of the second step of the analysis, with the entire electromagnetic loads acting at 4.2 K. The most stressed area of the steel casing is at half height of the inner leg, where the compression due to adjacent bodies occurs on the stiffest part of the structure, reaching the highest intensity. Fig. 3 reports the distribution of stress intensity on the left and of total displacement on the right. The sharp modelling of the inner edges of the casing introduced a geometrical singularity with a corresponding peak stress exceeding the material strength. This result was filtered out the following stress assessment, we postpone the investigation of this aspect to future 3D analyses with a proper representation of the edge fillets. Stress linearization is performed according to the procedure exposed in ITER magnet design criteria (2012). Linearized stresses along the chosen path of Fig. 4 left, lying on the equatorial cross section of the inner leg, shows that the primary stresses are still within acceptable levels (Fig. 4 right). Both membrane and membrane + bending stress intensities are below the strength limits of SS316LN, which are 667 MPa and 867 MPa respectively.