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Simulations & Innovative Projects

The "Projects and Calculations" labs (BPC) has two main roles : achievement of innovative projects and carrying out complex calculations in many physical fields for all ILL divisions.

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Mechanics

BPC is able to carry out very complex mechanical calculations on various samples and get in this way detailed information on stress, displacement, eigenfrequencies...

 

 

Structure Calculations

This example shows deformation calculation of the future IN16B movable aluminum vessel (under vacuum). Its internal volume is roughly 35m3 and its weight 8 tons

Optimisation of disk geometry for NIST institute

 

Thickness profile

The optimal profile for a disk without windows is a Gaussian profile. However, if we consider the disk with windows, the problem becomes more complicated.

The windows destroy the axisymetric equilibrium between radial and hoop stresses and essentially "cut" the continuity of hoop stresses. This leads to stress concentration at the lower part of windows.

 

The optimal design consists in finding a way to minimise the stresses in this region.

To solve this problem, another profile has been investigated, which allows the decreasing of the mass of material at the extremity of the disk, after the fillet.

The optimal solution is an hyperbolic profile.

The thickness of the disk in the central area is kept constant in order to optimise its mass and the total thickness of the disk at its centre regarding the maximum stress in the central hole.

Stress and deformations analysis

Method :

This disk has been modelled using the finite element software Ansys.

Due to its cyclic symmetric structure, and symmetry plane, only a portion of disk (1/24th) has been represented.

The model comprises 3D elements SOLID 45, and 2D elements SHELL43 which model the gadolinium.

The mesh is refined at the maximum stressed areas (window’s fillet and central hole).

The disk is submitted to a rotational speed of 18 000 revolutions per minute.

Results for stresses study:

 

For a periphery thickness of 2 mm, the maximum equivalent stress (Von Mises stress) is 258 Mpa in the fillet of the window. The security factor compared with the yield strength is about 2.

The study of the principal stresses shows that the circumferiential stresses sq are predominant compared with the radial stresses (259 Mpa versus 158 Mpa).

 

Example of a modal analysis of choppers

Modal analysis was performed to monitor the potential vibrations of the disks. The known excitations are the residual unbalance and the longitudinal active magnetic bearing of the shaft.

Experience on similar choppers showed that excitation due to the longitudinal active magnetic bearing is prevailing. So the modal analysis is focussed on the eigenmodes along z direction.

Method

The mesh of the model is specific: mapped mesh, 20 nodes tetrahedral elements SOLID186.

However the disk presents a cyclic symmetry, the modal analysis must be done on the entire disk, though only a small part of the searched spectrum would be calculated.

We will study particularly the eigenfrequencies, which the modal mass is significant compared with the total mass of the disk.

The boundary conditions are:

All degrees of freedom are fixed for 0 £ r £ 28.5 mm.

 

Study of the eigenfrequencies at standStill

The first eigenfrequency of the disk is at 395 Hz. It is a rotation mode about a w axis (in the xy plane). However the effective mass corresponding to this mode is very small: only 0.1% of the mass of the disk.

 

Eigenmodes along z:

 

  • The first eigenmode with a significant modal mass is at 451 Hz. It is a corolla mode. Its effective modal mass is about 2.56 kg, which represents 41.6% of the total mass of the disk.

  • The next significant eigenmode is at 942 Hz. Its effective mass is about 2.2kg (36% of the total mass).

Thetorsion mode around the z-axis is at 565 Hz.

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